3 Mineral Properties
- Mineral names derive from mineral appearance, composition, provenance, and many other things. Many are named after well-known mineralogists.
- A mineral’s common appearance (its habit) includes both crystal shape and the way that multiple mineral crystals may grow together.
- Mineral crystals have shapes related to the arrangements of atoms within them.
- We typically use physical properties such as luster, color, diaphaneity, crystal shape, streak, hardness, and cleavage to identify and distinguish different minerals.
- Other properties, including magnetism and reaction to hydrochloric acid are sometimes diagnostic.
3.1 Naming Minerals
Mineral names are based on mineral appearance, mineral chemistry, where the mineral is found, a famous scientist, or anything else deemed important by a mineral’s discoverer. The Commission on New Minerals and New Mineral Names of the International Mineralogical Association reviews proposed new names and descriptions and judges their appropriateness. The Commission also occasionally discredits old names. Absolute identification and classification of minerals require knowledge of their composition and atomic structure. Mineralogists must include such information when they submit names to the Commission for approval.
Determining mineral composition and structure may require time and equipment unavailable to most mineralogists or to mineralogy students, but fortunately we can use other methods to tell minerals apart. Differences in composition and structure lead to differences in appearance and in many other properties of minerals we use for identification. For example, the mineral halite, shown here, is most easily identified by its cubic, often clear crystals, by its softness, and by its salty taste.
Most of the properties discussed in this book fall into two general groups: hand specimen properties, which are easily determined using large samples, and optical properties, which we can only see with specially prepared slides and a polarizing light microscope, also called a petrographic microscope. This chapter reviews hand specimen properties and discusses their use in identification. We cover optical properties in the next chapter.
3.2 Mineral Identification
Give a mineral specimen to a nongeologist and ask them to describe it. Generally, they mention the appearance, especially color, first. With a little prodding, they may go on to describe the shape and nature of visible crystals.
For example, they might describe pyrite as metallic, being gold in color, and forming cubic crystals. (This is true of many pyrite crystals, but other crystal shapes are common.) They might describe a piece of the variety of quartz called rose quartz as hard, pinkish, glassy, and partially transparent.
Metallic and glassy are terms describing luster. Gold, clear, and pinkish describe color. Transparent describes diaphaneity. Cubic describes crystal shape, a property related to symmetry. These four properties (luster, color, diaphaneity, and shape) are basic for mineral identification. Other properties including streak (the color of a mineral when powdered), the way a mineral breaks (cleavage, parting, fracture), and hardness are also common keys to identification. Still, other properties can be important for specific minerals.
Given a single property, for example luster, we can sort minerals into groups. In the case of luster, we usually start by dividing minerals into those that are metallic and those that are nonmetallic. The pyrite seen above is metallic. The quartz is nonmetallic. There are, however, many metallic and many nonmetallic minerals; other properties must be considered if minerals are to be identified. Nonmetallic minerals can, for instance, be divided further based on more subtle luster differences.
Ultimately, we can identify minerals by name or at least place them into small groups based on their properties. It is tempting, then, to come up with a standard list of properties that we should evaluate when identifying minerals. However, most mineralogists know that, depending on the sample and circumstances, some properties are more important than others. Rather than going through a long list or filling out a standard table, experienced mineralogists focus on the properties that are most exceptional or unique. Sometimes, a single property, such as strong effervescence by hydrochloric acid (diagnostic of calcite), may serve for mineral identification. And, being magnetic usually identifies magnetite. (Metallic iron in meteorites is also magnetic.) At first, mineral identification may seem tedious, but with a little experience, it is possible to find shortcuts to make the process more efficient.
3.3 Crystal Shape
To most people, a crystal is a sparkling gem-like solid with well-formed faces and a geometric shape. For many scientists, including all mineralogists, crystal and crystalline also refer to any solid compound having an ordered, repetitive, atomic structure, which may or may not result in crystal faces and a gemmy appearance.
We use the term “crystal” in both ways. When a mineralogist refers to a garnet crystal, the reference may be to a dodecahedron, a twelve-sided crystal with diamond-shaped faces like the drawing seen here and the real garnet next to it. We call crystals, such as the well-formed garnet crystal with well-developed faces, euhedral.
On the other hand, petrologists and mineralogists may refer to crystals of garnet in a rock. The crystals may not have any smooth faces at all. The garnet seen in the photo here, which was picked out of garnet gneiss (a metamorphic rock), lacks crystal faces entirely. If no faces are visible, as in this photograph, the crystal is anhedral. Those crystals that fall between euhedral and anhedral are called subhedral.
Mineral crystals always have an ordered arrangement of atoms within them, but the crystals may not be geometrically shaped or smooth on the outside. All garnet crystals, for example, have the same highly ordered arrangement of atoms shown in this ball and stick drawing, but only some garnet crystals have visible crystal faces. In fact, most natural garnet is anhedral or, perhaps, subhedral. So, the garnet in the photo above is typical.
With just a few exceptions, all minerals are crystalline, but perfectly formed crystals with flat faces are rare. Nonetheless, because crystal shape reflects the crystal’s atomic arrangement, when faces on a mineral are fully or partially developed, crystal shape can be a powerful identification tool. When no faces are visible, we must rely on other properties to identify a mineral.
3.3.1 Crystal Forms
Mineralogists use the term form to refer to a group of identically shaped faces on a crystal. The faces of a form are related by crystal symmetry and have identical chemical and physical properties. If a crystal contains only one form, all crystal faces are the same size and shape. Euhedral garnet crystals, for example, generally have one form consisting of 12 identical diamond-shaped faces like the dodecahedron in Figure 3.5, earlier in this chapter.
The drawings in Figure 3.9 show common forms for six different minerals. Different samples of the same mineral may crystallize with different forms, but those shown here are typical. Like garnet crystals, chabazite crystals generally have only one form, typically containing six identical nearly (but not quite) square faces. The other five drawings show crystals with more than one form. In the ilmenite, corundum, vesuvianite and datolite drawings, the different forms have distinctive different shapes. In the gehlenite drawing, all faces are rectangular but not all are the same size. The gehlenite crystal contains three forms (three pairs of identical rectangular faces) with different sizes.
3.3.2 Mineral Habit
The photos above show examples of different mineral habits. Habit, a property closely related to crystal shape, includes shape and size of crystal faces, how forms combine, how well developed different forms are, and the way multiple crystals grow together. Habit, thus, is the characteristic appearance a mineral can have. Some examples of different habits are shown in the photographs above.
The most useful terms describing habit are self-explanatory. Common ones used to describe the habit of single crystals include equant (equidimensional), acicular (needlelike), tabular, and bladed. These and other terms are defined below.
|Terms Used to Describe Shapes of Individual Crystals (With Example Minerals)|
|equant||the same dimensions in all directions (garnet, spinel)|
|blocky||equant with a nearly square cross sections (halite, galena)|
|acicular||needlelike (actinolite, sillimanite)|
|tabular or platy||appearing to be a plates or a thick sheet (gypsum, graphite)|
|capillary or filiform||hairlike or threadlike (serpentine, millerite)|
|bladed||elongated and flattened in one direction (kyanite, wollastonite)|
|prismatic or columnar||elongated with faces parallel to a common direction (apatite, beryl)|
|foliated or micaceous||easily split into sheets (muscovite, biotite)|
For describing an assembly of multiple crystals, we use terms such as massive, granular, radiating, and fibrous. We list and define these terms and others in the table below.
|Terms Used to Describe Properties of Crystal Aggregates|
|massive||appearing as a solid mass with no distinguishing features|
|granular||composed of many individual grains|
|radiating or divergent||containing crystals emanating from a common point|
|fibrous||composed of fibers|
|stalactitic||appearing stalactite shaped|
|lamellar or tabular||appearing like flat plates or slabs growing together|
|stellated||containing an aggregate of crystals giving a starlike appearance|
|plumose||having feathery appearance|
|arborescent or dendritic||appearing like a branching tree or plant|
|reticulated or latticelike||net-like, composed of slender crystals forming a lattice pattern|
|colloform or globular||composed of spherical or hemispherical shapes made of radiating crystals|
|botryoidal||having an appearance similar to a bunch of grapes|
|reniform||having a kidney-shaped appearance|
|mammillary||having breastlike shape|
|drusy||having surfaces covered with fine crystals|
|elliptic or pisolitic||composed of very small or small spheres|
Unfortunately, although museum specimens and pictures of minerals in textbooks often show distinctive shapes and habits, most mineral samples do not. Small anhedral crystals without flat faces, or massive aggregates of many small crystals, are typical, often rendering shape and habit of little use for identification. Additional complications arise because some minerals, for example calcite, have different crystal shapes or habits, depending on how they grow. Nonetheless, shape and habit reflect the internal arrangement of atoms in a crystal and, when visible, can be important diagnostic properties. For example, the movie makers blew it in the movie Congo. See the Box 3-1 below.
●Box 3-1 What’s Wrong With This Picture?
In the 1995 movie Congo, an exploration team goes to Africa to seek large, flawless diamonds. When the diamonds are shown, the movie immediately loses credibility with mineralogists because the crystals are hexagonal prisms (long crystals with a hexagonal cross section). Mineralogists know that diamond crystals can never be hexagonal prisms. The photo shows actor Timothy Curry holding a quartz crystal, not a diamond crystal.
●Box 3-2 Asbestiform Minerals and Health Risks
We use the term asbestiform to describe a mineral habit characterized by small, strong, and flexible fibers, equivalent to hairs or whiskers. Asbestos is a commercial name for any marketable asbestiform mineral. For legal and regulatory purposes, however, the US Environmental Protection Agency has developed a more restrictive definition and defines asbestos as being one of six specific minerals. Other countries have similar legal definitions.
Mineralogists have described many asbestiform mineral varieties, but most are rare and only a few are produced for sale. The photo seen here shows “white asbestos,” composed of the mineral chrysotile. Chrysotile, which accounts for about 95% of the commercial market, is a member of the serpentine mineral group. It is a widespread but minor mineral in many altered ultramafic rocks.
Some commercial asbestos is composed of crocidolite (“blue asbestos”) or amosite (“brown asbestos”), varieties of the amphiboles riebeckite and grunerite, respectively. Other minerals that may be asbestiform include other amphiboles (anthophyllite, tremolite, actinolite), clays (sepiolite, palygorskite), and some members of the zeolite mineral group.
Historically, asbestos has had many uses. Since around 1880, it has been mined in large quantities because it is tough, flexible, and fire and chemical resistant. Between 1900 and 1986, builders sprayed asbestos on walls, ceilings, and pipes in many buildings in the United States. Industries have used asbestos in brake linings, roof shingles, and other applications. Unfortunately, asbestos easily crumbles to make a fine dust that people can inhale. Fibers can become embedded in lung tissue and cause asbestosis (a chronic breathing disorder that may be fatal), lung cancer, or mesothelioma (another form of cancer). For the most part, epidemiologists have documented these diseases in workers exposed to high levels of asbestos over long times.
In 1907 health workers reported the first asbestos-related diseases, but it was not until around 1960 that the threat posed by asbestos was accepted as serious. In 1974 the Environmental Protection Agency (EPA) banned asbestos for most commercial use in the United States, and soon afterward launched a vigorous program to remove asbestos from commercial structures. However, American companies still ship many products containing asbestos to developing countries. Despite the ban and efforts to eliminate asbestos from our environment, it is still common in many buildings and as a component in urban dust.
Click on the arrow below to be taken to a video that shows some spectacular images of crystals that have different crystal habits. This video is one of many produced by the Envisioning Chemistry Project.
For additional views of many different mineral habits with a discussion, go the video linked below:
blank▶️ Video 3-2: Examples of mineral habits (10 minutes)
3.4 Mineral Appearance
Luster refers to the general appearance or sheen of a mineral. It refers to the way in which a mineral reflects light.
For a good discussion, with examples, of many different mineral lusters, watch the video linked below:
blank▶️ Video 3-3: Examples of mineral lusters (7 minutes)
22.214.171.124 Metallic and Submetallic Lusters
Minerals that have the shiny appearance of polished metal are said to have a metallic luster. Some could be used as mirrors. Well-crystallized pyrite, is a good example. Other commonly metallic minerals include chalcopyrite (CuFeS2), bornite (Cu5FeS4), native copper (Cu), native gold (Au), hematite (Fe2O3), and magnetite (Fe3O4). The photo seen here is stibnite (Sb2S3). Most of minerals with a metallic luster are sulfides, oxides, or native elements.
Minerals that do not appear metallic have a nonmetallic luster. Those that appear only partially metallic are called submetallic. The chalcopyrite (CuFeS2; gold-bronze color) seen in this photo is metallic, the minor dark grey sphalerite (ZnS) in the photo might be considered submetallic, and the hard-to-pick-out fluorite (CaF2) (semi-clear and white) is nonmetallic. Other commonly submetallic minerals include, cinnabar (HgS), and cuprite (Cu2O).
126.96.36.199 Nonmetallic Lusters
Mineralogists use many terms to describe nonmetallic lusters. The most commonly used terms are listed in the table below.
|Terms Used to Describe Luster of Nonmetallic Minerals|
|vitreous||Having a glassy appearance||quartz, tourmaline|
|resinous||Having the appearance of resin||sphalerite, sulfur|
|greasy||Reflecting light to give a play of colors; similar to oil on water||chlorite, nepheline|
|silky||Having surfaces appearing to be composed of fine fibers||chrysotile (asbestos), gypsum|
|adamantine||A bright, shiny, brilliant appearance similar to that of diamonds||diamond, cerussite|
|pearly||Appearing iridescent, similar to pearls or some seashells||muscovite, talc|
|dull||Not reflecting significant amounts of light or showing play of colors||kaolinite (clay), niter|
Vitreous minerals are nonmetallic minerals that have a glassy appearance. The adjective is derived from the Latin vitrium, meaning glass. Quartz (SiO2) is an excellent example. Calcite (CaCO3), topaz (Al2SiO4F2), and fluorite (CaF2) are other minerals that may be vitreous. The two photos below show vitreous calcite and topaz.
Resinous minerals have a luster similar to violin resin or pine pitch. The most common mineral example is a resinous variety of sphalerite (ZnS) such as that shown in the photo below (Figure 3.26). Sphalerite, however, has other appearances. In fact, the name sphalerite is from the Greek sphaleros which means deceiving or treacherous. This name refers to the many different appearances that sphalerite may have. Its luster can be metallic, submetallic, resinous or adamantine. Some samples are transparent with a vitreous luster. The photo on the right below shows amber, which is fossilized tree resin, with an insect inclusion. Amber is a biomineral, not a true mineral.
Greasy minerals show a play of color – color change with angle of view – that resembles grease or maybe fat. Examples are the opal and cordierite seen in the two photos below. Besides opal and cordierite, jadeite and a few other minerals sometimes have a greasy luster. However, this luster occurs most commonly in opal (a mineraloid) and in minerals that contain many small inclusions.
A good short discussion of play of color can be found at:
blank▶️ Video 3-4: Play of colors (5 minutes)
Silky minerals appear to have a parallel arrangement of fine fibers, sometime making them have the luster of silk. If the fibers are coarse, we may describe minerals as fibrous instead of silky. Ulexite (a hydrated borate mineral shown in the photo on the left below) is a classic but rare example of a silky mineral. Satin spar, in the photo on the right below, is a variety of gypsum (CaSO4•2H2O). It gets its name from its fibrous appearance.
We use the term adamantine to describe crystals that sparkle or appear brilliant; diamond (C) is perhaps the best-known example. But, Herkimer diamonds, a variety of quartz (SiO2) from Herkimer, New York can also be adamantine. The two photos below show a Herkimer diamond and a real diamond. Other minerals that are sometimes adamantine include anglesite (PbSO4), cerussite (PbCO3), and corundum (Al2O3). Some synthetic minerals, including cubic zirconia (ZrO2) are also adamantine. All these natural and synthetic stones can have their sparkle enhanced with proper faceting.
Pearly minerals show a play of color that resembles that of pearls. Light reflecting from pearly minerals may appear to have washed out rainbow colors. The play of colors is due to a layered atomic arrangement, so pearly minerals generally have excellent planar cleavage. The two photos below show pearly muscovite and talc. These minerals are somewhat dichroic, which means their colors change with angle of view. But, that property cannot be seen in standard photos.
Dull and Earthy Minerals
Dull minerals show no remarkable luster – because they have non-reflective surfaces. Kaolinite (Figrure 3.36, below) is a good example – it is usually a fine-grained aggregate of small grains, white, and drab. Besides kaolinite, the other clay minerals, such as montmorillonite or illite, also have dull lusters.
We say that dull minerals are earthy if they have a brownish or reddish color resembling dirt. Common hematite is an excellent example (although some hematite may be metallic). The hematite seen in the photo below (Figure 3.37) can be described as being earthy, and Figure 3.14 showed another example of earthy hematite. Besides hematite, limonite and other metal oxides and hydroxides are commonly earthy.
Diaphaneity refers to a mineral’s ability to transmit light. Some minerals are transparent. When they are thick, some distortion may occur, but light passes relatively freely through them. For example, we can see the clouds in the distance through the clear Iceland spar (calcite) crystal in the photo seen here. Because it is so clear, Iceland spar has been used in some industrial applications. Unfortunately, it is not very durable because calcite is very soft. Very few minerals are as transparent as the calcite seen here. Thin sheets of muscovite and some quartz come close.
Minerals that do not transmit light as well as clear calcite may be translucent. Although it is not possible to see through them as with transparent minerals, if thin enough, translucent minerals transmit light. Both the whitish calcite and the darker colored orpiment in this photo (Figure 3.39) are translucent. Many other minerals exhibit this same property. For example, quartz comes in many different colors but, unless very finely crystallized, it is generally translucent to some degree. Calcite, gypsum, topaz, and many micas, are often commonly translucent.
Some minerals are neither transparent nor translucent, and are opaque instead. Opaque minerals, do not transmit light unless the mineral is exceptionally thin. Most opaque minerals have metallic lusters and belong to the sulfide, oxide, or native element groups. This photo (Figure 3.40) shows a hexagonal flake of opaque molybdenite (MoS2) on top of translucent quartz. Pyrite and magnetite are two more common examples of opaque minerals.
Color is often used for quick identification of minerals. Sometimes, it can be diagnostic, but for many minerals it is ambiguous or even misleading. The deep red color of rubies may seem distinctive. Ruby is, however, just one variety of the mineral corundum. Sapphires are different colored varieties of the same mineral. The photos show just a few of the many colors that corundum may have. To add to the confusion, other minerals, such as spinel or garnet, may have the same deep red color as ruby. Color is ambiguous because many things can give a mineral its color.
Color is one of the most misunderstood mineral properties. It is easy to look at a ruby illuminated by white light and say it has a red color. If the ruby is illuminated by light of a different color, it may not appear red. Color, then, is not a property of a mineral. It is instead the result we observe when light and a mineral interact. When we see that a mineral has color, what we are really observing is the color of the light that is being reflected or transmitted to our eye. Normal light, called white light, includes many different colors. When white light strikes a mineral surface, if all of the colors are reflected back to our eyes, the mineral will appear white. If none of the colors are reflected back to our eye, the mineral will appear black. Most minerals, like ruby, appear to have color because only one or a few wavelengths make it back to our eye. The other wavelengths of light are scattered in other directions or are absorbed or transmitted by the mineral in some way.
Metallic minerals, especially sulfides, tend to be constant in their coloration. So mineralogists commonly use color as a key tool for sulfide identification. However, metallic minerals easily tarnish, so we need a fresh surface to see the true color. The photos above show pyrite on top of dolomite (left photo) and chalcopyrite (right photo). These minerals are both sulfides, but pyrite is FeS2 and chalcopyrite has formula CuFeS2. Both sulfides are quite common and color usually distinguishes one from the other – pyrite is more golden or brass colored, and chalcopyrite generally has a yellowish tint and sometimes appears a bit greenish. Notice that the chalcopyrite is tarnishing. The pyrite specimen also contains a few crystals of dark colored hematite.
Color is often a poor property to use for identifying nonmetallic minerals because many things can cause minerals to have different colors. For example, quartz may be colorless, rosy (rose quartz), yellow (citrine), purple (amethyst), milky, smoky, or black. The photo seen here contains both light purplish amethyst and orangish citrine.
The most significant control on color is a mineral’s chemical composition. Elements that give a mineral its color are called chromophores. It does not take large amounts of chromophores to color a mineral. Minor amounts, less than 0.1 wt% of transition metals such as iron and copper, may control a mineral’s color because electrons in the d-orbitals of transition metals are extremely efficient at absorbing certain visible wavelengths of light. The remaining wavelengths are reflected and give minerals their color.
If the elements controlling the selective reflection of certain wavelengths are major components in a mineral, the mineral is idiochromatic, or “self-coloring.” Sphalerite (ZnS), for example, is an idiochromatic mineral. It changes from white to yellow to green to brown to black as its composition changes from pure ZnS to a mixture of ZnS and FeS. The two photos seen here are from the same mining district in New York but are different colors. Compare these with other photos of sphalerite earlier in this chapter (Figures 3.23 and 3.26). Sphalerite has a lot of looks!
Many copper minerals are idiochromatic and have green or blue coloration, while many manganese minerals are pinkish. These colors derive from selective absorption of certain colors by copper and manganese. Idiochromatic elements may have different effects in different minerals. Malachite and azurite, seen in the photo here, are both hydrated copper carbonates, but malachite is green and azurite is blue. In both minerals the color is due to copper.
Ruby and sapphire are examples of allochromatic varieties of corundum. In allochromatic minerals, minor or trace elements determine the color. Very small amounts of iron and titanium give sapphire a deep blue color. Small amounts of chromium give ruby and other gemstones deep red colors. Like the effects of idiochromatic elements, the effects of allochromatic elements may be different in different minerals. Allochromatic chromium is also responsible for the striking green color of emerald (a variety of the mineral beryl), chrome diopside, and some tourmalines.
Structural defects in minerals may also influence their color. Radiation damage gives quartz, for example, a purple, smoky, or black color. The purple color of many fluorites is caused by Frenkel defects (out of place ions in the atomic structure). Other causes of coloration include the oxidation or reduction of certain elements (especially iron), and the presence of minute inclusions of other minerals.
Although it would never occur to many people to check a mineral’s streak, streak is sometimes a key diagnostic property. It is not a useful property for identifying most silicates but is especially useful for distinguishing oxide and sulfide minerals. The streak of a mineral is the color it has when finely powdered. For mineral identification, it is much more reliable than mineral color, and it is easy to determine. The usual method of determining streak is to rub the mineral against a ceramic streak plate or other piece of unglazed ceramic. Figure 3.48 shows a red streak from the mineral hematite, and the photo below (Figure 3.49) shows a yellow streak from sulfur.
Steak color is a good diagnostic property because the mineral is finely powdered, so structural and other nonchemical effects are minimized. Calcite, for example, comes in many different colors, but calcite’s streak is always white. Pyrite (fool’s gold) is yellow but has a dark colored streak, as does chalcopyrite. Gold, which has a color similar to pyrite’s (in hand specimen), has a yellow-gold streak.
Mineralogists routinely use streak when identifying minerals, both in the laboratory and in the field, but it cannot be determined for minerals harder than the hardness of a streak plate. The table below lists some minerals that often have diagnostic streak colors.
|Examples of Minerals that Do Not Have a White Streak|
pale or light brown
yellow brown to ocher yellow
white to light brown or yellow
very pale blue to white
very pale blue to gray or tan
pale yellow to yellow
light yellow to yellow
orange or reddish yellow
dark red to scarlet
rust red to blood red
Streak can be extremely useful for telling dark-colored minerals apart, especially metallic ones. For example, hematite may be red, gray, or black in hand specimen and may or may not have a metallic luster. It always, however, has a diagnostic red streak that helps distinguish specular hematite from galena, or other kinds of hematite from similarly colored minerals. White and colorless streaks are considered the same because we cannot make the distinction using a standard streak plate, and (unfortunately) most minerals have a white or colorless streak.
Some minerals will emit light when they are activated by an energy form other than visible light. We call such an effect luminescence. Examples of luminescence include fluorescence, phosphorescence, and thermoluminescence.
This photo is a fluorescent specimen from the Franklin Mine in New Jersey. It is illuminated with short wavelength ultraviolet light and contains green willemite (Zn2SiO4), blue hardystonite (Ca2ZnSi2O7), orange clinohedrite (CaZnSiO4•H2O), and minor red calcite (CaCO3). Fluorescent minerals like the ones seen here give off visible light when they are struck by energy of a shorter wavelength. If the visible emission continues after the energy source is turned off, the mineral is phosphorescent. Pectolite is an example of a phosphorescent mineral.
Thermoluminescent minerals such as some tourmalines give off visible light in response to heating. Some varieties of fluorite, calcite, and apatite also have this property.
3.4.6 Play of Colors
We discussed play of colors earlier in this chapter (for example the pearly luster that is sometimes exhibited by micas or talc) but it is worth returning to the topic to consider some special examples. When it enters some crystals, white light can be separated into individual wavelengths of varying intensities emitted in different directions, somewhat like light coming out of a prism. The play of colors is a form of light scattering due to very fine particles in the minerals or to textures of mineral surfaces. The two photos on the right below (Figures 3.52 and 3.54) show examples of opalescence (most notably exhibited by opals, but also by a variety of K-feldspar called moonstone).
The limonite photo above (Figure 3.51) displays iridescence, sometimes described as being similar to the colors on top of an oil slick. This play of colors commonly appears when metallic minerals such as bornite tarnish or, in this case, when limonite tarnishes. The fourth photo (bottom left, Figure 3.53) shows labradorite, a feldspar, displaying labradorescence. Labradorescence is a variety iridescence.
Chatoyancy and asterism are two special scattering effects most easily seen in gemmy polished minerals, such as the two cabochons (gems that have been shaped and polished instead of faceted) and the polished tiger’s eye stone seen in the photos below.
Chatoyant minerals show a bright band of scattered light, often perpendicular to the long direction of a crystal. Such minerals are sometimes said to have a cat’s-eye (like the moonstone above) or tiger’s-eye (like the quartz above) appearance. The satin spar variety of gypsum is also chatoyant.
Asterism, a property sometimes visible in rubies, sapphires, garnets, and some other gems, refers to scattered light appearing as a “star.” The example shown above in(Figure 3.56 is a famous sapphire called the Star of India that is in the American Museum of Natural History in New York. It has spectacular asterism. Chatoyancy and asterism are caused by closely packed parallel fibers or inclusions of other minerals within a mineral crystal.
For some additional pretty examples of chatoyancy and asterism, see the video at:
blank▶️ Video 3-5: Chatoyancy and asterism (3 minutes)
3.5 Strength and Breaking
The color and shape of minerals are obvious to anyone, but mineralogists note other, more subtle, properties too. Several relate to the strength of bonds that hold atoms in crystals together. These properties are especially reliable for mineral identification because they are not substantially affected by chemical impurities or defects in crystal structure.
The term tenacity refers to a mineral’s toughness and its resistance to breaking or deformation. Those that break, bend, or deform easily have little tenacity. In contrast, strong unbreakable minerals have great tenacity. The photo here shows samples of the gemstone jade shaped and polished to produce a figurine and a cabochon. Gemmy jade may be either of two minerals: jadeite (a pyroxene) or nephrite (a rock containing amphibole). In either case, jade is one of the most tenacious natural materials known. It does not easily break or deform, even when under extreme stress. That is one reason, besides beauty, that it is prized as a gemstone.
The nature of its chemical bonds controls the tenacity of a mineral. Ionic bonding often leads to rigid, brittle minerals. Halite is an excellent example of a brittle mineral. It shatters into many small pieces when struck. Quartz, too, is brittle, although the bonding in quartz is only about half ionic. Many metallic minerals, such as native copper, are malleable, which means we can shape them with a hammer. Native copper is also ductile, which means we can stretch it out into wire-like shapes. Other minerals, such as gypsum, are sectile, which means they can be cut into thin pieces with a knife.
Some minerals, including talc and chlorite, are flexible due to weak van der Waals and hydrogen bonds holding well-bonded layers of atoms together. When force is applied, slippage between layers allows bending. When pressure is released, they do not return to their original shape. Still other minerals, notably the micas, are elastic. They may be bent but resume their original shape after pressure is released if they were not too badly deformed. In micas and other elastic minerals, the bonds holding layers together are stronger than those in chlorite or clays.
|Terms Used to Describe Tenacity|
|brittle||easily broken or powdered|
|malleable||capable of being hammered into different shapes|
|sectile||capable of being cut into shavings with a knife|
|ductile||capable of being drawn into a wire-like shape|
|flexible||capable of being bent into a different shape|
|elastic||a bendable mineral that returns to its original shape after release|
3.5.2 Fracture, Cleavage, and Parting
Fracture is a general term used to describe the way a mineral breaks or cracks. Terms used to describe fracture include even, conchoidal, splintery, and others defined in the table below. Because atomic arrangement is not the same in all directions within a crystal, and chemical bonds are not all the same strength, most crystals break along preferred directions. The orientation and manner of breaking are important clues to crystal structure. If the fractures are planar and smooth, we say that the mineral has good cleavage. Cleavage involves minerals breaking parallel to planes of atoms. We use geometric terms such as cubic, octahedral, rhombohedral, or prismatic to describe cleavage when appropriate.
|Terms Used to Describe Fracture|
|even||breaking to produce smooth planar surfaces (halite)|
|uneven / irregular||breaking to produce rough and irregular surfaces (rhodonite)|
|hackly||fracturing to produce jagged surfaces and sharp edges (copper)|
|splintery||forming sharp splinters (kyanite, pectolite)|
|fibrous||forming fibrous material (chrysotile, crocidolite)|
|conchoidal||breaking with curved surfaces as in the manner of glass (quartz)|
|Terms Used to Describe Cleavage|
|basal||well developed planar cleavage in one direction only; also sometimes called “platy” (micas)|
|cubic||three cleavages at 90̊ to each other (galena)|
||three cleavages not at 90̊ to each other (calcite)
|octahedral||four cleavages that produce 8-sided cleavage fragments (fluorite)|
|prismatic||multiple directions of good cleavage all parallel to one direction in the crystal (tremolite)|
If a mineral cleaves along one particular plane, a nearly infinite number of parallel planes are equally prone to cleavage. This is due to the repetitive arrangement of atoms in atomic structures. The spacing between planes is the repeat distance of the atomic structure, on the order of angstroms (1Å = 10-10 m) for mineral crystals. The whole set of planes, collectively referred to as a cleavage, represents planes of weak bonding in the crystal structure. The vast majority of minerals exhibit cleavage but the nature of the cleavage is highly variable.
Micas, including muscovite and biotite, are the best examples of minerals with one excellent cleavage. The photo below (Figure 3.59) shows cleavage in biotite; the mineral can be easily broken into very thin sheets. Molybdinite, too, has only one direction of cleavage and breaks into sheets (see the molybdenite in Figure 3.40). The micas, molybdenite, and many other minerals cleave into thick slabs or sheets. We say these minerals have basal cleavage.
The ball and stick model in Figure 3.60 shows the atomic arrangement in micas. It is similar to the arrangement in all sheet silicates. The layered structure explains why micas cleave as they do. Micas cleave into sheets because the bonds to the potassium ions are very weak compared to all bonding in other directions.
Minerals that have more than one direction of weakness will have more than one cleavage direction. The direction and angular relationships between cleavages, therefore, give valuable hints about atomic structure. Minerals that are equally strong in all directions, such as quartz and olivine, have no cleavage and fracture to form irregular surfaces. These minerals break along curved surfaces to form conchoidal fractures similar to what happens when a glass breaks. Figure 3.61 shows a sample of quartz fractured in many curving directions.
Feldspars, like the K-feldspar shown, have two cleavages, two planar directions that they break. One is very good (meaning it is easily observed) and the other better described as distinct (meaning it often shows). The angle between the two feldspar cleavages is about 90̊, which explains why the specimen in the photo appears to have a square cross section. Cleavages in feldspar are, however, generally not as well developed as cleavages in minerals like mica and, so, may be hard to discern. Feldspars are typical — in ideal specimens the cleavages and angles between them can be easily seen. But, in many specimens, they cannot.
The photo seen here shows albite, another kind of feldspar. Hints of cleavage can be seen, but it is not obvious how many cleavages are present or what their angular relationships are. But, if you click on the photo of albite, you will get to a 3-dimensional photograph that you can rotate so you see the cleavage. Look closely and you will see that the angle between cleavages is just about 90o.
Some minerals with two cleavages, such as kyanite, easily break into long splintery pieces. Anthophyllite, too, sometimes forms bladed crystals. The photos below show typical blue blades of kyanite and a cluster of bladed anthophyllite crystals.
Other minerals, including halite and calcite, may have three directions of cleavage. The ball and stick model below shows the atomic arrangement in halite. Atoms are evenly spaced and all bonds are perpendicular. Consequently, halite has cubic cleavage — three directions of cleavage at 90o to each other. In the blue/gray halite crystal below, the cleavages created a cube, and additional cleavage traces can be seen as fine cracks. Figures 3.2 and 3.10 also show halite. The cleavage cannot be seen in Figure 3.2 but is very clear in Figure 3.10
Like halite, calcite has three good cleavages. But, unlike halite, the cleavages are not perpendicular. So calcite cleaves into shapes called rhombs. This photo shows a bunch of calcite cleavage fragments. Compare the shape with the clear calcite shown in Figure 3.38. Besides calcite, other carbonate minerals, collectively belonging to the rhombohedral carbonate group, cleave the same way. Other minerals, such as fluorite, may have four, or more, cleavages.
The four fluorite crystals shown below in Figure 3.69 have the same atomic arrangements and so cleave the same way. All fluorite is essentially CaF2 but color varies due to minor chemical impurities.
The ease with which a mineral cleaves is not the same for all minerals or for all the cleavages in a particular mineral. Mineralogists describe the quality of a particular cleavage with qualitative terms: perfect, good, distinct, indistinct, and poor. Some minerals contain only poorly developed cleavage, other, like calcite have perfect cleavage. Whether perfect or not, when present, cleavage can be a good property to help identify minerals. For examples, some specimens of pyroxene and amphibole may appear as similar dense dark minerals. But, pyroxenes have two cleavages at about 90o to each other, and amphiboles have two cleavages at 60o to each other.
Click on the image below to explore a 3-dimensional model of cleavage in biotite, K-feldspar, and quartz.
Crystal faces and cleavage surfaces may be difficult to tell apart. In some minerals, principal cleavage directions are parallel to crystal faces, but in most they are not. A set of parallel fractures suggests a cleavage, but if only one flat surface is visible, there can be ambiguity. However, this problem is sometimes mitigated because crystal faces often display subtle effects of crystal growth. Twinning (oriented intergrowths of multiple crystals) and other striations (parallel lines on a face), growth rings or layers, pitting, and other imperfections make a face less smooth than a cleavage plane and give it lower reflectivity and a more drab luster.
The photo seen in Figure 3.71 is of a calcite crystal that shows visible striations on its crystal faces. The lines are artifacts of crystal growth and are not related to cleavage.
Some minerals exhibit parting, a type of breaking that is often quite similar to cleavage. Parting occurs when a mineral breaks along structural planes but, unlike cleavage, parting is not found in all samples of a particular mineral and does not repeat to form many parallel planes that are only a few angstroms apart. Several things can induce parting, perhaps most commonly it occurs because of twinning (when multiple crystals grow together and share atoms). Distinguishing parting from cleavage can, sometimes, be problematic.
Cleavage is an excellent property for mineral identification. Often the quality and number of cleavages may be seen in hand specimen. Sometimes we use a hand lens to identify the set of fine parallel cracks more irregular than twinning and striations, which suggest a cleavage that is too poorly developed to see with the naked eye. We may estimate angles between cleavages and, if we need accurate angular measurements, we can use techniques involving a petrographic microscope or a device called a goniometer.
For many good examples of mineral fracture and cleavage, watch the video linked below:
blank▶️ Video 3-6: Examples of mineral fracture and cleavage (8 minutes)
A good additional perspective on cleavage can be found at:
blank▶️ Video 3-7: Mineral cleavage (5 minutes)
Hardness is a mineral’s resistance to abrasion or scratching. Relative hardness (symbolized by H) is determined by trying to scratch a surface of one mineral with an edge or corner of a second mineral. If a scratch or abrasion results, the first mineral is the softer. Absolute hardness is not quite the same as relative hardness. It is the measure of a material’s ability to resist permanent deformation. Although rarely done by mineralogists, values of absolute hardness may be determined in several ways; the easiest is to use an indenting tool similar to ones used to measure the hardness of steel. The indenting tool measures the force necessary to produce a permanent indentation in a flat surface.
This table gives the relative hardness scale that is used by mineralogists. Based on ten well-known minerals, it is called the Mohs Hardness Scale, named after Austrian mineralogist Friedrich Mohs who developed it in 1812. The Mohs scale ranks minerals by their ability to scratch each other. The Mohs scale is related to absolute hardness but does not measure the same thing because resistance to scratching depends on additional factors.
If we compare the Mohs hardness scale with absolute hardness, we find that the Mohs scale is not linear and is close to being exponential. The hardnesses of the softest minerals are more similar than the hardnesses of the four hardest ones (quartz, topaz, corundum, diamond). Gypsum (H = 2) is only slightly harder than talc (H = 1), but diamond (H = 10) has a hardness five times greater than corundum (H = 9).
We can estimate relative hardness by conducting scratch tests to compare the hardness of an unknown mineral to the minerals in the Mohs hardness scale. Many labs are equipped with boxes of known minerals for this purpose.
Alternatively, we can approximate hardness by comparing mineral hardness with the hardness of a fingernail, penny, pocketknife, glass, or several other common objects – the most commonly used are listed in the table above. Figure 3.72 shows gypsum being scratched with a fingernail. Gypsum, one of the softest minerals known, has a hardness of 2 on the Mohs hardness scale; fingernails have a hardness of about 2½. A penny has hardness of 3½, iron has hardness of about 4½, a pocketknife has hardness of 5½, and a metal file has hardness of 6½.
Scratch tests are often straightforward, but there can be complications. Mineral specimens may be too small or too valuable to scratch. Large samples may consist of many grains loosely cemented together so that scratch tests are not possible. Others may cleave or fracture when we perform tests. In still other cases, the results of scratch tests may be ambiguous, especially if two minerals have the same, or nearly the same, hardness.
Most minerals have hardness greater than 2 and less than 7. The tables below, list some examples of relatively common minerals that fall outside this range.
|beryl||Be3Al2Si6O18||7½ to 8|
|spinel||MgAl2O4||7½ to 8|
|molybdenite||MoS2||1 to 1½|
|graphite||C||1 to 2|
|covellite||CuS||1½ to 2|
|orpiment||As2S3||1½ to 2|
|realgar||AsS||1½ to 2|
The hardness of a mineral relates to its weakest bond strength. So, because bonds are usually not the same in all directions in minerals, hardness may vary depending on the direction a mineral is scratched. In kyanite, for example, hardness varies from 4½ to 6½ depending on the direction of the scratch test. In most minerals, however, hardness is about the same in all directions. While the general relationship between hardness and bond strength is known, mineralogists have difficulty predicting hardness for complex atomic structures. For some simple ionic compounds, however, theoretical calculations match measurements well. Minerals with high density, highly charged ions, small ions, or covalent bonding tend to be hardest.
3.6 Density and Specific Gravity
The Greek letter ρ (rho) symbolizes density. We usually give the density of a mineral in units of grams/cubic centimeter (gm/cm3). Density varies slightly depending on pressure or temperature, but most minerals have values between 2 and 8 gm/cm3. Borax, shown in this photo, has density of about 1.8 gm/cm3, lower than all other common minerals.
The polymorphs diamond and graphite are both made of carbon (C), but due to differences in atomic arrangements, diamond has density of 3.5 gm/cm3, while graphite’s is 2.2 gm/cm3. Graphite forms under Earth surface conditions, but diamond, with its high density, only forms deep in Earth where pressures are great. The Laws of Thermodynamics tell us that high pressures favor dense minerals, which makes sense because at high pressure things are squeezed together.
Accurate determination of density can be difficult or impossible because it requires knowing the volume of a crystal – which can be difficult to measure with accuracy. A related property, specific gravity (G), is often used instead. Specific gravity (unitless) is the ratio of the mass of a mineral to the mass of an equal volume of water at 1 atm, and because mass and weight are proportional, we normally determine specific gravity by comparing weights. If a mineral is at normal Earth surface conditions, density and specific gravity have about the same values.
Because specific gravity varies greatly between minerals, we can easily distinguish minerals with high, moderate, or low specific gravity simply by picking them up. We use the term heft for estimations of G made by holding hand specimens; heft can be useful in mineral identification. For example, the mineral barite (BaSO4), such as the example in Figure 3.74, sometimes exists as massive white material that is easily confused with feldspars. However, its great heft, easily discerned by picking it up, helps identify it. Similarly, we can distinguish cerussite (lead carbonate) from other carbonate minerals by its heft.
Specific gravity differences can also help in the separation of minerals. In the laboratory, researchers separate crushed rock into mineral components by “floating” samples in liquids of different specific gravities. In these heavy liquids, which are much denser than water, minerals separate as some float and others sink according to their specific gravities. In mining operations, ore minerals are often separated from valueless minerals by using gravity separation techniques that depend on specific gravity differences. This occurs in natural systems, too. Placer gold deposits form when gold from weathered rock, because of its high specific gravity, concentrates in stream beds.
The lightest minerals have specific gravities on the order of 1.8 to 2. They are mostly borates (such as borax), halides (e.g., halite), and sulfates (e.g., gypsum). Silicates (including quartz and all the other common rock-forming silicates) and carbonates (e.g., calcite or dolomite) range from about 2.5 to 3.5. Oxides and sulfides may have specific gravities as great as 7 or 8 but are highly variable. And, native metals (e.g., copper, silver, or gold) range up to 19 or 20. The photo in Figure 3.75 shows a gold nugget with quartz beneath it. It came from a placer. Gold has specific gravity of about 19.3 and quartz has specific gravity of 2.7.
The specific gravity of a mineral depends, in part, on how its atoms are packed together within a crystal. For example, the left part of the table below lists the specific gravities for quartz and four other SiO2 polymorphs. Stishovite, coesite, quartz, cristobalite, and tridymite all have different densities because they do not have the same atomic arrangements. The densest two – stishovite and coesite – are only found in very high pressure rocks or in meteorite impact craters.
Specific gravity also depends on composition. This can be demonstrated by looking at isostructural minerals – minerals with the same atomic arrangement but with different compositions. Consider the garnet group minerals for example. The table below lists names, compositions and specific gravities for seven garnets. The specific gravity values vary from 3.54 for pyrope to 4.33 for almandine, reflecting the atomic weights of the garnets’ elemental constituents. Several examples of garnet can be seen in Figures 3.5 to 3.7.
|Specific Gravities of Some Silica Polymorphs|
|Specific Gravities of Some Garnet Minerals|
●Box 3-3 Graphite and Diamond
The photos in Figures 3.79 and 3.81 show a diamonnd crystal and a graphite crystal. Graphite and diamond are polymorphs (same compositions but different atomic arrangements).
Both minerals are made of carbon but they have different properties and appearances because the carbon atoms are arranged differently in the two minerals. The drawings in Figures 3.77 and 3.79 show how carbon atoms are arranged in each.
As shown in the atomic drawings, in diamond, each carbon atom is covalently bonded to four others, creating an overall cubic 3-dimensional network that has the same properties in all directions. In graphite, carbon atoms are covalently bonded to form sheets that contain interconnected 6-carbon rings; very weak van der Waals bonds hold the sheets together.
Diamond is a tenacious mineral that does not cleave easily. Graphite, because of its layered atomic arrangement, has excellent basal (planar) cleavage. Graphite has a hardness of 1½ and diamond has a hardness of 10. Diamond has a denser structure than graphite – the specific gravity of diamond is about 3.5, and graphite’s is 2.1 to 2.3. Because of its high density, diamond only forms deep within Earth. This is true for all high density minerals – they only form where pressure is great.
Magnetism derives from a property called the magnetic moment that results from the spinning and orbiting of electrons. The sum of all the magnetic moments of all the atoms in a mineral gives it magnetism. Minerals may be ferromagnetic, diamagnetic, or paramagnetic.
In ferromagnetic minerals, the moments of a mineral’s electrons are aligned in a constructive way and the minerals have properties similar to those of metallic iron. Ferromagnetic minerals may be magnetized. Magnetite (Figure 3.80) and pyrrhotite are examples of ferromagnetic minerals, but magnetite is much more ferromagnetic than pyrrhotite.
Diamagnetic minerals exhibit little magnetic character overall but may be weakly repelled by a strong magnetic field. Pure feldspars, halite, and quartz all exhibit weak diamagnetism. An impure feldspar, however, may contain iron, which results in paramagnetism, which means it is attracted to a strong magnet. Other commonly paramagnetic minerals include garnet, hornblende, and many pyroxenes.
For routine mineral identification, only a few minerals – for practical purposes only magnetite – can be identified because of their magnetism. In the field, rocks that contain magnetite will attract a magnet. This sometimes help distinguish different rock formations. And, in the laboratory, subtle differences in the magnetic properties of minerals are routinely used to separate different minerals in crushed rock samples. Figure 3.81 shows a device used for this purpose. Magnetism, thus, can be an important property of minerals.
3.8 Electrical Properties
Electricity can be conducted when a mineral’s electrons can move throughout its structure. So, minerals with metallic, or partially metallic bonds – like many sulfides — are good conductors. The native metals, such as copper, are the best examples. This photo (Figure 3.82) shows a “branch” of native copper with several small quartz crystals on it. The branch was extracted from a rock matrix. Sulfide minerals, because they commonly have partially metallic bonds, are also good conductors.
Small amounts of electrical conduction may also occur in minerals with defects or other imperfections in their structures. And, some minerals, while being unable to conduct electricity, may hold static charges for brief times. They may be charged by exposure to a strong electric field, a change in temperature, or an application of pressure. A mineral charged by temperature change is pyroelectric; a mineral charged by pressure change is piezoelectric. Because they are difficult to measure, however, electrical properties are not often used for mineral identification.
3.9 Reaction to Dilute Hydrochloric Acid
One chemical property, the reaction of minerals to dilute (5%) hydrochloric acid (HCl), is included here because it is diagnostic for calcite, one of the most common minerals of the Earth’s crust. Drops of acid placed on coarse samples of calcite cause obvious bubbling or fizzing, called effervescence, seen in the photo here (Figure 3.83).
Dolomite, a closely related carbonate mineral, effervesces when finely powdered but not when coarse. Other carbonate minerals, such as smithsonite (ZnCO3), aragonite (CaCO3), and strontianite (SrCO3), effervesce to different degrees. They are distinguished by crystal form, color, and other properties. Although acid tests have limited use, most mineralogy labs are equipped with small bottles of HCl and eyedroppers to aid in carbonate identification. Many geologists carry a small bottle of dilute hydrochloric acid when they go in the field so they may distinguish between rocks that contain calcite and rocks that do not.
3.10 Additional Properties
Minerals possess many other properties (for example, solubility, radioactivity, or thermal conduction). Because they are of little use for mineral identification in most cases, we will not discuss them individually here.
Uncredited graphics/photos came from the authors and other primary contributors to this book.
3.1 Calcite crystals on, James St. John, Wikimedia Commons
Video 3-1: Crystal habit, Envisioning Chemistry, YouTube