2.3 Mineral Properties

Minerals are universal. A crystal of hematite on Mars will have the same properties as one on Earth, and the same as one on a planet orbiting another star. That’s good news for geology students who are planning interplanetary travel since we can use those properties to help us identify minerals anywhere. That doesn’t mean that it’s easy, however; identification of minerals takes a lot of practice. Some of the mineral properties that are useful for identification are as follows: colour, streak, lustre, hardness, crystal habit, cleavage/fracture, specific gravity (density), and a few others.

Colour

For most of us, colour is one of our key ways of identifying objects. While some minerals have particularly distinctive colours that make good diagnostic properties, many do not, and for many, colour is simply unreliable. The mineral sulphur (2.3.1 left) is always a distinctive and unique yellow. Hematite, on the other hand, is an example of a mineral for which colour is not diagnostic. In some forms hematite is deep dull red, but in others it is black and shiny metallic (Figure 2.3.2). Many other minerals can have a wide range of colours (e.g., quartz, feldspar, amphibole, fluorite, and calcite). In most cases, the variations in colours are a result of varying proportions of trace elements within the mineral. In the case of quartz, for example, yellow quartz (citrine) has trace amounts of ferric iron (Fe³⁺), rose quartz has trace amounts of manganese, purple quartz (amethyst) has trace amounts of iron, and milky quartz, which is very common, has millions of fluid inclusions (tiny cavities, each filled with water).

Streak

In the context of minerals, “colour” is what you see when light reflects off the surface of the sample. One reason that colour can be so variable is that the type of surface is variable. It may be a crystal face or a fracture surface or a cleavage plane, and the crystals may be large or small depending on the nature of the rock. If we grind a small amount of the sample to a powder we get a much better indication of its actual colour. This can easily be done by scraping a corner of the sample across a streak plate (a piece of unglazed porcelain) to make a streak. The result is that some of the mineral gets ground to a powder and we can get a better impression of its “true” colour (Figure 2.3.2).

Lustre

Lustre is the way light reflects off the surface of a mineral, and the degree to which it penetrates into the interior. The key distinction is between metallic and non-metallic lustre. Light does not pass through metals, and that is the main reason they look “metallic” (e.g., specular hematite in Figure 2.3.1 and pyrite in Figure 2.3.4b). Even a thin sheet of metal—such as aluminum foil—will not allow light to pass through it. Many non-metallic minerals may look as if light will not pass through them, but if you take a closer look at a thin edge of the mineral you can see that it does. If a non-metallic mineral has a shiny, reflective surface, then it is called “glassy” (Figure 2.3.4a). If it is dull and non-reflective, it is “earthy” (see earthy hematite in Figure 2.3.2). Other types of non-metallic lustres are “silky,” “pearly,” and “resinous.” Lustre is a good diagnostic property since most minerals will always appear either metallic or non-metallic. There are a few exceptions to this (e.g., hematite in Figure 2.3.2).

Hardness

One of the most important diagnostic properties of a mineral is its hardness. In 1812 German mineralogist Friedrich Mohs came up with a list of 10 reasonably common minerals that had a wide range of hardnesses. These minerals are shown in Figure 2.3.3, with the Mohs scale of hardness along the bottom axis. In fact, while each mineral on the list is harder than the one before it, the relative measured hardnesses (vertical axis) are not linear. For example apatite is about three times harder than fluorite and diamond is three times harder than corundum. Some commonly available reference materials are also shown on this diagram, including a typical fingernail (2.5), a piece of copper wire (3.5), a knife blade or a piece of window glass (5.5), a hardened steel file (6.5), and a porcelain streak plate (6.5 to 7). These are tools that a geologist can use to measure the hardness of unknown minerals. For example, if you have a mineral that you can’t scratch with your fingernail, but you can scratch with a copper wire, then its hardness is between 2.5 and 3.5. And of course the minerals themselves can be used to test other minerals.

Crystal Habit

When minerals form within rocks, there is a possibility that they will form in distinctive crystal shapes if they formed slowly and if they are not crowded out by other pre-existing minerals. Every mineral has one or more distinctive crystal habits, but it is not that common, in ordinary rocks, for the shapes to be obvious. Quartz, for example, will form six-sided prisms with pointed ends (Figure 2.3.4a), but this typically happens only when it crystallizes from a hot water solution within a cavity in an existing rock. Pyrite can form cubic crystals (Figure 2.3.4b), but can also form crystals with 12 faces, known as dodecahedra (“dodeca” means 12). The mineral garnet also forms dodecahedral crystals (Figure 2.3.4c).

Because well-formed crystals are rare in ordinary rocks, habit isn’t as useful a diagnostic feature as one might think. However, there are several minerals for which it is important. One is garnet, which is common in some metamorphic rocks and typically displays the dodecahedral shape. Another is amphibole, which forms long thin crystals, and is common in igneous rocks like granite (Figure I5).

Mineral habit is often related to the regular arrangement of the molecules that make up the mineral. Some of the terms that are used to describe habit include bladed, botryoidal (grape-like), dendritic (branched), drusy (an encrustation of minerals), equant (similar in all dimensions), fibrous, platy, prismatic (long and thin), and stubby.

Cleavage and Fracture

Crystal habit is a reflection of how a mineral grows, while cleavage and fracture describe how it breaks. Cleavage and fracture are the most important diagnostic features of many minerals, and often the most difficult to understand and identify. Cleavage is what we see when a mineral breaks along a specific plane or planes, while fracture is an irregular break. One particularly distinct type of fracture, common in quartz, is called conchoidal fracture (top left photograph in Figure 2.3.5). Some minerals tend to cleave along planes at various fixed orientations (Figure 2.3.5), some do not cleave at all (they only fracture). Minerals that have cleavage can also fracture along surfaces that are not parallel to their cleavage planes (Figure 2.3.6).

As we’ve already discussed, the way that minerals break is determined by their atomic arrangement and specifically by the orientation of weaknesses within the lattice. Graphite and the micas, for example, have cleavage planes parallel to their sheets (Figure 2.1.1), and halite has three cleavage planes parallel to the lattice directions (Figure 2.1.2). Quartz has no cleavage because it has equally strong Si–O bonds in all directions, and feldspar minerals have two cleavages at 90° to each other (Figure 2.3.6). When a mineral has more than one cleavage plane or cleavage direction, it is important to specify the number of cleavage planes and the approximate angle between them.

Tips for recognizing cleavage in mineral samples

There are a few common difficulties that students encounter when learning to recognize and describe cleavage. One of the main difficulties is that cleavage is visible only in individual crystals. Most rocks have small crystals and it’s very difficult to see the cleavage within those crystals. Use your hand lens to magnify your field of view, and make sure you have an adequate light source nearby. If crystals are very small, it may not be possible to see cleavage at all.

Some minerals have perfect cleavage, meaning that the cleavage planes are perfectly flat, they glint light back at you as you rotate the mineral around, and are generally easy to recognize. Mica and feldspar minerals commonly have perfect cleavage. Some minerals, on the other hand, have poor cleavage, meaning that the planes are not perfectly flat and may be harder to recognize. Talc is an example of the latter. Talc has one cleavage plane, but with a Mohs hardness of just 1, any recognizable plane is often scratched and uneven, making it more difficult to recognize that talc has cleavage at all!

It can also be easy to misidentify flat crystal faces, or even smooth flat fractures, as cleavage planes. As already noted, crystal faces are related to how a mineral grows while cleavage planes are related to how it breaks. In most minerals cleavage planes and crystal faces do not align with one-another.  An exception is halite, which grows in cubic crystals and has cleavage along those same planes (Figure 2.1.2).  But this doesn’t hold for most minerals. For example, fluorite forms cubic crystals like those of halite, but it cleaves along planes that differ in orientation from the crystal surfaces.  This is illustrated in Figure 2.3.7.

Remember that cleavage planes are controlled at the molecular level by the crystal lattice, and so they tend to repeat themselves at different depths throughout the mineral. Planes that are parallel are considered the same direction of cleavage and should only count as one. If are unsure whether the flat surface you are examining is a cleavage plane, try rotating the mineral under bright light, like a desk lamp. If the mineral has cleavage, you will generally find that all of the cleavage surfaces of a given cleavage direction will glint in the light simultaneously (Figure 2.3.8). Crystal faces will also glint under the light, but do not repeat themselves at depth throughout the mineral. In some minerals, crystal faces have striations, as shown by the faint parallel lines on the faces of the quartz crystal in Figure 2.3.4a. Finally, if you have identified more than one cleavage plane or direction within a mineral, it can take practice to describe the angle between those cleavage directions. To help visualize the angle between two cleavage planes try extending the planes using both hands. Place one finger from each hand flat on each cleavage plane, and examine the angle between your fingers. Is the angle between your fingers close to 90°, or definitely not 90°? Remember, for the purposes of this course you do not need to describe cleavage with an exact angle. Geology students have to work hard to understand and recognize cleavage, but it’s worth the effort since it is a reliable diagnostic property for most minerals.

Density and Specific Gravity

Density, reported in units of grams per cubic centimetre (g/cm³), is a useful diagnostic tool in some cases. Specific gravity (SG) is a related measure that geologist’s use to describe the density of a mineral. For the purposes of this course, the specific gravity of a mineral can be described as “low”, “moderate”, or “high”. Most minerals you will encounter in this course like quartz (2.65), feldspar, calcite, amphibole, and mica have “moderate” SG between 2.6 and 3.4, and it would be difficult to tell them apart on the basis of their SG alone. In comparison to these minerals, galena, for example, has distinctly high SG (7.5), while graphite has distinctly low SG (1.75). To determine this qualitatively in the lab, try comparing samples of quartz and galena of roughly the same size by hefting them in your hands. The sample of galena should feel much heavier than the similarly-sized sample of quartz. A limitation of using density (or SG) as a diagnostic tool is that one cannot assess it in minerals that are a small part of a rock that is mostly made up of other minerals.

Other Properties

Several other properties are also useful for identification of some minerals, including:

• Calcite reacts (fizzes vigorously) with dilute acid and will give off bubbles of carbon dioxide.
• Magnetite is strongly magnetic, and some other minerals, like pyrrhotite, are weakly magnetic.
• Halite tastes salty – please do not lick the lab samples; there are other diagnostic properties you should use to identify halite!
• Sphalerite has a pale yellow streak that gives off a sulphurous (rotten egg-like) smell.
• Talc feels soapy to the touch.
• Plagioclase feldspar commonly has striations (Figure 2.3.9).
• Some potassium feldspars have exsolution lamellae.
  

  

Image Descriptions

Figure 2.3.8 image description: As you rotate a mineral with good cleavage (top) under a light source, you will see the light glint back at you all at once, as the rays of light are reflected by the mirror-smooth cleavage plane. Even if the cleavage plane causes the mineral to break along “steps”, you will still see a single glint off of these steps all at once. Minerals with poor cleavage (bottom) do not glint all at once, as their cleavage planes are rough and uneven, causing light rays to scatter. [Return to Figure 2.3.8]

Media Attributions

• Figures 2.3.1, 2.3.7: © Steven Earle. CC BY.
• Figure 2.3.2: © Karla Panchuk. CC BY.
• Figures 2.3.3 and 2.3.6: © Siobhan McGoldrick. CC BY. Derivatives of Figures 2.6.3 and 2.6.5 © Steven Earle. CC BY.
• Figure 2.3.4a, 2.3.4b, 2.3.9: © Candace Toner. CC BY-NC.
• Figure 2.3.4c: Almandine garnet © Eurico Zimbres (FGEL/UERJ) and Tom Epaminondas (mineral collector). CC BY-SA.
• Figure 2.3.5: © Lyndsay Hauber and Joyce M. McBeth. CC BY. Adapted from Randa Harris and M.C. Rygel. CC BY-SA 3.0.
• Figure 2.3.8: © Siobhan McGoldrick. CC BY.