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 colour, streak, lustre, hardness, habit, cleavage/fracture, and density.
For most of us, colour is a key way to identify an object. 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 (Figure 5.21 left) is always a distinctive and unique yellow. Hematite, on the other hand, is an example of a mineral for which colour is not necessarily diagnostic. In some forms hematite is deep dull red (a fairly unique colour; Figure 5.21 middle), but in others it is a metallic silvery black (5.21 right; many minerals have a similar colour).
Figure 5.21 Examples of minerals for which colour is and is not a useful diagnostic property. [KP with photos by R. Weller/Cochise College (permission for non-commercial educational use).]
For other minerals, the problem is that a single mineral can have a wide range of colours. In most cases, the variations in colours are a result of varying proportions of trace elements within the mineral. In the case of quartz (FIgure 5.22), 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). Smoky quartz gets its colour by being exposed to natural radiation.
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. A way to get around this problem is to grind a small amount of the sample to a powder and observe the streak– the colour of the powder. This is done by scraping the sample across a streak plate (a piece of unglazed porcelain; Figure 5.23). Figure 5.23 shows the same reddish brown streak for two samples of hematite, even though one sample is metallic and the other is deep red.
Streak is an especially helpful property when minerals look similar. In Figure 5.24 all of the minerals are dark in colour, with varying degrees of metallic lustre. The streaks of the minerals are much more distinctive.
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 lustres. Light does not pass through metals, and that is the main reason they look metallic (e.g., the hematite on left of Figure 5.23). Even a thin sheet of metal — such as aluminum foil — will prevent light from passing 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, it is said to have a glassy lustre. The quartz crystals in Figure 5.22 are examples of minerals with glassy lustre. If the mineral surface is dull and non-reflective, it has an earthy lustre (like the hematite on the right of Figure 5.23). Other types of non-metallic lustres are silky, pearly, and resinous. Lustre is a good diagnostic property because most minerals will always appear either metallic or non-metallic, although as Figure 5.23 shows, there are exceptions.
One of the most important diagnostic properties of a mineral is its hardness. In practical terms, hardness determines whether or not a mineral can be scratched by a particular material.
In 1812 German mineralogist Friedrich Mohs came up with a list of 10 minerals representing a wide range of hardnesses, and numbered them 1 through 10 in order of increasing hardness. These minerals are shown in Figure 5.25 with the Mohs scale of hardness along the bottom axis. 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 (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. The minerals themselves can be used to test other minerals.
When minerals form within rocks, there is a possibility that they will form in distinctive crystal shapes if they are not crowded out by other pre-existing minerals. Every mineral has one or more distinctive crystal habits determined by their atomic structure, although 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 5.26 left), 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 5.26 centre), but can also form crystals with 12 faces, known as dodecahedra. The mineral garnet also forms dodecahedral crystals (Figure 5.26 right).
Some of the terms that are used to describe habit include bladed, botryoidal (grape-like), dendritic (branched), drusy (an encrustation of crystals), equant (similar size in all dimensions), fibrous, platy, prismatic (long and thin), and stubby.
Cleavage and Fracture
Cleavage and fracture describe how a mineral breaks. These characteristics 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. Some minerals tend to cleave along planes at various fixed orientations, some, like quartz, 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.
The way that minerals break is determined by the arrangement of atoms within them, and more specifically by the orientation of weaknesses within the lattice. Graphite and the micas, for example, have cleavage planes parallel to their sheets (Figure 5.27).
Other minerals have two directions of cleavage, classified as two planes at 90° (Figure 5.28 upper left) and two planes not at 90° (Figure 5.28 lower left). The minerals in Figure 5.28 both have two planes of cleavage that are very close to 90°. The white dashed lines mark the edges of the planes, as with Figure 5.28. See if you can find the planes repeated in the images. The images are very close-up views of the minerals, only a few cm across. Sometimes you must look very carefully to find cleavage planes.
Some minerals have many directions of cleavage. Figure 5.29 shows examples of minerals with three directions of cleavage. Halite (Figure 5.29 top) has three planes at 90° and calcite (Figure 5.29 bottom) has three planes not at 90°.
There are a few common difficulties that occur when students are learning to recognize and describe cleavage. One is being confused about whether a flat surface on a crystal is a cleavage plane, a crystal face, or simply a surface that happens to be flat. Another is that it might be necessary to look very closely at a sample to see mineral cleavage. The key features in Figure 5.28, for example, are only cm or mm in scale. If crystals are very small, it may not be possible to see cleavage at all. Sometimes cleavage is present, but it is poor, meaning the cleavage surface isn’t perfectly flat. The best way to overcome all of these problems is to look at lots of examples. It’s worth it to be able to identify cleavage and fracture, because cleavage is a reliable diagnostic property for most minerals.
Density is a measure of the mass of a mineral per unit volume, and it is a useful diagnostic tool in some cases. Most common minerals, such as quartz, feldspar, calcite, amphibole, and mica, are of average density (2.6 to 3.0 g/cm3), and it would be difficult to tell them apart on the basis of their density. On the other hand, many of the metallic minerals, such as pyrite, hematite, and magnetite, have densities over 5 g/cm3. If you picked up a sample of one of these minerals, they would feel much heavier compared to a similarly sized sample of a mineral with average density. A limitation of using density as a diagnostic tool is that one cannot assess it in minerals that are a small part of a rock with other minerals in it.
Several other properties are useful for identification of some minerals. For example:
- Calcite dissolves in dilute acid and will give off bubbles of carbon dioxide.
- Magnetite is strongly magnetic, and other minerals are weakly magnetic.
- Sphalerite ((Zn,Fe)S) gives off a smell of sulphur when drawn across a streak plate.
- Halite tastes salty.
- Talc feels soapy to the touch.
- Plagioclase feldspar has striations (parallel razor-thin lines etched on the surface) and some varieties show a play of colours when light hits them at the right angle.