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Identifying Minerals

written by:

David Dillon,
Department of Earth Sciences,
The University of Western Ontario


One question that confronts a teacher of geology is: “How do I do this with the minimum expenditure and the greatest influence on the ability of my students to learn?”  The following is a set of suggestions that may be of value to you as a teacher.

1) Go on a field trip to collect rocks. Go your own, or take your class. Including students can excite their interest.
2) Once this has been done, the next task is to categorise your samples into those that are readily identifiable and challenging specimens. (In an ideal world, everybody picks up textbook examples of the various rock species. I find that this is much less common than I would prefer. This is because rocks are often subjected to physical and chemical alteration after their initial hardening. When this is the case, a teacher has to rely on someone with greater experience.)
Integral to this task is the process of learning how to identify rock and mineral species.
The leftover samples of this task form a collection either to challenge your developing prowess, or to be discarded.
3) You may wish to place the rocks into the context of where and how they were formed. This is a major aspect of what academic geologists do.

There are over 3000 mineral species. Of these, 350 are abundant enough to find in crustal rocks. Few are abundant enough to be common constituents in rocks. They include: talc, gypsum, calcite, fluorite, halite, quartz, garnet, amphibole, pyroxene, orthoclase, plagioclase, muscovite, biotite, hematite, magnetite, pyrite, chalcopyrite, serpentine, sphalerite, galena, graphite, apatite, corundum, olivine, chlorite, epidote and dolomite.  Many of these minerals have economic value:
Talc is the chief constituent in talcum powder.
Gypsum, until the latter part of the twentieth century was the source of wallboard and plaster.
Calcite is the major component in limestone and marble. Limestone is quarried to make quicklime for the production of Portland cement and flux for the steel industry.
Halite is used as table salt and for de-icing highways.
Quartz is a major component in the making of glass, as well as electronic due to its piezo-electrical property.
Garnet and corundum are used as abrasives. Their gem varieties are used for jewellery.
Muscovite and biotite have been used for insulation and even furnace windows.
Hematite, magnetite, sphalerite, chalcopyrite, and galena have been mined for their metal contents.
Pyrite was once mined for its sulphur.
One variety of serpentine, chrysotile is used for asbestos.

Rocks collected by teacher, or even better, by students.

Procedure (Identifying Mineral Properties):
Once you’ve got your rocks and minerals, try to look for identifiable characteristics. These are usually in terms of mineral species and textures.
It’s important to understand that when the word “texture” is used in a geological context, it refers to how the rock is constituted, rather than how it feels.
The mineral species are recognised and identified via the physical properties they exhibit. True, they are chemical compounds and a chemical analysis will help you identify them, but such analysis is beyond the practical capability of most teachers.  The physical properties of minerals are varied. Some provide good means to differentiate mineral species.

1) Lustre
When geologists and mineralogists look at a mineral, they consider what the material looks like. The first lustre type we’ll address is “metallic”. That is, does the mineral look like a metal. Consider if the it looks like it could be converted into a coin, mirror or something commonly made of metal. The following minerals are metallic: magnetite, pyrite, chalcopyrite, galena, and graphite. Hematite is metallic when it occurs in visible crystals, but it tends to be earthy when extremely fine-grained.
Earthy lustre is like the appearance of dried mud. Light dispersed in all directions from the minute grains.
Pearly is a lustre that invokes the memory of pearls. It is due to the reflection of light off parallel internal planes. Talc, gypsum, muscovite, biotite and chlorite can have pearly lustre.
Greasy lustre is reminiscent of cold hardened fat. It is also similar in some ways to solid wax. Talc, serpentine, and corundum can be greasy.
Resinous is a lustre that appears like congealed tree sap. The internal reflections where the material is broken look similar to those in pine gum or plastic. Sphalerite, and to a lesser extent apatite and garnet have resinous lustre.
Vitreous lustre is the look of glass. It doesn’t matter whether it is transparent, translucent or opaque. Gypsum, calcite, fluorite, halite, quartz, garnet, amphibole, pyroxene, orthoclase, plagioclase, muscovite, biotite, apatite, olivine, epidote and dolomite can exhibit vitreous lustre.
Silky is a lustre that speaks of the fibrous nature of the mineral. Chrysotile (a variety of serpentine) is silky.

2) Streak
When a mineral is rubbed along a surface than it is, (such as a porcelain plate) it leaves some of itself behind as a thin film. To test for streak rub the sample across a streak plate, wipe away the excess powder and observe the colour of the film left.  Most metallic minerals will give a coloured streak, most non-metallic minerals lack a coloured streak.     Amphibole and chlorite often leave a light grey-green streak. Biotite streaks light brown. Magnetite, pyrite, chalcopyrite, graphite and galena leave black streaks. Sphalerite streaks can range from cream through yellow to brown. Hematite leaves a distinct red-brown streak on a plate.

3) Diaphaneity
Diapheneity is the ability to transmit light. The associated terms are: transparent, translucent or opaque.
Transparent minerals allow light to pass through and you can see well enough to read through them. For example: gypsum, calcite, fluorite, halite, quartz, apatite, olivine, epidote, dolomite, muscovite, garnet, corundum and biotite can be transparent. For some of these minerals you would have to see thin specimens or those of gem quality.
Translucent minerals allow light to pass through but you cannot see objects through them. This is the majority of minerals. Talc, gypsum, calcite, fluorite, halite, quartz, garnet, amphibole, pyroxene, orthoclase, plagioclase, muscovite, biotite, serpentine, sphalerite, apatite, corundum, olivine, chlorite, epidote and dolomite tend to be translucent.
Opaque minerals absorb and reflect light. to pass through them and you cannot see through them. Hematite, magnetite, pyrite, chalcopyrite, sphalerite, galena and graphite are opaque.

4) Mineral hardness
An Austrian by the name of Fredrick Mohs established a scale based on resistance to being scratched. This scale goes from 1 (softest) to 10 (hardest). For the purposes of identifying minerals, it is only necessary to see if an unknown falls within a particular hardness range. Geologists usually take the following tools with them to do hardness tests.

These are:
    porcelain plate, 7.0
    glass, 5.5
    a knife blade, 5.0
    copper coin, 3.0
    fingernail, 2.5.

Mohs’ hardness scale is shown here.

The minerals of our set are laid out similarly.
corundum  9.0
garnet  6.5-7.5
quartz  7.0
olivine  6.5-7.0
epidote  6.0-7.0
pyrite  6.0-6.5
hematite  5.5-6.5
magnetite  6.0
apatite  5.0
serpentine  2.0-5.0
fluorite   4.0
calcite 3.0
chlorite 2.0-2.5
gypsum 2.0 
graphite 1-1.5
talc 1.0

5) Cleavage
When minerals break, they do so in either an irregular fashion or in ways that are predictable. When breakage is irregular, it is called a fracture. When it is along planes of weakness, it is called cleavage.
We can count the number of unique directions in which there is planar failure as well as the angle between cleavage directions and the quality of the cleavage.   Cleavage is often the most difficult of the physical properties to identify.  It takes practise to  identify cleavages and their angles of intersection.

Cleavage is indicated when:
a) The sample has a simple geometric shape e.g. a cube or simple prism.
b) Several broken samples of the same mineral show common angles or shapes.
c) The mineral has surfaces  (including internal ones) that reflect light as single planes.
d) The mineral has parallel sets of flashing surfaces.

A mineral with no cleavage typically shows
a) Non-geometric or very complex geometric shapes.
b) Several samples have different shapes.
c) Mineral surfaces are curved or pointed.
d) Mineral surfaces are dull.
e) The top of a mineral flashes, but the parallel surface does not. This may be the case if the shiny surface is a crystal face.
Light tends to be dispersed from fractured surfaces, whereas light tends to be reflected in a planar fashion from a cleaved surface.

Basal cleavage
One direction of cleavage (basal) results in flake-shaped pieces and often a pearly lustre.   Minerals having basal cleavage include talc, muscovite, biotite, graphite and chlorite. Gypsum exhibits one perfect cleavage with two less distinct cleavages.

Prismatic cleavage
Two directions of cleavage  (prismatic) result in pieces that are often rod-like. Amphibole, pyroxene, orthoclase and plagioclase have prismatic cleavage. Epidote has one perfect cleavage with a second that is well developed (good). The angle between cleavage directions is important especially when attempting to distinguish between amphibole and pyroxene. Both have similar hardness and colour range. Pyroxene however, cleaves at 90º, while amphibole cleaves near 120º and 60º.

Cleavage types with more than two planes
Three directions of cleavage can be divided into those at 90º (cubic) and those at something else (rhombohedral).  Halite and galena exhibit cubic cleavage. Dolomite and calcite show rhombohedral cleavage.  Only one mineral in the set cleaves in four directions (octahedral).  This is fluorite.  The largest number of cleavages is exhibited by sphalerite. There are six directions of cleavages and it is work to see them all except in very large crystal grains.

Poor cleavage or no cleavage
The mineral apatite cleaves into irregular pieces.  For our purposes, it might as well be thought of as a mineral that fractures.  The rest of the mineral set: quartz, garnet, apatite, corundum, olivine, hematite, magnetite, pyrite and chalcopyrite break irregularly.

6) Crystal form
Sometimes it is possible to see the external form of crystals. This not common. As a result crystal faces, when seen, can be misidentified as cleavages.  The following properties can be helpful in distinguishing crystal faces from cleavage surfaces.
a) While crystals and cleavage faces are similar (they are flat surfaces that, may be parallel and reflect light as a single plane), crystals commonly have complex shapes and with more surfaces than would be produced by cleavages.
b) Crystal faces are not necessarily reproduced when the crystal is broken. In the case of quartz, there is no cleavage. Flat surfaces are therefore cleavage faces.     Of the mineral set, there are some which may be found as crystals more frequently than others. Quartz forms hexagonal prisms topped by six-sided pyramids. Garnet forms dodecahedrons. Magnetite crystals are octahedrons. Pyrite commonly forms as cubes, but is also seen as “pyritohedrons” - dodecahedrons with five-sided faces. Chalcopyrite forms tetrahedrons that are slightly elongated. Galena commonly forms cubes. Apatite forms hexagonal prisms with rounded or complex ends. Corundum occurs as barrel-shaped hexagonal prisms. Dolomite forms rhombohedral crystals.

7) Specific gravity
This is a way of measuring the density of a mineral. It is specifically the ratio of the mass of a mineral divided by the mass of the same volume of water. For our purposes, it is important to be familiar with minerals that are relatively light versus those that tend to be heavy. For example, both graphite and galena are metals that are grye and have a metallic lustre. However graphite is extremely light in comparison with feels light or heavy.  The metallic minerals are usually heavy, most non-metallic minerals are light.

8) Other diagnostic properties
Some minerals have characteristics that make them easy to identify. For example, halite has the characteristic taste of table salt. It is better to look for some of the other characteristics of halite before tasting your rock and mineral samples. Indiscriminate tasting can be catastrophic to your health! Consider that some of the minerals in our set contain a lot of lead, zinc, copper and trace amounts of cadmium, mercury and maybe arsenic!

Calcite (CaCO3) reacts readily with dilute HCL (Hydrochloric acid) and will fizz.  The mineral dolomite (CaMg(CO3)2) is like calcite but will only fizz with HCl when it is powdered.

Only two minerals in our set exhibit twinning striations. Twinning striations are due to wafer-like inter-grown crystals with different structural orientations. Plagioclase typically shows these straight, parallel lines (best seen on cleavage faces). Orthoclase, also a feldspar, lacks these. Calcite sometimes exhibits striations but not as often as feldspars.

Magnetite is the only mineral in our set to be attracted to a magnet. Other minerals show magnetic attraction, but they are less common, the attraction is weaker, and they are not members of our mineral set.

Additional Comments:
Observations of mineral properties should be made on fresh surfaces of the mineral sample.  Only include the physical properties that you have measured for yourself.  Do not copy mineral descriptions from a textbook.  This is self-defeating, as you do not learn to do the tests or learn the diagnostic properties.  One or two diagnostic properties will often clinch the identification for you.  For example, magnetite has the strongest magnetic attraction of all of the minerals. Hematite has a distinctive red-brown to go with its metallic to earthy lustre range.

The table provided here summarises what you’ve already read and adds a few remarks, plus the chemical formulae of the simpler minerals.