The Hardness of Minerals and Rocks

By Dr. William S. Cordua, University of Wisconsin Dr Bill Cordua's Home Page
Originally Published in Three Installments through the Lapidary Digest Mail List

Hardness tests of minerals are among the easiest and most useful tests to perform. What rockhounds speak of as hardness is more accurately described as resistance to abrasion. We are testing how easily one substance will scratch another. As an example, copper is relatively easy to scratch, but would you bet on diamond or copper standing up better to blows from a hammer? Hammer blows measure the ease with which something fractures or its tenacity. There are other hardness scales than are based on ease of indentation, resistance to twisting and so forth. For the sake of simplicity and standard usage, in these articles, hardness will refer to the resistance to abrasion as given by Mohs' Scale.

The classic scale for hardness was published in 1822 by Frederick Mohs, an Austrian mineralogist who got the basic concept from scratch tests performed routinely by miners. Since Mohs published the scale, it bears his name rather than that of the unknown genius who thought of it. The scale selects 10 minerals as standards, arranging in order of increasing hardness. These are, as most of you probably know:

1 = Talc
2 = Gypsum
3 = Calcite
4 = Fluorite
5 = Apatite (fluorapatite)
6 = Orthoclase
7 = Quartz
8 = Topaz
9 = Corundum
10 = Diamond

These minerals were selected for their abundance, as well as their differing hardness. The scale is uneven. For example. diamond at 10 is much harder then corundum at 9, while fluorite at 4 is only slightly higher than calcite at 3.

A more limited but practical scale can be easily and cheaply obtained by observing that
....your fingernail has a hardness of 2.5,
....a penny has a hardness of about 3.5,
....glass and a steel nail have nearly equal hardnesses of 5.5 and
....a streak plate has a hardness of 6.5.
If I carry a nail and streak plate with me and can scrounge up a penny, I've got a handy, light weight mineral testing lab in my pocket.

More expensive sets can be bought. A set with small samples of all of Mohs' minerals allows a bit more precision in testing. The specimens do lose their usefulness the more they are scratched up in various tests. As an alternative, one can custom build their own Mohs set through collecting or purchasing small fragments of the needed minerals. Other venders provide sets of hardness pencils with tips of two natural or artificial substances of measured hardness. These are handy in that they are very precise and allow one to test a small surface easily.

Most mineralogy texts give tables of mineral hardness. Particularly complete and useful tables appear in John Sinkankas' "Gemstone and Mineral Data Book."

Doing hardness tests requires some technique. You need to find a good surface or edge on your unknown to test. Take care to make sure you are testing the right grain - not the bit of quartz right next to it. In some case it is easier to scratch the unknown across the standard. (the point of a unknown mineral grain across a calcite cleavage). In other cases it is easier to test the standard across the unknown ( tip of a nail across cleavage surface of the unknown grain). In an ideal case, you should try to do both, to double check your findings. You need to press hard enough to good effect, but not so hard as to fracture either sample. Practice will help you get the proper level of stress to exert.

As a result of your test, you will look for a scratch. Rub aside any powder to see if a distinct scratch has been left. Calcite will leave a trail of powder across quartz. Rub away the powder and you'll see the quartz is unharmed. A hand lens will help you see the scratch. In this way you can bracket the hardness of your unknown between two of your standards (harder than a fingernail, softer than a penny). The ease with which one substance scratches another is also useful. Quartz easily scratches calcite, telling you of a large hardness difference. Quartz will scratch feldspar with much more difficulty. When testing a standard against an unknown that is of equal hardness, both substances will leave shallow scratches on each other.

The hardness of a particular mineral may vary with direction within the same grain. Kyanite is a good example. Kyanite generally occurs in long bladed crystals. The hardness taken the short way across the blade has a hardness of 7 the hardness taken the longway along the same grain will be 4.0. Muscovite is another good example of this. Its hardness is 2.5 when taken across a the flat surface of a cleavage sheet, but 4 when taken across the grain of a book.

The reason hardness varies in this way is that the phenomenon depends on the strength of the bonds holding the mineral together. The bond strength can be significantly different in different directions in the mineral, giving the different hardness. In most minerals this difference with direction is minor and doesn't affect the test. In the case of kyanite, this difference in hardness is a confirming test by itself.

Some minerals' hardness may vary from sample to sample depending on that mineral's exact chemical composition. Hornblende's hardness can vary from 5 to 6, meaning some hornblende is softer than glass, some harder. This reflects the fact that hornblende can accommodate varying amounts of sodium, calcium, iron and magnesium in its structure, which affect the details of its chemical bonding, hence its hardness.

Testing the hardness of rocks is less effective than testing the hardness of minerals. A rock is basically a mixture of various minerals, although it can contain non-mineral materials such as natural glass and fossils. (Fossils aren't minerals because they are organic, while glass isn't a mineral because it lacks an internal crystalline structure). Let's take a granite pegmatite for example. This might contain grains of topaz (H= 8), quartz (H=7), feldspars (H=6) and muscovite mica (H= 2.5). You could thus get a range of hardness depending on which grain you tested. In a coarse grained rock, identifying the individual minerals allows you to identify the rock. If the rock is fine-grained, it's harder to interpret the results.

The hardness of fine-grained rocks tends to reflect the average hardness of the minerals in them. Shales are made mostly of clay and tend to be soft. Limestones and dolostones are also soft, with a hardness of 3-4. Just watchout if quartz sand is present mixed with the carbonates! Quartzite and chert being made mostly of quartz are both very hard. The hardness of sandstone may be difficult to test. If the sand grains have not been cemented well or have been cemented by calcite, the sandstone will seem softer. The individual quartz sand grains will still have a hardnessof 7, but the rock may crumble or disaggregate in your hand, making it look soft. If you think it is really soft, trying dragging the disaggregated sand grains across a piece of glass and you'll readily see the effects. Most igneous and metamorphic rocks contain much feldspar, quartz, pyroxenes and amphiboles. Their hardness is thus going to be between 6 and 7. This means hardness is not a good way to distinguish one of these rocks from another. Volcanic glass will typically have a hardness of 5.5 - 6.0, depending on its particular chemical composition.

The hardness or rocks and minerals is also dependent on the degree of weathering. Weathering may convert feldspars (H=6) to clay minerals (H = 2 -3) Even corundum (H=9) can alter and have rims of softer minerals such as margarite (H= 3.5-4.5 ) around it. This is why it is important to testas fresh or unweathered a surface as you can while doing hardness tests.

Mohs' scale has stood the test of centuries as a useful tool for mineral identification. Its simplicity and effectiveness will likely assure its relevance well into the future.

Mineralogical Meanderings

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Bob Keller "The Hardness of Minerals and Rocks" © 1997 by Dr. William S. Cordua