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THE HARDNESS OF ROCKS AND MINERALS
Written especially for Lapidary Digest byDr. Bill Cordua
University of Wisconsin-River Falls
Copyright 1998. This document may be copied and used in mineral and gem club
newsletters without asking permission, given that the article is reprinted
"in toto" and that credit is given Lapidary Digest as the source. Others
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the e-mail form on the first page.
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 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 long way 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 watch
out 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 hardness of 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 test as
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.
Dr. Bill Cordua
william.s.cordua@uwrf.edu
http://www.uwrf.edu/~wc01/welcome.html
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