Most of the tests that are used
to determine the chemical composition of a mineral require special equipment and
chemicals. We describe five relatively simply tests here: the HCL
acid, smell, taste, flame,
and bead tests.
Some of these tests involve dangerous materials. Adult supervision of
children attempting them is strongly recommended.
HCl acid test is highly diagnostic for
carbonate minerals. The acid used is a very weak solution, consisting of
at least five parts water to one part muriatic acid. (WARNING!
although a solution of this strength should not burn your skin, it can be
harmful if it gets in your eyes or mouth, and it will damage clothing!) A few
drops of this weak acid solution dropped on the clean surface of calcite will
produce bubbling ( called effervescence)
similar to that which occurs in a carbonated soft drink, as some of the
molecules of calcite recombine with the acid to form new compounds, including
carbon dioxide gas. The chemical formula for this reaction is:
CaCO3 + 2HCl = CO2 + H2O + Ca2+ +
Most of the carbonate minerals will
effervesce but the degree of the effervescence varies, as shown in the table
||Response to dilute
|Calcite and Aragonite
colors the acid green (malachite) or blue (azurite)
||Effervesces only when
|Effervesces only if
the acid solution is heated
||Does not effervesce
Other acids are occasionally used for
distinguishing between certain minerals.
"smell" test is highly diagnostic for
two common minerals. Native Sulfur (S) has a very distinctive "rotten
eggs" smell. Arsenopyrite (FeAsS) when heated gives off a very strong
odor of garlic. (WARNING! Heating
arsenopyrite is DANGEROUS! Arsenic is
toxic even in very small quantities.)
taste test is diagnostic for two minerals (halite
and sylvite), in general, licking mineral samples is not recommended. It
rarely yields helpful information and it can be unhealthy. Some minerals
are poisonous, but of greater danger is the possibility that your tongue may
pick up things that it is best not to ingest (bird, insect or animal droppings,
Halite and sylvite, both of which have
distinctive tastes, are salts that are highly soluble in water. Table salt
is pure halite (NaCl). Sylvite (KCl) is one of the ingredients in salt
substitutes used by people who are trying to reduce the amount of sodium in
their diets. If you want to try the taste test, use a few grains of table
salt as an example of halite, and Mr. Salt or some other low-sodium salt
substitute as an example of sylvite. Sylvite has a bitter or sour taste
compared to halite.
While you are tasting the salts, take
this opportunity to examine the three directions of cleavage at right angles of
the isometric minerals halite and sylvite.
test is less often used today than in the past, since other more
precise tests are available. The purpose of the test is to check for the
presence of certain volatile elements that may occur in some minerals. Each
of these elements imparts a distinctive color to a flame when a fine-grained
powder composed of the host minerals is inserted into the flame of a Bunsen
burner. One problem with this test is that sodium contamination of the
surfaces of minerals is common and may obscure the presence of other elements.
|Calcium (Ca) in
flame test is extremely useful for distinguishing some isostructural minerals,
including the carbonates witherite (BaCO3) and strontianite (SrCO3),
sulfates barite (BaSO4), celestite (SrSO4), and anhydrite
test, which also is rarely used these days, can be used to determine
whether specific elements are present in a mineral. The bead test
typically requires several steps. An example of the bead test for tungsten
follows. 1) The mineral is powdered. 2) Heat is used to fuse the powder
into a solid mass (this is the "bead"). 3) The fused mass is
dissolved in HCl; if tungsten is present, a yellow precipitate (WO3)
forms. 4) As an added check, grains of metallic tin or zinc are added to
the solution. 5) The solution is then boiled and tungsten-bearing solutions
should turn blue.
Additional information about the flame
and bead tests is available in the twentieth edition of Klein and Hurlbut's Manual
of Mineralogy (but not in later editions).