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Chemical Properties

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.

Warning:  Some of these tests involve dangerous materials.  Adult supervision of children attempting them is strongly recommended.

Smithsonite and CalciteThe 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+ + 2Cl-1.

Most of the carbonate minerals will effervesce but the degree of the effervescence varies, as shown in the table below.

Carbonate mineral Response to dilute HCl acid
Calcite and Aragonite (CaCO3)
Smithsonite  (ZnCO3)
Strontianite  (SrCO3)
Witherite  (BaCO3)
Effervesces
Malachite  (Cu2CO3(OH)2)
Azurite  (Cu3(CO3)2(OH)2)  
Effervesces, and colors the acid green (malachite) or blue (azurite)
Dolomite  (CaMg(CO3)2) Effervesces only when powdered
Magnesite  (MgCO3)
Rhodochrosite (MnCO3)
Siderite  (FeCO3)
Effervesces only if the acid solution is heated
Cerussite   (PbCO3) Does not effervesce

Other acids are occasionally used for distinguishing between certain minerals.

ArsenopyriteThe "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.)

Although the 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, viruses, bacteria).

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.

The flame 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.

Element Flame Color
Strontium  (Sr) Crimson
Lithium  (Li) Crimson
Barium  (Ba) Yellow-green
Boron  (B) Yellow-green
Sodium  (Na) Yellow
Calcium  (Ca) in Anhydrite Orange-red

Barite and CalciteThe 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 (CaSO4).

The bead 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).


 

 

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