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THE CHEMICAL PROPERTIES OF MINERALS
Topics: Atoms, Molecules, and Elements Every substance on earth is made of atoms, the building blocks of all matter. There are many different types of atoms, which are classified as elements. Molecules join together to form all matter. Different atoms bond together to form molecules. Molecules can be composed of two atoms of one element (known as molecules) or can be a combination of diatomic atoms of different elements. Molecules form mixtures and compounds. Mixtures are groups of molecules that can be separated by physical means. Compounds are groups of molecules, each of whom contains a definite specific arrangement, and can only be separated by chemical means. Every mineral type contains defined amounts of specific molecules in its structure. Most minerals are composed of molecules that contain an exact amount of certain atoms. For example, every water molecule contains two hydrogen atoms and one oxygen atom. Grouped water molecules form bodies of water. Minute amounts of other molecules mixed among the water molecules are known as impurities. Impurities can slightly alter physical properties such as color. Atoms join together based on their positive and negative charges, caused by the amounts of protons (positive charges) or electrons (negative charges) they contain. Chemical Formula 92 elements occur naturally on earth. Most are very rare. About 20 elements make up over 95 percent of material on the earth (including the earth itself). Each mineral has a unique arrangement and the number of elements contained in its crystal structure. This arrangement of atoms determines a mineral type. All minerals have a chemical formula, which is an analysis of the types and amounts of elements present in a mineral. Every element has a one or two letter abbreviated term, such as oxygen, which is "O", and gold, which is "Au". The chemical formula of the mineral hematite is "Fe2O3". The letters describe the element type (Fe = iron, O = oxygen; see the elemental listing for a list of all the elements). The small numbers describe how many atoms are in each molecule. Thus, a hematite molecule has 2 iron atoms and 3 oxygen atoms. If there is no number written after an element, it means there is only one atom of that element present. Some minerals have much more complex formulas, which will be discussed further. Radicals, or polyatomic ions, are special types of compounds. They act as if they were a single element when they join other elements to form molecules. There are many radicals, some of the most familiar are the:
Radicals are treated like any element when written in a
chemical formula. Just as oxygen is abbreviated as "O" in the chemical formula
of hematite, if a carbonate radical were to be present in hematite instead of the
oxygen, its chemical formula would look like this: Fe2CO3. If there were three carbonate radicals present in Hematite, the
"CO3"
would be in parenthesis, and a subscripted three would be written after the parenthesis,
and it would look like this: Fe2(CO3)3. Hydrous Minerals Minerals containing water in their structure are known as hydrous minerals. The hydrous mineral gypsum has a chemical formula of "CaSO4 · 2H2O". The large number 2 in front of the H2O signifies that there are two water (H2O) molecules for every molecule of CaSO4. The dot in between CaSO4 and 2H2O indicates that these are two separate molecules, but they are rationally proportionate. The letter "n" is used to describe a variable amount of water in the structure of a mineral. For example, the hydrous mineral opal has an inconsistent amount of water in its structure, thus its formula is written as "SiO2 · nH2O". Some minerals, such as torbernite, have a varying amount of water within fixed limits. The formula for torbernite is "Cu(UO2)(PO4)2 · 8-12H2O". The 8-12H2O indicates that there can be 8 to 12 water molecules for every Cu(UO2)(PO4)2 molecule. Mineral Series A number of minerals contain a varied amount of two or more elements. The mineral aurichalcite, which has a chemical formula of "(Zn,Cu)5(CO3)2(OH)6" contains a varying amount of zinc (Zn) and copper (Cu). This is indicated by comma separating the Zn from the Cu. If a chemical formula with two elements in parenthesis is separated by a comma, the number of those elements vary. Aurichalcite can contain any variable where both the copper and the zinc add up to five. This can result in theoretical formulas such as "Zn3Cu4(CO3)2(OH)6", "Zn2Cu3(CO3)2(OH)6", "ZnCu4(CO3)2(OH)6", or any other blend where the zinc and copper together amount to 5 atoms per molecule. Theoretically, aurichalcite can contain five molecules of zinc and no copper and the opposite also holds true. Usually, when the elements in a mineral vary, a series is formed. A series consists of a mineral in which one (or more) of the elements varies; such is the case with spinel, which will be discussed. The formula of spinel is written by most as "MgAl2O4 ". However, this is only the formula of common spinel. Spinel forms a series, with its formula being "(Mg,Zn,Fe,Mn)Al2O4". There are different types of Spinel — some contain one, two, three, or even all of the elements in the parenthesis (magnesium, zinc, iron, and manganese). Certain varieties contain only one of those elements, and not the other three. The end members, or members that form all mixtures are:
Some intermediary members of the spinel group (i.e. minerals in the spinel group that are a mixture of two or more end members) have designated names, such as gahnospinel (Mg,Zn)Al2O4, which is a mixture of common spinel (MgAl2O4) and (gahnite ZnAl2O4) while other mixtures are not, and are just called "spinel". Most mineral series form solid solutions. In a solid solution, there are intermediate members between the two end members. Individual intermediary members of some series may be given names, while others may not. An example is the olivine group, with its formula being (Mg,Fe)2SiO4. It contains forsterite, Mg2SiO4, and fayalite, Fe2SiO4 as the end members. The intermediary member is chrysolite, which is a combination of the two (Mg,Fe)2SiO4. Some solid solution series classify different intermediary members based on the percentage of the anchor minerals they contain. [More information on mineral series] Atomical Variations There are certain elements that come in slight modifications. Some elements have different amounts of electrons in different atoms of the same element. Some minerals, such as babingtonite, contain two types of the same element. Its formula is written as "Ca2Fe2+Fe3+SiO14(OH)". The superscript form of 2+ and 3+ next to the iron (Fe) distinguishes the different types of iron. 2+ means the iron has 2 more electrons than protons, and 3+ means it has three more electrons. The amount of electrons affects the chemical bonding of iron or any other element with variable electrons. If there is more than one variable atom in a molecule, a superscript number representing the type of atom (i.e. how many electrons over protons) is written after the element symbol, and a subscript form of how many atoms of that element is written after that. For example, 2 atoms of +2 iron is written Fe2+2. Formula Variables The formula of many minerals can be written in different formats. Some references may have one or two elements in the formula of a mineral that does not exist in the formula of a different reference. The formula of zincite is written as "ZnO" in some references, and "(Zn,Mn)O" in others. Both formulas are correct. Zincite is made up of zinc and oxygen, but contains indefinite traces of manganese that may be too insignificant to be listed in its formula in some guides. In some references, the formula of the same mineral may be listed differently, but they mean the same thing. The formula of dioptase is usually written as "CuSiO2(OH)2". However, its formula may also be written "H2CuSiO4". Both formulas mean the exact same thing. There is an equal amount of all the atoms in both formulas. We see this if we break up the hydroxyl (OH) radical in the first formula. We are left with 2 oxygen (O) and 2 hydrogen (H) atoms. We add the two oxygen atoms to the oxygen in the silica (CuSiO2 + O2 = CuSiO4), and put the hydrogen in the beginning of the formula, making it H2CuSiO4. Sometimes a formula may be reduced or expanded by using multiplication or division. (A reduced formula is known as an empirical formula in chemistry.) For example, the chemical formula of sodalite is commonly written as "Na4Al3Si3O12Cl". The silicon and oxygen form a radical, so they are bound together as a single atom. Since the amount of silicon and oxygen is divisible by three (Si3÷3O12÷3 = SiO4), the formula can be written as Na4Al3(SiO4)3Cl. The subscripted three after the parenthesis tells us to multiply every atom in the parenthesis by three, thus the amount of silicon for each atom is three (3 x 1) = 3 and the amount of oxygen is twelve (3 x 4) = 12. Variable Formulas Some minerals may contain small traces of an element or even large amounts partially replacing a standard element. For example, the mineral adamite, Zn2(AsO4)(OH), often contains small amounts of Cu (copper) and Co (cobalt) replacing some Zn (zinc). These elements are not mentioned in the chemical formula, as they do not compromise a significant portion and are not always present. Therefore, an additional formula, known as the variable formula has been developed for this guide to state the occasional presence of these elements. We hope this variable formula will be a new assessment tool in recognizing and categorizing a mineral. Each mineral that often contains an additional element is given a variable formula. The variable formula displays the regular formula with the additional elements that might be present. The element always present is listed first in the parenthesis, and the elements that might be present are listed after, underlined, to state that they are variable. That said, the variable formula of Adamite is (Zn,Cu,Co)2(AsO4)(OH). The Zn (zinc) is always present, and the Cu and Co may be absent. The Zn, which is not underlined, must be greater in percentage to all underlined elements. Note that the variable formula only displays elements that regularly replace part of a standard element in a given mineral. Some minerals without a variable formula may still have variable elements, but those elements only occur insignificantly or rarely. For example, the mineral fluorite occasionally contains traces of Ce (cerium) and Y (yttrium), but there is no variable formula, for these combinations are uncommon.
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