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THE CHEMICAL PROPERTIES OF MINERALS


Atoms, Molecules, and Elements

Every substance on earth is made of atoms, the building blocks of all matter. There are all different types of atoms, and each type of atom is classified as an element. Atoms will bond together to form molecules, which are organized agglomerations of different types of atoms. There are ninety two elements that occur naturally on earth. About twenty of the elements make up over 95 percent of all matter on earth, the others are all rare.

Molecules form mixtures and compounds. Mixtures are groups of molecules that can be separated by physical means. Compounds are groups of molecules with a definitive arrangement, and can only be separated by chemical means.

Every mineral contains a defined ratio of specific molecules in its structure. For example, a water molecule is composed of H2O - two hydrogen atoms and one oxygen atom. When water molecules are grouped together, they form a body of water. Trace amounts of foreign molecules not inherent to a mineral's makeup are known as impurities. Impurities can slightly alter physical properties such as color.

Atoms join together based on their positive and negative charges. This is caused by the amounts of protons (positive charges) or electrons (negative charges) they contain.


Chemical Formula

Every mineral has a unique arrangement of elements within its inherent 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. For example oxygen is "O", and gold is "Au". (See the periodical table for a list of all the elements).

The chemical formula of the mineral Hematite is Fe2O3. The letters describe the element type (Fe = iron, O = oxygen), and the subscripted numbers describe the amount of those atoms in each molecule. A Hematite molecule has 2 iron (Fe) atoms and 3 oxygen (O) atoms. If there is no number written after an element, it means there is only one atom of that element present.


Radicals

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:

Carbonate radical CO3
Sulfate Radical SO4
Chromate Radical CrO4
Hydroxyl Radical OH

Radicals are treated like any element when written in a chemical formula. Radicals are sometimes surrounded with a parenthesis when written in a chemical formula, and the number after the parenthesis describes how many of these radicals are present within each mineral's molecular breakdown. For example, the mineral Talc has a chemical formula of Mg3Si4O10(OH)2.The subscripted 2 at the end of the (OH) describes that there are two hydroxyl radicals within each molecule of Talc. If there is no subscripted number written after a radical, that means there is only one radical present in that mineral's molecule.


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 a loosely defined composition with an inconsistent amount of water in its structure, thus its chemical formula is written as "SiO2 · nH2O".

Some minerals, such as Torbernite, have a varying amount of water within fixed limits. The chemical 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 varying amount of two or more elements. For example, the mineral Aurichalcite, which has a chemical formula of (Zn,Cu)5(CO3)2(OH)6 contains an unspecific 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 has a variable amount of zinc of copper where the combination of both these elements totals five. The more dominant element is usually listed first.

Often, when the elements in a mineral vary, a series is formed. A series consists of a group of mineral in which one of the elements varies. For example, the Spinel series contains four members, with a series formula of (Mg,Zn,Fe,Mn)Al2O4. There are four end members of this series, which are listed below. Intermediary forms that are a combination of two or more also exist.

Spinel End Members
Spinel MgAl2O4 Magnesium Spinel
Gahnite ZnAl2O4 Zinc Spinel
Hercynite FeAl2O4 Iron Spinel
Galaxite MnAl2O4 Manganese Spinel

Some intermediary members of the spinel group have designated names, such as Gahnospinel (Mg,Zn)Al2O4, which is a mixture of Spinel (MgAl2O4) and Gahnite (ZnAl2O4); while other mixtures lack a designated name and are just called under the umbrella of Spinel. Intermediary members are sometimes scientifically recognized as individual mineral species by the IMA, though they are more often not recognized (Gahnospinel is not recognized as a scientifically distinct mineral by the IMA).

Many mineral series form solid solutions. In a solid solution, there are intermediate members between the two end members. Individual intermediary members may be given names, while others may not. An example is the Olivine group, with a chemical formula of (Mg,Fe)2SiO4. It contains Forsterite, (Mg2SiO4), and Fayalite, (Fe2SiO4) as the two end members. Very rarely are the Olivine members pure; most are somewhere along the intermediary scale within the solid solution series, with a varying percentage of Mg and Fe.


Atomical Variations

There are certain elements that come in slight modifications. Some elements have different amounts of electrons in atoms of the same element. Some minerals, such as Babingtonite, contain two types of the same element. The chemical formula for Babingtonite is written 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 any element with variable electrons. Anytime 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.


Chemical Formula Variables

The chemical formula of some minerals may be written in different formats and styles. It is not uncommon to see chemical formulas written differently for the same mineral. For example, the chemical formula for the mineral Dioptase is usually written CuSiO2(OH)2. However, its formula may also be written H2CuSiO4. Both formulas mean the exact same thing, as there is an equal amount of all the atoms in both formulas.

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 chemical formula can also be written as Na4Al3(SiO4)3Cl.


Variable Formulas

Some minerals may contain an element that partially replaces an inherent 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 only occasionally present. Therefore, an additional formula, known as the variable formula has been developed for this guide to state the occasional presence of these elements.

The variable formula displays the regular formula with the additional elements that are occasionally present. The inherent element is listed first in the parenthesis, and the occasional replacement elements are listed after and underlined. For example, 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 present in small amounts or may not be.

Some minerals without a variable formula may still have variable elements, but those elements are too rare or insignificant to be reckoned. For example, the mineral Fluorite occasionally has contains traces of Ce (cerium) and Y (yttrium), but there is no variable formula since these combinations are too uncommon.



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