Classes of Minerals
Minerals are classified according to their chemical properties. Except for the native element class, the chemical basis for classifying minerals is the anion, the negatively charged ion that usually shows up at the end of the chemical formula of the mineral. For example, the sulfides are based on the sufur ion, S2–. Pyrite, for example, FeS2, is a sulfide mineral. In some cases, the anion is of a mineral class is polyatomic, such as (CO3)2–, the carbonate ion. The major classes of minerals are:
- native elements
Based on the polyatomic anion, (SiO4)4–, which has a tetrahedral shape. Most minerals in the earth’s crust and mantle are silicate minerals. All silicate minerals are built of silicon-oxygen tetrahedra (SiO4)4– in different bonding arrangements which create different crystal lattices. You can understand the properties of a silicate mineral such as crystal shape and cleavage by knowing which type of crystal lattice it has.
- In nesosilicates, also called island silicates, the silicate tetrahedra are separate from each other and bonded completely to non silicate atoms. Olivine is an island silicate.
- In sorosilicates or paired silicates, such as epidote, the silicate tetrahedra are bonded in pairs.
- In cyclosilicates, also called ring silicates, the silicate tetrahedra are joined in rings. Beryl or emerald is a ring silicate.
- In phyllosilicates or sheet silicates, the tetrahedra are bonded at three corners to form flat sheets. Biotite is a sheet silicate.
- In single-chain inosilicates the silicate tetrahedra are bonded in single chains. Pyroxenes are single-chain inosilicates.
- In double-chain inosilicates the silicate tetrahedra are bonded in double chains. Amphiboles are double-chain inosilicates.
- In tectosilicates, also known as framework silicates, all corners of the silicate tetrahedra are bonded to corners of other silicate tetrahedra, forming a complete framework of silicate tetrahedra in all directions. Feldspar, the most common mineral in earth’s crust, and quartz are both framework silicates.
These are based on the sulfide ion, S2–. Examples include pyrite, FeS2, galena, PbS, and sphalerite, ZnS in its pure zinc form. Some sulfides are mined as sources of such metals as zinc, lead, copper, and tin.
These are based on the carbonate ion, (CO3)2–. Calcite, CaCO3, and dolomite, CaMg(CO3)2, are carbonate minerals. Carbonate minerals tend to dissolve relatively easily in water, especially acid water, and natural rain water is slightly acid.
These are based on the oxygen anion, O2–. Examples include iron oxides such as hematite, Fe2O3 and magnetite, Fe3O4, and pyrolusite, MgO.
These have a halogen element as the anion, whether it be fluoride, F–, chloride, Cl–, bromide, Br–, iodide, I–, or astatide, At–. Halite, NaCl, is a halide mineral.
These have the polyatomic sulfate ion, (SO4)2–, as the anion. Anhydrite, CaSO4, is a sulfate.
These have the polyatomic phosphate ion, (PO4)3–, as the anion. Fluorapatite, Ca5(PO4)3F, which makes your teeth hard, is a phosphate mineral.
These are made of nothing but a single element. Gold (Au), native copper (Cu), and diamond and graphite, which are made of carbon, are all native element minerals. Recall that a mineral is defined as naturally occurring. Therefore, elements purified and crystallized in a laboratory do not qualify as minerals, unless they have also been found in nature.
Mineral Classification Tables
In tables 1–3, hardness is measured on Mohs Hardness Scale. As you read through the tables, you can click on the images of minerals to see a larger version of the photo.
|Table 3. Metallic Luster
|Photo of Mineral
|black or dark gray
|3 planes at right angles
How to Identify Minerals
First, you need good light and a hand lens or magnifying glass. A hand lens is a small, double-lens magnifying glass that has a magnification power of at least 8× and can be purchased at some bookstores and nature stores.
Minerals are identified on the basis of their physical properties, which have been described in the the previous section. To identify a mineral, you look at it closely. At a glance, calcite and quartz look similar. Both are usually colorless, with a glassy luster. However, their other properties they are completely different. Quartz is much harder, hard enough to scratch glass. Calcite is soft, and will not scratch glass. Quartz has no mineral cleavage and fractures the same irregular way glass breaks. Calcite has three cleavage directions which meet at angles other than 90°, so it breaks into solid pieces with perfectly flat, smooth, shiny sides.
When identifying a mineral, you must:
- Look at it closely on all visible sides to see how it reflects light
- Test its hardness
- Identify its cleavage or fracture
- Name its luster
- Evaluate any other physical properties necessary to determine the mineral’s identity
In the minerals tables that accompanies this section, the minerals are grouped according to their luster and color. They are also classified on the basis of their hardness and their cleavage or fracture. If you can identify several of these physical properties, you can identify the mineral.
A simple lesson on how to identify minerals is seen in this video.