18 Classifying 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, S^2–. 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:

• silicates
• sulfides
• carbonates
• oxides
• halides
• sulfates
• phosphates
• native elements

Silicates

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.

An example of common silicate mineral is olivine: (Mg, Fe)₂SiO_4. ; in this case either cation (Mg or Fe) or both can be present in the mineral.

Sulfides

These are based on the sulfide ion, S^2–. 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.

Carbonates

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.

Oxides

These are based on the oxygen anion, O^2–. Examples include iron oxides such as hematite, Fe₂O₃ and magnetite, Fe₃O₄, and pyrolusite, MgO.

Halides

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.

Sulfates

These have the polyatomic sulfate ion, (SO₄)^2–, as the anion. Anhydrite, CaSO₄, is a sulfate.

Phosphates

These have the polyatomic phosphate ion, (PO₄)^3–, as the anion. Fluorapatite, Ca₅(PO₄)₃F, which makes your teeth hard, is a phosphate mineral.

Native Elements

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. Each table contains a group of minerals according to a specific luster. Luster is one of the first main identification keys.

As you read through the tables, you can click on the images of minerals to see a larger version of the photo.

Table 1. Nonmetallic Luster—Light Color
Typical Color	Hardness	Cleavage/Fracture	Mineral Name	Photo of Mineral
colorless	7	conchodial fracture	quartz
variable	7	conchodial fracture	chalcedony (chert, etc.)
pink or white	5–6	2 planes at right angles	orothoclase (feldspar)
white	5–6	2 planes at right angles	Na-plagioclase (feldspar)
white to gray	5–6	2 planes at right angles	Ca-plagioclase (feldspar)
variable	4	4 planes	fluorite
colorless or white	3	3 planes at odd angles	calcite
pink or white	3	3 planes at odd angles	dolomite
colorless or white	2.5–3	3 planes at odd angles	halite
colorless or white	2.5	1 plane	muscovite
colorless or white	2	2 planes at right angles	gypsum
variable	1	1 plane	talc
white	< 1	uneven (turns to powder)	kaolinite

Table 2. Nonmetallic Luster—Dark Color
Typical Color	Hardness	Cleavage/Fracture	Mineral Name	Photo of Mineral
green	5–6	irregular	olivine
red	5–6	irregular	garnet
red	3–6	irregular	hematite
dark green	3–6	2 planes at right angles	pyroxene
black	4.5–6	2 planes at odd angles	hornblende (amphibole)
black	2.5	1 plane	biotite
green	2	1 plane	chlorite

Table 3. Metallic Luster
Typical Color	Hardness	Cleavage/Fracture	Mineral Name	Photo of Mineral
black or dark gray	6	irregular	magnetite
brassy yellow	6	irregular	pyrite
coppery yellow	4	irregular	chalcopyrite
silver	3	3 planes at right angles	galena

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:

1. Look at it closely on all visible sides to see how it reflects light
2. Test its hardness
3. Identify its cleavage or fracture
4. Name its luster
5. 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.

An example of mineral identification:

Online minerals and their properties guide, see: https://geology.com/minerals/