Reading: Classifying Minerals

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:

  • 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.

  • 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 singele-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.

Sulfides

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.

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, O2–. 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. 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 three varieties of quartz
variable 7 conchodial fracture chalcedony (chert, etc.) three varieties of chalcedony
pink or white 5–6 2 planes at right angles orothoclase (feldspar) two varieties of orothoclase
white 5–6 2 planes at right angles Na-plagioclase (feldspar) Na-plagioclase
white to gray 5–6 2 planes at right angles Ca-plagioclase (feldspar) Ca-plagioclase
variable 4 4 planes fluorite two varieties of fluorite
colorless or white 3 3 planes at odd angles calcite two varieties of calcite
pink or white 3 3 planes at odd angles dolomite dolomite
colorless or white 2.5–3 3 planes at odd angles halite halite
colorless or white 2.5 1 plane muscovite muscovite
colorless or white 2 2 planes at right angles gypsum two varieties of gypsum
variable 1 1 plane talc talc
white < 1 uneven (turns to powder) kaolinite kaolinite
Table 2. Nonmetallic Luster—Dark Color
Typical Color Hardness Cleavage/Fracture Mineral Name Photo of Mineral
green 5–6 irregular olivine olivine
red 5–6 irregular garnet garnet
red 3–6 irregular hematite two varieties of hematite
dark green 3–6 2 planes at right angles pyroxene pyroxene
black 4.5–6 2 planes at odd angles hornblende (amphibole) hornblende
black 2.5 1 plane  biotite biotite
green 2 1 plane  chlorite Chlorite
Table 3. Metallic Luster
Typical Color Hardness Cleavage/Fracture Mineral Name Photo of Mineral
black or dark gray 6 irregular magnetite magnetite
brassy yellow 6 irregular pyrite pyrite
coppery yellow 4 irregular chalcopyrite chalcopyrite
silver 3 3 planes at right angles galena 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.

A simple lesson on how to identify minerals is seen in this video.


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