minerals

Mineral Classification

The Dana System

Mineral classification can be an organizational nightmare. With over 3,000 different types of minerals a system is needed to make sense of them all. Mineralogists group minerals into families based on their chemical composition. There are different grouping systems in use but the Dana system is the most commonly used. This system was devised by Professor James Dana of Yale University way back in 1848. The Dana system divides minerals into eight basic classes. The classes are:

Mineral Classification
	Native Elements This is the category of the pure. Most minerals are made up of combinations of chemical elements. In this group a single element like the copper shown here are found in a naturally pure form.
	Silicates This is the largest group of minerals. Silicates are made from metals combined with silicon and oxygen. There are more silicates than all other minerals put together.The mica on the left is a member of this group.
	Oxides Oxides form from the combination of a metal with oxygen. This group ranges from dull ores like bauxite to gems like rubies and sapphires. The magnetite pictured to the left is a member of this group.
	Sulfides Sulfides are made of compounds of sulfur usually with a metal. They tend to be heavy and brittle. Several important metal ores come from this group like the pyrite pictured here that is an iron ore.
	Slufates are made of compounds of sulfur combined with metals and oxygen. It is a large group of minerals that tend to be soft, and translucent like this barite.
	Halides form from halogen elements like chlorine, bromine, fluorine, and iodine combined with metallic elements. They are very soft and easily dissolved in water. Halite is a well known example of this group. Its chemical formula is NaCl or sodium chloride commonly known as table salt.
	Carbonates are a group of minerals made of carbon, oxygen, and a metallic element. This calcite known as calcium carbonate is the most common of the carbonate group.
	Phosphates are not as common in occurrence as the other families of minerals. They are often formed when other minerals are broken down by weathering. They are often brightly colored.
	Mineraloid is the term used for those substances that do not fit neatly into one of these eight classes. Opal, jet, amber, and mother of pearl all belong to the mineraloids.

BASIC DEFINITIONS

A mineral is a naturally occurring inorganic substance with a characteristic chemical composition and definite crystal structure. The composition and crystalline structure determines the properties of a mineral. The main mineral properties used for identification are color, streak, hardness, specific gravity, cleavage and fracture. Other properties such as luster and habit are also important.

Rocks are aggregates of minerals. Rocks exhibit not only different mixture of minerals, but also certain textures. The texture depends upon the size, shape, and arrangement of the minerals composing the rock. Mineral assemblage and texture serve in determining the origin and identification of a rock.

MINERAL IDENTIFICATION PROCESS

MINERAL PROPERTIES

COLOR - The color of a mineral serves to narrow down the number
of possible choices since it is the first and most obvious
property noticed. However, because most minerals may
exhibit a variety of colors, color is not a reliable diagnostic
property.

HARDNESS - Hardness is a measure of resistance to scratching. The hardness of a mineral is based upon comparisons of scratching tests. Mohs Scale of Hardness is the basic comparison test.

Mohs Hardness Scale Hardness of Test Materials

1 - Talc 2.5 - Fingernail
2 - Gypsum 3.5 - Copper penny
3 - Calcite 5.5 - Masonry nail
4 - Fluorite 5.5 - Glass
5 - Apatite
6 - Orthoclase feldspar
7 - Quartz
8 - Topaz
9 - Corrundum
10 - Diamond

Example of hardness determination: If an unknown mineral cannot be scratched by a masonry nail but can be scratched by orthoclase feldspar, then the hardness of the unknown mineral would be between 5.6 and 5.9.

STREAK - The color of the powder of a mineral is the streak. Rub the mineral on a piece of unglazed porcelain (streak plate) to obtain the streak.

CLEAVAGE - Certain minerals have a tendency to part, producing smooth flat surfaces. An example is mica: it cleaves along one plane thus it has one direction of cleavage. Galena breaks into cubes; a cube has three directions of cleavage that intersect at 90 degree angles.

FRACTURE - If a crystal does not break along a cleavage surface, it exhibits fracture. Quartz shows no cleavage when it is broken, instead it fractures in a shell shape known as conchoidal fracture. Other minerals with well-developed cleavage may fracture along other surfaces. Common descriptions: conchoidal, splintery, fibrous and irregular.

LUSTER - The degree or manner in which the surface of a mineral reflects light is luster. Terms used to describe luster include earthy, glossy, metallic, pearly, greasy, waxy, and vitreous (glassy).

SPECIFIC GRAVITY - The weight of a mineral compared to the weight of an equal volume of water is the specific gravity. Gold has a specific gravity of 19. This indicates that a cubic centimeter of gold weighs 19 times as much as a cubic centimeter of water. (One cubic centimeter of water weighs 1.0 grams) A relative comparison (of what is light and what is heavy) is satisfactory for this laboratory.