METHODS USED TO IDENTIFYING
MINERALS
More than 4,000 minerals are known to man, and they are identified by their physical and chemical
properties. The physical properties of minerals are determined by the atomic structure and crystal
chemistry of the minerals. The most common physical properties are crystal form, color, hardness,
cleavage, and specific gravity.
CRYSTALS
One of the best ways to identify a mineral is by examining its crystal form (external shape). A crystal is
defined as a homogenous solid possessing a three-dimensional internal order defined by the lattice
structure.
Crystals developed under favorable conditions often exhibit characteristic geometric forms (which are
outward expressions of the internal arrangements of atoms), crystal class, and cleavage. Large, well-
developed crystals are not common because of unfavorable growth conditions, but small crystals
recognizable with a hand lens or microscope are common. Minerals that show no external crystal form but
possess an internal crystalline structure are said to be massive.
A few minerals, such as limonite and opal, have no orderly arrangement of atoms and are said to be
amorphous.
Crystals are divided into six major classes based on their geometric form: isometric, tetragonal,
hexagonal, orthorhombic, monoclinic, and triclinic. The hexagonal system also has a rhombohedral
subdivision, which applies mainly to carbonates.
CLEAVAGE AND FRACTURE
After minerals are formed, they have a tendency to split or break along definite planes of weakness. This
property is called cleavage. These planes of weakness are closely related to the internal structure of the
mineral, and are usually, but not always, parallel to crystal faces or possible crystal faces. Minerals may
have one, two, three, four, or six directions of cleavage. These cleavage forms are cubic, octahedral,
dodecahedral, rhombohedral, prismatic, and pinacoidal. Minerals that break easily along these lines of
weakness have shiny surfaces. Many crystals do not cleave, but fracture or break instead. Quartz, for
example, forms well-developed crystal faces but does not cleave at all; instead, it fractures or breaks
randomly with a conchoidal fracture.
COLOR
The color of a mineral is, for the amateur mineralogist, the most important identifying characteristic. Many
minerals exhibit various colors; the varieties are mainly caused by impurities or a slight change in
chemical composition. For example, calcite can be white, blue, yellow, or pink. Surface tarnish may have
changed the color of a specimen, so a fresh surface should be examined.
HARDNESS
The hardness of a mineral can be measured by its resistance to scratching or abrasion. The Mohs scale is
a set of 10 common minerals chosen for comparative hardness. The minerals are arranged in order of
increasing hardness; each mineral will scratch all that precede it, and be scratched by all that follow it.
The Mohs scale is as follows:
1. talc
2. gypsum
3. calcite
4. fluorite
5. apatite
6. orthoclase
7. quartz
8. topaz
9. corundum
10. diamond
STREAK
The streak of a mineral is the color of the powder produced when the mineral is rubbed against an
unglazed porcelain plate or other fine-grained, hard, abrasive surface. The color of a particular mineral
may vary, but the streak is generally constant. The streak may be the same color as the mineral or an
entirely different color, but the streak of all white minerals, including calcite, is white.
LUSTER
Luster refers to the brightness of light reflected from the mineral's surface. The main types of luster are
metallic and nonmetallic. Some of the more important nonmetallic lusters are:
Adamantine: brilliant, like that of a diamond
Earthy: dull, like kaolin
Silky: having the sheen of silk, like satin spar, a variety of gypsum
Greasy: oily appearance
Resinous: waxy appearance, like sphalerite
Vitreous: the appearance of broken glass, like quartz
Nacreous (pearly): like mother of pearl; for example, pearly luster on fossil gastropods and cephalopods
SPECIFIC GRAVITY
The specific gravity (relative density) of a mineral is its weight compared to the weight of an equal volume
of water; thus, a mineral with a specific gravity of 4 is four times heavier than water. Special instruments
are needed to measure specific gravity.
TENACITY
Tenacity is the measure of a mineral's cohesiveness or toughness. Tenacity terms are:
Brittle: breaks or powders easily; for example, pyrite or marcasite
Ductile: can be drawn into a wire; for example, copper
Elastic: bends and resumes its original position or shape when pressure is released; for example, biotite
or muscovite
Malleable: can be hammered into thin plates or sheets; for example, gold or copper
Sectile: can be cut or shaved with a knife; for example, gypsum or galena
ACID TEST
When carbonates (especially calcite) are treated with cold, dilute hydrochloric acid, they will effervesce
(foam and bubble) and give off carbon dioxide gas. When sulfides, such as galena, pyrite, and sphalerite,
are treated with dilute hydrochloric acid, they will give off the rotten-egg odor of hydrogen sulfide.
MAGNETISM
A few minerals, such as magnetite and pyrrhotite, are attracted by a magnet and are said to be magnetic.
Magnetic minerals are rare in Kentucky, but do occur in the kimberlite in Elliott County. If you find a large
piece of highly magnetic material, it may be a meteorite or a furnace product.
FLUORESCENCE
Some minerals, such as calcite, gypsum, halite, uranium minerals, and fluorite, will fluoresce in brilliant
colors when viewed with an ultraviolet light. UV light is not normally visible to the human eye, and you
should avoid looking directly at the UV source, because it can damage eyesight.
