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.