Identifying Minerals

Structure and composition are the defining marks of a mineral. True mineral identification requires x-ray analysis to determine structure, and chemical tests to determine compositionl. These tests use expensive equipment, expert know-how and often destroy the specimen BUT structure and composition affect certain physical properties and appying these properties allows "rock hounds" the reliably identify specimens. 

An idealized physical property is one that will give a unique result for each mineral and will always give the same result, again and again, for any and every specimen of that mineral. Mineralogists are usually content to have a property that simply is consistent in providing the same result for every specimen of a certain mineral. Hopefully, this property also has a good range of possible results so that two similar minerals stand a good chance of having different results. A collector then uses the the results of several know physical property tests to identify an unknown mineral.

Physical Properties Used to Identify Minerals

  1. Color -  It is the first thing we notice but generally color is not a good property to be used in mineral identification. Many minerals have different colors and some minerals' colors are identical to other minerals' colors. It is important to understand what causes color in minerals in order to understand this mineral property.
  2. Streak - This is the color of the powder of a mineral. The proper way to test for streak is to rub a mineral across a tile of white unglazed porcelain and to examine the color of the "streak" left behind. Streak is closely related to color, but a different property because streak color may be a different color than the mineral. It is a very useful property because it is generally very consistent from specimen to specimen for a given mineral.
  3. Luster- This describes the way light interacts with the crystals surface. It is how you describe to someone how a mineral looks. For example it is metallic or waxy or dull. It has nothing to do with color or shape, but is related to transparency , surface conditions, crystal habit and index of refraction. The terms used are meant to be descriptive.
  4. Transparency- Also known technically as diaphaneity, It is a function of the way light interacts with the surface of a substance. There are only three possible descriptions: Transparent where light enters and exits the surface relatively undisturbed. Translucent if light can enter and exit the surface of the substance, but in a disturbed and distorted fashion. Opaque when light cannot even penetrate the surface of the substance.
  5. Cleavage- A mineral breaks either by fracturing or by cleaving. Cleavage is a smooth break producing what appears to be a flat crystal face. Cleavage is reproducible, meaning that a crystal can be broken along the same parallel plane over and over again. All cleavage must parallel a possible crystal face.  All cleavage planes of a mineral must match that mineral's symmetry. And finally, the same mineral will always, always have the same cleavage. Cleavage is described in terms of how easy cleavage is produce: perfect (easiest), imperfect, good, distinct, indistinct, and poor (hardest).
  6. Fracture- Describes the way a mineral tends to break. It is different from cleavage which are generally clean flat breaks along specific directions. Fracture occurs in all minerals even ones with cleavage. Different minerals will break in different ways and leave a surface that can be described in a recognizable way. Fracture is described as conchoidal, subconchoidal, jagged, uneven, splintery, and earthy.
  7. Hardness- One of the better physical properties for minerals. Specimens of the same mineral may vary slightly from one to another, but generally they are quite consistent. Hardness is one measure of the strength of the structure of the mineral relative to the strength of its chemical bonds. Minerals with small atoms, packed tightly together with strong covalent bonds throughout tend to be the hardest minerals. Hardness is generally consistent because the chemistry of minerals is generally consistent.
  8. Crystal Systems-A mineral's crystalline structure, the arrangement of its component atoms and/or ions, is responsible for the outward shape of the crystal Minerals generally form crystals that are consistent with the symmetry class that the mineral falls into, based on its own structure. Understanding what symmetry class a mineral belongs to is very helpful in identifying its crystals.
  9. Technical Crystal Habits- Crystal habit describes the shapes and aggregates that a certain mineral is likely to form. This is often the most important characteristic to examine when identifying a mineral. Although most minerals do have different forms, they are sometimes quite distinct and common only to one or even just a few minerals. Understanding them can greatly increase the chance of correctly identifying minerals. There are either open forms or closed forms.
  10. Descriptive Crystal Habits- These terms are more descriptive and are easier to understand. There are basically two types: single crystal forms and aggregate forms. The single crystal terms are used to describe individual crystals and so terms like platy or prismatic are used. Aggregate terms are for minerals whose crystals come in groups of crystals and form a unique shape. Individual crystals in aggregates are usually hard to discern. All these terms are subjective and most minerals form more than one.
  11. Twinning- Twins form because of an error during crystallization. Instead of a "normal" single crystal, two crystals appear to be growing out of or into each other, like Siamese twins. There are two general types of twin styles; contact and penetration. Contact twins have a composition (twin) plane that forms at the boundary between the two twins and acts like a mirror. Penetration twins which look like two crystals that grew into and out of each other. These twins can form crosses, 3-D star shapes and complex structures.
  12. Mineral Associations- Minerals often form in specific environments, associated with specific minerals. Sometimes a mineral is only associated with a certain groups or a 'suite' of minerals. Identifying these associated minerals can help identify an unknown mineral.
  13. Locations- There exists certain places around the world, areas that for one reason or another, produce the most amazing assortment of minerals. These sites are well known to mineral collectors everywhere. They may be the type locality for a number of rare minerals. They can be where literally hundreds of different mineral species are found.

This information compiled from sources including www.mineral.galleries.com