Identifying Minerals
Minerals have certain mineral properties that are determined by their crystal structure and chemical composition. These properties can assist in identifying many minerals. Below is a brief overview of some key topics/terms relating to mineral identification.
COLOR:
Some minerals have a characteristic color; Malachite has a distinct green, Azurite a blue, Sulfur is yellow, etc.. Some minerals occur in a variety of colors, for example, fluorite comes in almost every color. These kind of minerals are better identified by properties other than just color.
Colors in minerals is caused by the absorption of refraction of light of particular wavelengths. This can happen several different ways:
The presence of 'trace' elements - "foreign" atoms that are not part of the basic chemical makeup of the mineral in crystal structure. As few as just three atoms per one million can absorb enough of the visible spectrums of light to give color to some minerals.
Color can also be the result of the absense of an atom or ionic radical from a place that it would normally be in a crystal. The structure of the mineral itself, without any defect or foreign element , may also cause color.
Opal is composed of very small spheres of silica that diffract light.
LUSTER:
"Luster" is the appearance of a mineral's surface in reflected light. There are two broad 'types' of luster, metallic and non metallic. Metallic luster is like an untarnished metal surface, like copper or gold. These minerals are usually opaque (no light shines through). Minerals with a non metallic luster are often transparent or translucent. Vitreous describes the luster of a piece of broken glass. A diamond's luster is called adamantine. Resinous, pearly, greasy, silky and dull are also 'lusters'.. We'll talk more about each luster later.
STREAK:
Ok, so imagine this. You have an unidentified mineral and an unglazed porceilain surface (like the bottom of a mug or plate, even). You take your mineral and draw it across an unglazed porceilain surface. A powder is created from the force of the friction of the two materials. The color of this powder is considered your 'streak'. A minerals streak is consistent, therefore it is a better diagnostic indicator than just color is, as color can vary. Streak can help to distinguish between two easily confused minerals. For example, iron oxide hematite has a red streak, while magnetite , another iron oxide, gives a black streak.
The streak of a mineral is consistent from specimen to specimen, as long as an unweathered surface is used/tested. It is the same as the color of the powdered mineral.
CLEAVAGE:
The ability of a mineral to break along flat (planar) surfaces is called cleavage. This happens in the crystal structure itself, where the forces that bond atoms together are the weakest. Cleavage surfaces are usually smooth and reflect light evenly. Cleavage is described by its direction relative to the orientation of the crystal, and by how easily it's produced. If cleavage easily produces smooth, lustrous surfaces, it is called 'perfect cleavage'. Distinct, imperfect, and difficult are less 'easy' kinds of cleavage. Minerals can have different quality cleavages in different directions, some have no cleavage at all.
FRACTURE:
Some minerals break in directions other than those of the cleavage planes. These breaks - or fractures - can help identify different minerals. For example, fractures with jagged edges are often found in metals, while conchoidal (shell-like) are common in quartz. Other terms used to described fracture are even (rough, but more or less flat), uneven (rough and irregular), and splintery (with partially seperated fibers).
TENACITY:
Gold, silver and copper are maleable and can be flattened without crumbling. Sectile minerals can be cut smoothly with a knife; flexible minerals bend easily and stay bent when pressure is removed. Ductile minerals can be drawn into a wire, brittle minerals are prone to breaking, and elastic minerals return to their original form after they are bent.
HARDNESS:
The relative ease or difficulty of a mineral to be scratched. A harder mineral can scratch a softer one, but not vice versa. Minerals are assigned a number between 1 and 10 on Moh's Scale. Hardness differs from strength or 'toughness' - very hard minerals can be very brittle. Most hydrous minerals - those that contain water molecules - are soft, as are phosphates, carbonates, sulfates, halides and most sulfides. Anhydrous oxides - those without water molecules - and silicates, are rather hard.
REFRACTIVE INDEX:
As light passes througha transparent or translucent mineral, it changes velocity and direction. The extent of this change is measured by the refractive index: the ratio of light's velocity in the air to its velocity in the crystal. A high index causes dispersion of light into its component colors. Refractive indices can be found using specialized liquids or inexpensive equipment.
FLUORESCENCE:
Some minerals exhibit fluorescence - they emit visible light of various colors when subjected to ultraviolet radiation. Fluorescence is an imperfect indicator of a minerals identity because not all specimens of a mineral show fluorescence , even if they look identical and come from the same location.
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