Mineral grains make up the building blocks of every kind of rock, whether it’s a crumbling sandstone pebble or the smooth, polished granite of expensive countertops. Earth scientists use the minerals present to identify rock types, which is one part of the puzzle for defining a place’s geologic history. More important, perhaps, is that some minerals are rich in certain elements, which means that they can be mined to produce metals like copper and iron or non-metals like silicon and sulfur. The first step in identifying a rock, whether it is a mineral ore or a chunk of building stone, is to identify the minerals it contains.
By definition, a mineral is a naturally-occurring inorganic substance with a specific chemical composition and a characteristic crystal structure. There are thousands of minerals, however, and many look very similar to the naked eye. If you want to definitively identify one of these minerals from a specimen, you’ll need a well-equipped laboratory where you can perform a chemical analysis to determine your sample’s composition and a special X-ray machine to determine the shape of its crystal lattice. Fortunately, most of the common rock-forming minerals are easier to recognize and can be identified in the field or lab using basic observation, combined with simple tests to differentiate minerals based on their characteristic physical properties..
The simplest way to identify the common minerals is to use some kind of dichotomous key. There are many such keys, some of which are in pictorial. A dichotomous key asks you to consider a series of questions to “zero in on” the mineral by producing a smaller group of possibilities at each step. In order to use your key, you’ll need to make some simple observations (the order depends on the key and not all questions are asked on all keys):
* Lustre: Is the specimen shiny like metal or dull like dirt?
* General color: Is your specimen dark or light?
* Cleavage: Does the mineral naturally break along parallel planes? If yes, how many planes and at what angles? Be careful not to confuse cleavage and crystal faces.
Note: Except for light vs. dark, color is not particularly useful for mineral identification because the same mineral’s color can vary widely depending on the impurities in its chemistry.
You will also need to make simple tests of the mineral’s physical properties:
* Hardness: Is the mineral harder than a copper penny? Harder than glass or a knife blade? You’re measuring relative hardness, which is one of a mineral’s most useful diagnostic properties.
* Streak: What color is the mineral in powdered form? Rock labs keep small squares of unglazed porcelain for testing streak color, but you can also crush a bit with a knife blade. Even black minerals can have a white streak, while a shiny piece of hematite (iron ore) has a dull, brick-red streak.
*Other properties: Some minerals can be identified by unusual properties. For instance, magnetite is attracted to a magnet, calcite bubbles in dilute acid, and halite tastes like salt (because it is).
Once in a laboratory setting, you can perform other non-destructive tests to make sure of your identification. These include measuring cleavage or crystal-face angles and calculating specific gravity. Small mineral grains or crushed minerals can be inspected under a microscope to determine the index of refraction, which is also a diagnostic feature. Destructive tests can also be performed; such as cutting a wafer-thin section for electron microscope to determine crystal lattice shape.
While highly precise identification of a mineral is possible in a laboratory setting, for almost all applications in the field the use of a simple dichotomous key is good enough to identify most minerals one finds. Rock hounds and earth scientists usually commit a key to memory and carry with them the simple items – a knife, a copper penny and a hand lens – needed for basic identification of the common minerals.
