The chemical elements bond together in various ways to form the basic building blocks of rocks -- minerals. Minerals, in geologic terms, must fit a very restricted set of rules.
They must be natural in origin. Synthetic gems and crystals grown in space are not minerals.
Minerals are inorganic in character; though they may be produced by plants and animals. Shells, bones and teeth are commonly preserved in rock after the "complex organic molecules" (tissue) has decomposed.
Minerals are crystalline in nature. The atoms within a mineral have a definite internal atomic structure that repeats itself in a symmetrical pattern. When a mineral crystallizes or grows in a void (an "unrestricted" environment), the mineral shape reflects the internal structure and a crystal forms. In most instances, however, the mineral is growing in a complex area with other crystals. This causes "interference" of growth; the minerals intergrow and "distort" the outer appearance of the minerals. The internal arrangement remains true but the outer form takes on the "look" of the "restricted" growth and appears as an irregular shape. This shape is commonly called...a rock.
Any given mineral will have a limited chemical composition. If the element in that crystal are changed, it will no longer be the same mineral. Sodium chloride is the mineral halite; potassium chloride is the mineral sylvite. Calcium carbonate is the mineral calcite; calcium sulfate is the mineral gypsum.
There is some minor flexibility, known as ionic substitution. Some of the ions are similar in size and ionic charges (Ex.: Fe +2 and Mg+2). If the mineral is crystallizing out of a liquid that is rich in both elements, the crystal may incorporate some of the "substitute" ions instead of the more appropriate ion.
This substitution is common in the silicate families.
Given the these limitations, mineral characteristics follow predictable patterns. Certain mineral groups are soluble because of their chemical compositions; minerals break in predictable fashions because of weaknesses in their crystal habits; some minerals are more dense (heavier) than others (lead composition verses aluminum). Several physical properties, in combination, can be used to identify common rock forming minerals.
Each of the following physical properties has a simple test that can be performed "in the field" to narrow the choices for mineral identification. Most "handbooks" on mineral identification will list the major physical properties common to any given mineral. Not all the physical properties apply to all minerals.
If all minerals grew in a pure state (with no inclusions or contaminants) and formed perfect crystals (unrestricted growth) there would be no difficulty identifying minerals. They don't ordinarily come that way -- that's why you spend big bucks at gem shows! A limited number of minerals commonly grow as well formed crystals; use them when available. Most minerals grow into irregular masses of interlocking crystals; other physical properties must be used.
Weaknesses in bonding in the atomic structure form predictable patterns of breakage in a mineral. If a mineral has cleavage it will always cleave in the same way within a single "crystal". Some minerals will cleave in one direction; others cleave in two, three or more directions at specific angles. Some of the cleavages are perfect (clean, crisp and flat planes); some are imperfect (stepped and jagged surfaces). In the field, cleavage is tested by striking a mineral with a hammer and observing how it breaks. In the lab, look for flat surfaces that reflect light well.
This property is recognized as a random, irregular pattern along the surface of a mineral. All minerals have fracture, but not all minerals have cleavage. Look for duller surfaces and breaks that are non parallel in character. Many "glassy" minerals often have only fracture; note how the breaks occur in random patterns.
The way a mineral looks in reflected light is known as luster. Hundreds of terms are used to describe luster, the most important of which are metallic and nonmetallic types. Either metallic or nonmetallic may be shiny or dull in character. Metallics resemble common metal pieces: shiny "foils" of silver, gold, brass, etc. or dull, rusty reds, grays and yellows. Nonmetallics range from glassy colors to greasy or waxy lusters and dull, "earthy", muddy colors. (Black shiny minerals are commonly nonmetallic and glassy in texture! Think patent leather!)
Though most "handbooks" on mineral identification separate minerals by colors, it is the most unreliable property to use for most common rock forming minerals. Very few minerals have characteristic colors. Most minerals contain inclusions and contaminants of other minerals which mask or discolor the true shade of a mineral. Use caution when attempting to identify a mineral by its color.
The true color of mineral becomes apparent when it is crushed to powdered form. This color is usually determined by rubbing the mineral against an unglazed porcelain slab called a "streak plate". This process can only be used for softer minerals; i.e. most non silicates. Because the streak plate is harder than these minerals, rubbing the mineral across the plate leaves a powder residue. When the excess powder is blown away, what remains is the color of the streak. Metallic minerals have the most distinctive streak colors; nonmetallics are often white (and therefore indistinguishable), or, in the case of many silicate minerals, too hard to perform the test.
Minerals are commonly tested to see how well they resist being scratched. Most gemstones are minerals that resist abrasion - they hold up well to being bumped and abused. To determine hardness, minerals are compared to materials of known hardness, referred to as Moh's Scale of Hardness. The materials are ranked from one to ten with one representing the softest mineral (talc) and ten, the hardest (diamond). (See text for figure.) Common field tools used for comparison include fingernails (H=2.5); a copper penny (H=3.5); and glass (H=5.5-6.0).
This is the "heft" of a mineral sample or how heavy it is relative to water. When a sample has a specific gravity of 5, it is 5 times heavier than an equal amount of water. This test can only be properly performed in a lab setting and a "density test" is often used as a substitute for measuring true specific gravity. (See the lab manual for directions on how to measure density). When in the field, use the "this one feels heavier" test.
Other Properties (also referred to as Special Properties):
Some minerals dissolve when placed in water (halides and some sulfates); others effervesce (dissolve) when placed in acid (carbonates: especially calcite; dolomite will if powdered first).
Only one mineral is attracted to a magnet: magnetite; one of the iron oxides.
Minerals that are soluble in water often have a taste; especially the halides.
Check for a slippery (clays when wet), soapy (talc; i.e. baby powder!) or greasy (graphite) feel.
Check for sulfur smells (rotten eggs) in elemental sulfur and sulfides (a light scratch with a little acid will bring out the smell). Clays will smell "earthy" (wet desert) when moistened.
These are fine parallel etchings or "scratches" on flat reflective surfaces of minerals. They are common on some crystal faces (quartz and pyrite, running across the crystal face) and on cleavage planes in plagioclase feldspars (running along the cleavage surface). This is NOT the change in colors present in some feldspars (called laminae); it is visible only in reflected, glassy surfaces.
Calcite, in the form of "Icelandic spar", will cause "double images" (double-refraction) when placed over letters on a piece of paper.
Keep in mind that not all minerals will have all the above mentioned physical properties. When working with metallic lusters be sure to check streaks and hardness, cleavage is not common in these minerals. Nonmetallic lusters are distinguished by their cleavage, hardness and occasional special properties; their streaks are not considered important properties.
Common Mineral Assemblages:
- Silicates: nine in particular
- Ferromagnesian silicates:
*Pyroxenes (Ex.: Augite)
*Amphiboles (Ex.: Hornblende)
- *Orthoclase (K-spar)
*Quartz (all coarse crystalline varieties, Ex.: smoky quartz, milky quartz)
- rock fragments of all varieties weathered silicates:
Iron oxides (Ex.: *Hematite, *Limonite)
- *Quartz (both coarse crystalline and cryptocrystalline varieties, Ex.: *Chert, Jasper, Flint and Agate)
*Clays (Ex.: Kaolinite)
- all igneous silicates except Olivine, Ca-plagioclase in addition:
Each mineral forms in a very specific environment: the correct chemical composition at the right temperature and right pressure. (It's kind of like cooking: you can make cakes, donuts or breads with the same ingredients. It depends on the mix and how you cook it.) The rock that is created will be "stable" only where it was formed. If it is moved through erosional or tectonic processes it will become "unstable" and alter to new minerals. This concept is known as mineral stability.
The rock cycle (see text for figure) is a summary of some of the processes that rock material can go through as it is altered to new rocks. This class will review one of the many possible cycles that rock material can "follow" as it reacts with various geologic processes. The rock cycle is actually much more complex and the rocks need not make a "complete" cycle. What a rock becomes and when it is altered will depend entirely on its location during Plate Tectonic events. (Side Thought: Remember that the Scientific Method is based on the collection of data (cold, hard facts) and interpretations of that data. The problem with data collection in geology is...the rock cycle. The data (rocks) are continuously destroyed. Geologist must work with incomplete and "tampered" data to interpret the earth's geologic history!)