Minerals are the “stuff” of which all rocks are made. Whether Igneous, Metamorphic, or Sedimentary, all rocks are composed of assorted sizes and unique combinations of minerals.
Every rock tells a story. Geologists are scientists who have learned to read and share the stories that rocks tell.
The unique recipe of minerals in rocks tells geologists the story of the origin, history, and environments of rocks and the changes to which rocks have been subjected over time. These changes in rocks and minerals enable scientists to reconstruct and study the past environments of the Earth.
Petrology is the geological study of rocks. Petrology's sister discipline Petrography looks very closely at the combination, distribution, form, and abundance of minerals that each rock type or individual rock specimen contains.
Mineralogy is the geological science that characterizes, defines, and examines the chemical and physical properties of the various minerals, which either comprise rocks, or which sometimes stand alone in their mineralogical forms. Dana’s Handbook of Mineralogy in its various revisions has been a standard reference work in Geology and Mineralogy since the 1800s.
Most minerals have a crystalline form and the science of Crystallography is the geological discipline, which examines the physical properties of mineral grains and crystals. Thus, the identification of minerals is important to the science of Geology overall as well as to the geological sub-disciplines of Petrology, Petrography, Crystallography, and Mineralogy.
A mineral is defined as, any naturally occurring inorganic (non-carbon-containing) substance characterized by a unique crystalline structure and chemical formula. Minerals are classified according to the way that atoms and molecules are arranged into unique three-dimensional geometric solid crystalline structuressuch as cubic, tetrahedron, pyramidal, or bipyramidal. The metallic mineral Galena (lead ore) commonly has a cubic (6-sided) crystalline structure while the gemstone Garnet often takes on a rhombic dodecahedron (12-sided) crystalline structure.
The identification of minerals is carried out from several perspectives including detailed and expensive laboratory analysis of chemical composition by various methods and by studying a mineral’s crystalline structure and light properties by creating and examining microscopic thin-sections of rocks and minerals. Polarized light is often used to make microscopic observations of mineral crystals. Diffraction, reflection, and refraction of electromagnetic waves such as light or x-rays through crystals are used to make detailed laboratory observations and measurements.
In addition to detailed laboratory analyses, a variety of cursory field and workbench methods are used to characterize the physical properties of minerals and to zero-in on their identification. A good handbook or field guide on rock and mineral identification in addition to Dana’s Handbook of Mineralogy, will serve students and “rock hounds” by listing common physical and chemical properties and field test results for a variety of common minerals. Peterson’s Guides, Golden Guides, Audubon Guides, and an assortment of dichotomous keys available are all excellent resources when used either individually or in combination.
The basic physical and chemical properties used in mineral identification include the following:
- Cleavage. Cleavage is the way a mineral breaks along planes of weakness in its crystalline structure. The mineral Gypsum cleaves along parallelogram planes and creates smaller parallelograms when it is broken.
- Color. The color of a mineral often tells the story of its formation, is key to its identification, and is often brought about by various chemical alterations of the base mineral. Diamonds come in clear white, blue, black, and other colors based upon their unique chemical compositions. Quartz comes in clear, smoky gray, and milky white. When Quartz is in its purple form, it is called Amethyst.
- Crystal Form. The three-dimensional (3-D) geometric form of a mineral crystal is highly indicative of its identification but is not always reflected in its cleavage pattern. Quartz crystals frequently have rhombohedral crystal structures but are amorphous (without form) when you accidentally or intentionally break one.
- Fracture. Fracture is the breakage pattern of a mineral crystal but NOT along natural planes of weakness as in cleavage.
- Hardness. The hardness of a mineral is based upon a scale of one through ten called the “Mohs Hardness Scale” and is defined as a mineral’s resistance to breakage. Talc, the softest of minerals, is 1.0 on the Mohs scale; and Diamond, the hardest of minerals, is assigned a Mohs Hardness Scale number of 10.
- Luster. Is a measure of a mineral’s reflectivity of light… its shininess or dullness in appearance.
- Magnetism. Minerals containing ferrous Iron are often magnetic. The mineral Magnetite is named for its magnetic properties. Loadstones contain iron minerals and actually can become magnets. Meteorites often contain magnetic iron minerals and magnetism is important in their identification.
- Acid Test. A few drops of dilute hydrochloric acid (HCl) or vinegar (acetic acid) will sizzle (react) when placed on carbonate minerals such as Calcite (calcium carbonate) or upon Limestone, a sedimentary rock, which is composed largely of the mineral Calcite derived from the fossilized shells of marine organisms.
- Specific Gravity. Specific Gravity is the ratio between the mass of a rock or mineral sample and the mass of the equal volume of water it displaces in a tube. Specific Gravity is similar to Density but takes into account the force of gravity by the Earth (weight). Density is the absolute mass per unit of volume without regard to the force of gravity.
- Streak. Streak is the color of a powdered form of a mineral and is usually determined by scratching an unglazed ceramic tile with a sample of the mineral. A mineral’s streak color is often different from its overall (unpowdered) color. Hematite is a black appearing mineral, which leaves a red streak in its powdered form.
- Taste and Odor. Tasting rocks and minerals is NOT recommended in general for safety reasons. The mineral Halite (rock salt) can be distinguished from gypsum by its salty taste. Experienced geologists can sometimes make field inferences based upon the taste and smell of some rocks and minerals. Amateur collectors and students should not use the Taste method for mineral identification. Iron and Sulfur compounds frequently give off characteristic odors.
- Fluorescence. Some minerals reflect characteristic colors that are quite different from their gross apparent colors when subjected to ultraviolet light. Some rock and mineral collectors have whole displays of specimens that look very plain and ordinary under normal light but glow brilliantly and colorfully under UV light.
- Radioactivity. A few minerals and rocks emit natural radioactivity, which can be measured with a Geiger Counter. Rocks and minerals containing Uranium or Uranium salts are common in some areas. Some dinosaur bone fossils are radioactive because they accumulated in low-lying areas where Uranium salts and minerals also accumulated in the geologic past.
Use all of these physical and chemical properties of rocks and minerals along with several good identification guides and assorted dichotomous keys to identify the minerals that you find and keep in your rock and mineral collection.