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Home>Learn and Explore!>Rocks, Minerals and Prospecting>Rock and Mineral Identification
 

Rock and Mineral Identification

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Field identification of rocks and minerals can be an approximate, best-guess sort of business. If you have a good field guide in hand, however, noting a few basics about a specimen will get you started in the right direction.

Minerals

Because they occur in a pure form, minerals are relatively easy to identify. Some of the characteristics that can be used to make this determination are:

Hardness:

Moh's hardness kit

This is measured with the Mohs scratch test. Minerals range from very soft, like talc (Mohs 1), to exceedingly hard, like diamond, (Mohs 10). You can carry a small Mohs Hardness Testing Kit with you into the field, or you can use a more informal method. Fingernails are about Mohs 2˝, so if you can scratch your sample with your nail, it's Mohs 2 or less. A piece of glass or a knife blade is about Mohs 5˝, and a steel file is between Mohs 6 and 7.

Color:

Sometimes, as with the vividly green mineral malachite, color is diagnostic. Other minerals may show a number of different colors due to trace minerals or other conditions. Pure quartz crystals are clear as water, but a minute amount of iron will tint them purple making them amethys, while exposure to natural radiation will turn them a deep, smoky brown also known as citrine.

 The color a mineral leaves when scratched across an unglazed porcelain surface (most commonly the back of a bathroom tile, though commercial streak plates are also available) may be different than its surface appearance. Hematite, which is a silvery gray, leaves a red streak. This test is not effective with minerals harder than the streak plate, which is about 6˝ on the Mohs scale.

Luster:

This describes the way the mineral reflects light. Some common terms are: metallic, vitreous (glassy), oily, pearly, and earthy (dull).

Flourite cubeCrystal Habit:

Minerals are crystalline by nature, although with some of them this may be very hard to see without a microscope. If your specimen shows a definite crystalline structure, this is a good clue to its identity. There are six basic kinds of crystal structure, with a number of variations. Fluorite is generally isometric, forming a cube. Quartz is hexagonal, or six-sided. The other crystal forms are tetragonal, orthorhombic, monoclinic, and triclinic -- your field guide will describe them in detail, along with their variations.

Cleavage and Fracture:

If your specimen breaks cleanly, producing a smooth plane, it is said to have cleavage. Mica has cleavage in one direction; halite shows three directions of cleavage, and will produce a cube. If the specimen doesn't have cleavage, the fracture is generally described as conchoidal (shell-like), fibrous, or irregular. Opal shows a conchoidal fracture, spar gypsum a fibrous one, and quartz fractures irregularly.

Specific Gravity:

This compares the density of the mineral to that of water. A rough test is to feel how heavy a specimen weighs in your hand. Minerals containing a lot of iron or lead, for example, will feel very heavy for their size. Practicing with equal-sized samples of differing, known specific gravities will help. For example, halite (2.1) is considered light, quartz (2.7) is medium-weight, barite (4.5) is heavy, and galena (7.6) is very heavy. But to get the exact numerical value you'll need a good beam balance or a similar scale.

Fluorescence:

Many minerals will glow vividly under short- or long-wave light. The color a particular specimen fluoresces can help identify not only what it is, but sometimes where it comes from (if you didn't find it yourself). To make the test you'll need a short- or long-wave lamp (or a combination lamp), a guide to fluorescent minerals, and a dark place. Not all specimens of a particular mineral will fluoresce, however, so if you get no response, go on to another test!

Unusual Properties:

These are specific to particular minerals. White vinegar will fizz when dropped on calcite and aragonite, but not the related mineral dolomite. Magnetite will attract iron, while several other iron-based minerals will attract a magnet. Halite (rock salt) tastes salty, while kaolinite smells earthy when wet. Some minerals are radioactive, which can be useful if you have access to a Geiger counter.

 

Rocks

Rocks are more difficult. When you set out to identify them (again, with field guide in hand!), the first thing to know is that there are basically three kinds, classified by how they were formed. These are Igneous, Sedimentary, and Metamorphic: Rock Cycle Kit

  • Igneous rocks, which get their name from the Latin word for fire (ignis), are cooled and hardened magma, a molten material that comes from far below the Earth's surface. If the magma hardens while still underground, the rock is called intrusive; if aboveground, as in a volcanic eruption, it's called extrusive.

  • Sedimentary rocks are the result of the wearing away of older rocks by wind and water. The resulting debris is deposited in layers that in turn become cemented into new rock. They get their name from the Latin word sedere, meaning "to settle."

  • Metamorphic rocks are transformed from older rocks by heat and pressure -- hence the name, which comes from the Greek word for transformation, metamorphosis.


Mahogany Obsidian is igneous rockIgneous rocks tend to be harder than Mohs 5˝, and usually lack any sign of layering. If volcanic or extrusive, the rock may be very dense and fine-grained (basalt and rhyolite), glassy (obsidian), or full of gas bubbles (notably pumice and scoria, though some rhyolites or basalts also show this characteristic). The intrusive (or plutonic) igneous rocks tend to have an interlocking coarse-grained (granite, gabbro, and diorite) to medium-grained texture (diabase). Small, rounded crystals are usually scattered throughout, and sometimes flecks of mica.

Sedimentary rocks usually have a layered appearance. Hardness varies from Mohs 1 (lignite coal) up to about Mohs 7 (chert, and some breccias and conglomerates). Fossils (limestone, sandstone, and shale, and of course coal is itself a fossil!) are common, as are ripple marks and mudcracks (shale). Grain size can be anywhere from large pebbles in conglomerates and breccias down to very fine in chemical precipitates such as limestone.

Because metamorphic rocks can start out as igneous, sedimentary, or other metamorphic rocks, they show a wider range of characteristics than do the other two rock types. Still, there are a few indications that you can use. The minerals are more likely to show as large, well-formed, and well-aligned (not interlocking) crystals than they were in the parent rock. Layered structure (gneiss; slate; some serpentines) is common; sometimes the rock will easily split along these layers (slate and schist); sometimes it will not (gneiss). Layers that appear bent or crumpled are a good clue to a metamorphic origin (schist; some serpentines). And if your specimen doesn't appear to be either sedimentary or igneous, you may well start searching for it under the metamorphic heading!


Primary Minerals: Once you classify a rock as one of the three main types, determine what the primary minerals are. This will help pin down the particular rock or group of rocks: gabbro and diabase, which are intrusive igneous rocks, contain little quartz, whereas granite, which is in the same class, is primarily quartz. Examine embedded crystals, with a hand lens such as a jeweler's loupe if needed, to determine the structure and luster. A Mohs scratch test can help also, as can an examination of the color or colors in the specimen.

Work on recognizing common rocks and minerals to start with. Scoria, the rough, red or black gravel used in Southwestern landscaping, may be common in your area. Granite and marble are both used as building stones, and may be easily distinguished -- granite generally has a speckled, pepper-and-salt appearance, and is usually harder than glass, whereas marble is usually softer than glass. Talc, or soapstone, is used to make pancake griddles and is placed on the outside of many woodstoves; it is so soft you can scratch it with a broomstraw.*

With practice, the range of rocks and minerals you can identify will grow. It's a long study, but a rewarding one!

***

Much of the information in this article comes from the Audubon Field Guide to North American Rocks & Minerals.

 

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