Characteristics of Rocks and Minerals
A Featured Snippet Guide: Identifying rocks and minerals involves observing physical characteristics like hardness, color, luster, streak, specific gravity, and cleavage. These structural properties allow geologists to classify specimens accurately.
Rocks are identified by several different qualities or physical properties. Among these are hardness, color, luster, streak, specific gravity or weight, and cleavage. Another characteristic of some rocks is fluorescence.
Hardness
Hardness of rocks is determined by a more or less arbitrary scale developed so that anyone with materials found in any home or laboratory can classify a specimen. This is called the Moh Scale, and it is graduated in steps from 1 to 10. Each step does not necessarily mean that the one above or below it is twice as hard or half as hard. The difference between Step 9 and Step 10, for example, is very great as compared with the difference between Step 1 and Step 2.
In simple layman's language, the Moh Scale can be described thus:
Step 1: TALC. The softest mineral, capable of being scratched by every other mineral, and also easily scratched by the fingernail. Feels greasy.
Step 2: GYPSUM. Scratched by the fingernail, but not easily.
Step 3: CALCITE. Capable of being scratched by a sharp penny. Very easily cut with a steel knife blade.
Step 4: FLUORITE. Easily scratched with a knife blade.
Step 5: APATITE. Can be scratched with difficulty by a steel point, and it will itself, also with difficulty, scratch glass.
Step 6: FELDSPAR. A steel blade barely leaves a mark, but a file will scratch it easily. It will scratch glass readily.
Step 7: QUARTZ . This mineral will very easily scratch glass, copper, steel, or almost any common substance.
Step 8: TOPAZ . Topaz will scratch quartz easily, and is harder than almost any common substance.
Step 9: CORUNDUM. Easily scratches both quartz and topaz . Several man-made products are as hard as corundum, but differ chemically. Carborundum is one of these.
Step 10: DIAMOND . This is the hardest substance found in nature or in man-made materials. It easily scratches corundum. Because of its hardness, diamond is very important in industry as a cutting and grinding material.
In determining hardness in the field, quartz , feldspar, and calcite are very common almost everywhere, and every field trip should include a knife and some kind of glass in its equipment. Even an old bottle will serve admirably.
Color
Color determinations in the field are relatively easy to make-you simply look at the specimen-except in instances where the specimen may be covered with a layer of a different mineral, such as clay, indurated volcanic ash, or some other concealing addition. Oxidized layers are the most common concealing layer, and when they are present, one can reveal the color of the specimen by chipping the surface to expose the inner mineral. All t
he nodules of Brazilian agate imported to this country have been chipped, not to identify the material as agate, but to see if the inside is more colorful than normal. If it is more colorful, the price is raised as premium material. Unfortunately, this chipping more often than not causes the nodule to crack in one or more places, thus ruining much of the agate for cutting purposes.
The color of a specimen does not necessarily indicate that it is a solid piece of one element or another. Color is most often caused by minute-to-large quantities of an accidental impurity. The importance of color in identification is the fact that many minerals are found only with some impurity or other which colors that particular mineral. Other minerals are never found with any color other than that which is characteristic of that substance. Malachite and azurite , for example, are always a deep green or blue, opaque, and are easily recognized as copper minerals.
In transparent minerals, the color may be rich and deep or so pale as to be almost undetectable. Amethyst is a good example of this variable staining during the formation of the cr
Many minerals have their color found in zones. Others are dichroic . There is a difference in the two terms. Zoned color is found most dramatically in tourmaline of the "watermelon" variety. Here the outer layers of the crystals are a deep, bright green, while the inner core is pink to rose or deep red, resembling a slice of watermelon; hence the name.
This zoning of color is apparent no matter which way you look at the specimen, while dichroism is another matter entirely. Here again, tourmaline is a good example of this phenomenon. Viewed through the sides of the crystal, a specimen may show a bright-green color, but viewed through the ends of the same crystal, the color may be a dirty brownish-green to almost black.
Luster
Luster is another way of identifying or partly identifying a mineral. This is the appearance of the freshly exposed surface. Rocks and minerals are divided into two rough categories as to luster-metallic and nonmetallic. Metallic luster appears like metal, sometimes with a color overlaying the surface, usually yellow or yellowish, since the luster is usually due to the inclusion of sulphides in the specimen.The nonmetallic luster is further broken down into several categories: silky, in the case of asbestos and other fibrous materials; vitreous, if the surface looks porcelaneous, such as smithsonite; resinous, as in the case of amber, and in some of the wood opals; adamantine, if it has a hard, somewhat greasy look, as is seen in diamond, mercury, lead, and other elements; pearly, where cleavage planes show as flakes or cracks beneath the surface; glassy, as in garnets . All these properties are indications of certain kinds of minerals, and all serve to help identify the specimen to a certain degree. At the very least, they place the specimen within a certain group where positive identification becomes more sure.
Streak
Related to color, and a test that is easily performed in the field, is the streak test. This is nothing more than scratching the mineral on a dull, unglazed surface, such as the back of a bathroom tile, or on the surface of a special streak tile sold for the purpose, and observing the color of the powder left by the streak. Often the color of the streak is entirely different from the color of the specimen, and this makes the streak test a valuable one for identification.The hardness of tile is about 7, so minerals harder than this will not leave a streak, but will scratch the tile. This in itself is an indication of identity.
Specific Gravity
Specific gravity determinations are not easily made in the field, since one must use a specific gravity scale and make accurate measurements. In essence a specific gravity scale is one which has a double pan on the specimen end, one pan suspended below the other. In use, a beaker of water is placed under the pans, and the lower one immersed int he water until the specimen is entirely submerged. The scale is balanced with the lower pan in the water and nothing on the other pans. Now the specimen is weighed on the pan still in the air and the exact weight noted on a pad. The specimen is then removed ans placed on the pan under water and weighed a second time.The weight of the specimen under water is now subtracted from the weight of the specimen in the air. This gives you the weight of the amount of water displaced by the weight of the displaced water. The result is the specific gravity of the specimen.
Cleavage and Fracture
Cleavage and fracture are often confused by the beginner. They are really manifestations of different properties. Cleavage is the parting plane of a mineral, and it is related to the atomic structure of the crystal. This permits a crystal to part along the atomic planes in a smooth, flat surface. The direction of a cleavage plane is always identical in every crystal of the same mineral, in relation to the axis of that crystal.Fracture, on the other hand, is the way a mineral or crystal will break if subjected to pressure or sudden impact. Fracture is usually a tentative way of placing a specimen into a certain group and is not a definite identification test. In the case of testing a piece of obsidian, however, a blow at the edge of a specimen will always result in a conchoidal fracture, leaving shell-shaped surfaces which are typical of this mineral. Most rocks and minerals merely break with a rough surface, without definite identifying characteristics.
Location
Location is also a help in mineral identification. Certain minerals are found or not found in certain locations. For example, it is useless to look fro diamonds in the Unite States, except within a limited area in the Southeast.By: P. Villiard