ROCKS AND SOILS 



69 



whereas the physical product is a mineral called 

 calcite. Meteorites are also rocks. 



Two additional terms, gem and stone, are fre- 

 quently applied to rocks and minerals. Gems are 

 precious and semiprecious minerals or rocks; their 

 chemical composition is not considered. Stones can 

 be almost any kind of rock or mineral; there is no pre- 

 cise definition of the term. 



ROCK-FORMING MINERALS 



All minerals are found either within or closely 

 associated with rocks. However, only eight minerals 

 are frequently encountered. These are the common 

 rock-forming minerals. In the following account, 

 consideration is limited to the eight common types 

 and their diagnostic features. The purpose is not to 

 provide any real discussion of minerals, but rather to 

 identify them as an aid to rock identification. 



QUARTZ 



Quartz is the hardest of the common minerals. Its 

 appearance is glassy but it can be clear, milky white, 

 or any light to dark color. Structurally, it can be 

 massive (like a chunk of glass), granular (like sand 

 grains), or crystalline (in six-sided, geometrically or- 

 ganized crystals). When quartz is struck, curved 

 chips are broken off, leaving a clamshell-like de- 

 pression. Light rays sparkle and scatter in all direc- 

 tions from its surface. The optical properties do not 

 include a deep or mirrorlike reflection of light. 



FELDSPARS 



Feldspars, a group of similar minerals, can be 

 scratched by quartz but not by a knife. They are 

 glassy like quartz, but have a satiny sheen not found 

 in that mineral. Common feldspars are usually white, 

 cream, gray, or tan, but light to dark feldspars of 

 other colors are also found. They can be massive or 

 crystalline. When struck, a feldspar may fracture into 

 curved chips like quartz, but it also cleaves into 

 slightly slanted blocks whose surfaces have a satiny 

 sheen. The latter kind of breaking is related to the 

 structure of the mineral and is called cleavage. Min- 

 erals that cleave have cleavage surfaces that, when 

 properly oriented to light, give a deep, mirrorlike 

 shine. The satiny sheen of feldspars is a very 

 conspicuous mirrorlike shine. 



OLIVINE 



Unweathered olivine is too hard to be scratched by 

 quartz; however, in nature it is usually weathered to 

 the point where even feldspars might scratch it. Oli- 

 vine is a glassy, green mineral. It is usually found as 

 embedded grains in igneous rocks. Commonly, it has 

 been altered to serpentine. When it occurs as large, 

 pure masses, it is also dunnite, a rock. It rarely oc- 

 curs as crystals. Olivine fractures like quartz and 

 feldspars, but unlike either, it usually appears in the 

 form of glassy, green grains. Cleavage is poor. 



PYROXENES 



Minerals in the pyroxene group may or may not 

 be scratched by a knife. The commonest mineral in 

 this group is dark greenish to black, glassy augite, 

 which commonly occurs as grains or as a few 

 scattered, small, short and blocky crystals in igneous 

 rocks. Like all pyroxenes, augite is brittle, fractures 

 unevenly, and cleaves in two directions almost at 

 right angles to one another. Augite's crystals and 

 cleavage distinguish it from hornblende. 



HORNBLENDE 



Hornblende, the commonest mineral of the amphi- 

 bole group, may or may not be scratched by a knife. 

 It is usually green or black and generally occurs as 

 grains in igneous rocks, as parallel layers of long, 

 thin crystals in schist, or as pure solid masses of 

 hornblendite. It breaks, fracturing somewhat like 

 the other common minerals, but cleaving in two 

 directions at angles of either 56 or 124 degrees. 

 Therefore, cleavage angles alone distinguish horn- 

 blende from augite. 



DOLOMITE 



Dolomite, common in sedimentary rocks and rare 

 in metamorphic rocks, can easily be scratched with a 

 knife but cannot be scratched by a penny. It is color- 

 less, white, pinkish, or has light tints of color. Like 

 calcite, it fractures into chips and cleaves into the 

 form of a rhombus (resembling a cube, but with the 

 corner angles not right angles). However, dolomite 

 is harder than calcite and will fizz in acid only after 

 being scratched or powdered. 



