MINERALS 257 



soft . usually dark green or greyish green in mass, and gives a grey or greenish grey 

 streak. Chlorite has micaceous cleavage, and the plates are mostly flexible and 

 inelastic. The greenish color of many igneous rocks is due to the presence of 

 chlorite formed by the alteration of pyroxenes, amphiboles, etc. 



Dolomite. CaMg(COs)2. H. 3.5-4; Sp. gr. 2.8. A mineral closely resembling 

 calcite. Dolomite occurs in some of the older limestones, in some marbles, and to 

 a small extent in veins. It is a rather soft mineral, showing a cleavage like calcite 

 when crystals of sufficient size are seen. Some dolomitic rocks are so fine grained 

 that the individual crystals cannot be detected without the aid of the microscope, 

 and in such specimens no cleavage is apparent. Some dolomite crystals exhibit 

 peculiarly curved faces, which are rare in calcite. Most dolomite is milky white, 

 brownish, or pink, but it varies greatly in color with impurities of various sorts. 

 It is most easily distinguished from calcite by its behavior with dilute acids. 

 Calcite effervesces vigorously even in quite dilute acids, while dolomite is only 

 very feebly attacked by such acids. 



Garnet. A complex silicate, different varieties having different composition. 

 H. 6.5-7.5; Sp. gr. 3-I5-4-3. 



Garnet is rarely found outside of metamorphic rocks, in which it is in some 

 instances so abundant as to be the chief constituent. It is a very hard mineral 

 about as hard as quartz; but when it has been exposed to weathering for a long 

 time it may in some cases be scratched with steel. Garnet has no cleavage, but 

 breaks with conchoidal fracture like glass or quartz. In color most garnets are 

 red or brown, but other colors, as pink and green, are known. Garnet has rather 

 high specific gravity compared with the rest of the common silicates. Garnet 

 . is used in large amounts as an abrasive, and to a small extent as a gem stone. 



Graphite (carbon). H. 1-2. Sp. gr. 2.1. 



Graphite is one of the crystalline modifications of carbon. It occurs in meta- 

 morphic rocks mostly in metamorphosed sedimentary rocks. It is very soft 

 and has a marked greasy feel, like that of talc. It is black, and will mark paper 

 with a black streak, a property of which use is made in common lead pencils. 

 Graphite also is used in the manufacture of crucibles, as a lubricant, and as an 

 adulterant. 



Gypsum. Hydrated calcium sulphate; CaSO^zHuO; H. 2; Sp. gr. 2.3. 



Gypsum occurs in beds among other sedimentary rocks, in some places as a 

 residue from the evaporation of saline lakes, and in crystals scattered through 

 shales. It occurs sparingly in veins. It is a very soft mineral and commonly 

 has a very good cleavage, resembling mica, except that the thin leaves are not 

 clastic nor strong like those of mica. It has not the greasy feel characteristic of 

 talc. A number of varieties of gypsum are recognized. Selenite, the common 

 variety of gypsum, has been described above. Satin spar is a variety that has a 

 fibrous structure, and a bright satin-like luster. Alabaster is white massive gypsum; 

 that is, gypsum which is made up of minute crystals which cannot be readily dis- 

 tinguished as individual crystals. Gypsum is characteristically white, but may be 

 reddish, brownish or gray when it is impure. Gypsum is largely used in the 

 manufacture of various sorts of plaster. Alabaster of high grade is sometimes 

 used for ornamental vases, etc. 



Kaolin. A hydrous aluminum silicate. H. 2-2^; Sp. gr. 2.6. Kaolin 

 occurs in igneous rocks as an alteration produce of feldspars, and in sedimentary 

 rocks. Shales and clays are made up largely of kaolin, and it is present in varying 



