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Pari. § it] ROCK-FORMING MINERALS.  —_—S_ 85 
nature, occurs only in comparatively small quantity. Its most abun- 
dant compound is with Calcium as the common mineral Fluorite. 
Fluorite (Fluor-spar). Isometric, usually in cubes; also massive ; 
colour ranging most commonly through many shades of yellow, blue, 
and green. H.4. Gr. 3:1—3:2. Composition—fluoride of calcium 
(= fluorine 48°72, calcium 51:28). Occurs generally in veins, 
especially in association with metallic ores. 
CHLORIDES. There is only one chloride of importance as a con- 
stituent of rocks—sodium chloride or common salt. As it occurs 
chiefly in beds as a rock-formation, it is described among the 
' rocks at p. 111. 
SULPHIDES. Sulphur is found united with metals in the form of 
sulphides, many of which form common minerals. The sulphides of 
lead, silver, copper, zinc, antimony, &c., are of great commercial 
importance. ‘I'he sulphide of iron, however, is the only one which 
merits consideration here as a rock-forming substance. It occurs 
in two forms, Pyrite and Marcasite. 
Pyrite (Hisenkies, Schwefelkies). Isometric, abundant in cubes ; 
also globular, with internal radiating fibrous structure, and amorphous. 
Colour, pale brass yellow, with splendent metallic lustre. H. 6—6°5. 
Gr. 49--5:2._ Composition—iron disulphide, Fe 8, (=sulphur 53:33, 
iron 46°67), but usually with traces of other metals. Occurs dis- 
seminated through almost all kinds of rocks, often in great abundance, 
as among dolerites and diabases ; also frequent in veins or in beds. 
Tron disulphide is formed at the present day by some thermal springs, 
and has been developed in many rocks as a result of the action of 
infiltrating water in presence of decomposing organic matter and 
iron salts. In microscopic sections of rocks, pyrite appears in small 
cubical, perfectly opaque crystals, which with reflected light show 
the characteristic brassy lustre of the mineral, and cannot thus be 
mistaken for the isometric magnetite, of which the square sections 
exhibit a characteristic blue-black colour. Pyrite when free from 
marcasite yields but slowly to weathering. Hence its cubical crystals 
may be seen projecting still fresh from slates which have been 
exposed to the atmosphere for several generations. 
Marcasite (Hepatic pyrites). Orthorhombic, but frequently also 
in fibrous, rounded or encrusting masses, or in amorphous aggregates. 
Colour palerthan pyrite. H.6—6°5. Gr. 465—4°88. Composition 
same as pyrite. Occurs abundantly among sedimentary formations, 
sometimes diffused in minute particles, sometimes segregated in layers, 
or replacing the substance of fossil plants or animals; also in veins 
through crystalline rocks. This form of the sulphide is especially 
characteristic of stratified fossiliferous rocks, and more particularly of 
_ those of Secondary and Tertiary date. It is extremely liable to de- 
composition. Hence exposure for even a short time to the air causes 
it to become brown, free sulphuric acid is produced, which attacks the 
surrounding minerals, sometimes at once forming sulphates, at other 
times decomposing aluminous silicates and dissolving them in con- 
