ROCK-FORMING MINERALS 27 
microscope, this white powder may be seen to consist largely 
or entirely of minute transparent or translucent plates, with 
pseudo-hexagonal symmetry. Before the blowpipe it is 
infusible, and is insoluble in acids; hardness=1; specific 
Sravity=2-5. It is a common alteration-product of many 
rock-forming aluminous silicates, notably orthoclase, albite, 
and lime-soda felspars. When moistened with water, it is 
highly plastic. Impurities are usually present, particularly 
iron-oxides, which give it a yellow, red, or brown colour; 
other colours met with are grey, blue, and green. Lzthomarge 
is merely an impure compact kaolin; it is often mottled red 
owing to the presence of ferric hydrate. 
Mi 2 ALOrDs 
Fluor-spar or Fluorite (calcium fluoride) is hardly entitled to be 
called a rock-former. It occurs rarely as rounded grains in granitic, 
syenitic, and gneissic rocks, where it is apparently of secondary origin. 
It is met with, however, frequently as a gangue-mineral in lodes, 
particularly in association with lead- and tin-ores. The common form of 
the crystallised mineral is a cube, and interpenetrating twins often occur. 
The colour is variable—violet, blue, green, yellow, and occasionally pink. 
Thick veins of granular fluor-spar appear now and again, traversing 
crystalline schistose rocks, especially in the neighbourhood of granite 
masses. The mineral has a hardness of 4 and a specific gravity of 3-2. 
It is decomposed by sulphuric acid, but hardly attacked by other acids, 
and fuses with some difficulty before the blowpipe. 
Rock-salt (sodium chloride) crystallises in the form of cubes, but occurs 
massive as a rock in beds, associated with anhydrite and gypsum. It 
is met with also as a product of sublimation in volcanic regions, along with 
calcium- and magnesium-chlorides and calcium-sulphate. 
View CR ELUDES 
Pyrite (disulphide of iron) commonly crystallises in cubes 
and octahedra, but not infrequently occurs as irregular 
ageregates. It has a very uniform, brass-yellow colour. 
Hardness = 6 to 6-5 ; specific gravity = 4-9 to 5-2; streak = black. 
Before the blowpipe pyrite gives off sulphur, burning with a 
bine flame, It is: decomposed Dy -nitric acid: The only 
minerals with which pyrite might possibly be confounded are 
chalcopyrite (an ore of copper), magnetic pyrite, and perhaps 
gold. Gold, however, is malleable, and the others are not. 
Pyrite is paler and considerably harder (6 to 6-5) than 
