ROCK-FORMING MINERALS 29 
tary rocks. As a secondary product, it appears commonly in 
the minute pores and capillaries of many different minerals 
and rocks; it also occupies cracks, fissures (see Plate XV.), 
and cavities of all kinds—being a common gangue-mineral in 
lodes. It is the chief petrifying agent, and, next to quartz, 
the commonest of all minerals. 
Aragonite has the same composition as calcite, but crystallises in 
the orthorhombic system. Its hardness (3-5 to 4) and specific gravity 
(2-9 to 3) are both somewhat greater than those of calcite. It is a more 
soluble form of calcium-carbonate than calcite, and not nearly so 
common as that mineral. Sometimes it is met with in beds associated 
with gypsum and iron-ore, and not infrequently in cracks and cavities 
in recent eruptive rocks. It is often a deposition from hot-springs. 
Dolomite or Bitter Spar (calcium and magnesium carbonate) crystal- 
lises in the hexagonal system—the faces of the crystals being frequently 
curved. Hardness = 3-5 to 4-5; specific gravity = 2-85 to 2-95. It is 
only slightly affected by cold dilute hydrochloric acid, but is dissolved 
when the acid is heated. It may be variously coloured, but white and 
yellow varieties are most common. Magnesian limestone is composed 
in large part of this mineral. 
Siderite or Chalybite (carbonate of iron) occurs usually in rhombo- 
hedral forms, often with curved faces. It is colourless or pale yellow 
when freshly exposed, but soon becomes tarnished brown orrusty. Hard- 
ness = 3:5 to 4:5; specific gravity = 3-7 to 3-9; the mineral is infusible 
before the blowpipe, but effervesces with weak acids. It occurs in 
lodes along with various ores. Sfherosiderite is the name given to a 
compact siderite often showing a concentric, radiating, fibrous structure. 
It occurs as nodules and nodular masses in veins and cavities in 
crystalline schists, etc. C/ay-zronstone is an impure variety of sphzro- 
siderite mixed with clay, which occurs as nodules, bands, and beds in 
various geological formations. A&lackband-tronstone is a clay-ironstone 
containing a notable amount of carbonaceous matter. [Clay-ironstone 
and blackband-ironstone are rather rocks than minerals. | 
Vin SULEEATES 
Anhydrite (calcium sulphate) crystallises in the orthorhombic system, 
but usually occurs massive or in granular and fibrous aggregates. It 
is often associated with rock-salt and gypsum. Hardness = 3 to 3:5; 
specific gravity =2-9 to 3. It is slightly soluble in hydrochloric acid, and 
fuses before the blowpipe with difficulty to a white enamel, colouring the 
flame reddish-yellow. 
Gypsum (hydrated calcium sulphate) crystallises in 
monoclinic forms, its hardness (1-5 to 2) and specific gravity 
