ROCK-FORMING MINERALS 23 
greenish colour to the alteration of their original ferro- 
magnesian constituents into chlorite. [Here also may be 
included G/lauconzte—a hydrous silicate of aluminium, potas- 
sium, and iron—which occurs in the form of small rounded 
granules of a greenish colour in certain sandstones of 
Cretaceous and Tertiary age; it is also met with in amyg- 
daloidal cavities in igneous rocks. | 
THE TALC AND SERPENTINE GROUP 
Talc (hydrous silicate of magnesium) is a white or pale 
greenish mineral, readily cleavable into non-elastic folia, and 
so soft that it can be scratched with the finger-nail. Hard- 
ness= 1, specific gravity=2-7 to 2-8. It has a pearly lustre and 
a pronounced greasy feel; is fusible on thin edges to a white 
enamel, but is not decomposed by acids. It never assumes 
a crystalline form. In igneous rocks it occurs rarely, and 
always as a secondary product, usually in the form of foliated 
plates and scales, replacing non-aluminous magnesian silicates. 
It is met with chiefly in the crystalline schists, being the chief 
ingredient of talc-schist. 
Steatite (soap-stone) is a cryptocrystalline to compact variety of talc. 
Potstone is another but very impure variety. Sefzolite or Meerschaum is 
a closely allied mineral of essentially the same chemical composition. It 
is amorphous, occurring in irregular shaped nodules and masses, which 
are compact and finely porous. When dry it floats in water, which it 
absorbs greedily. Like talc, it is eminently a product of the alteration of 
magnesian silicates. As a rock-former it is of no importance. 
Serpentine (hydrous silicate of magnesium, often contain- 
ing iron; hardness = 3 to 4; specific gravity = 2-5 to 2-7), 
like talc, never assumes a crystalline form, but occurs in 
compact or granular masses and in aggregates with a lamellar, 
scaly, or fibrous structure. The colour is some dark shade of 
green, red, or yellow, often mottled or variegated. The finely 
fibrous variety is known as Chrysotdle (sce Plate VI.). [Most 
of the “asbestos” of commerce is not true asbestos, but 
chrysotile.] Serpentine is fusible with difficulty on thin 
edges, and is decomposed by hydrochloric acid. It is always 
a secondary mineral—a product of the alteration of ferro- 
magnesian minerals, as olivine, pyroxenes, amphiboles, etc. 
