~~ + 
Te 
=. 
 Parrii.gii] | ROCK-FORMING MINERALS. 73. 
cayities of a felspathie rock affords good ground for suspecting the_ 
presence in the latter of a lime-felspar.* 
Saussurite, a compact, finely granular, not definitely crystallized, 
greyish to greenish-white, faintly translucent to opaque mineral, 
having an average composition of silica 43—49, alumina 25—382 per 
cent., with variable proportions of ime and soda. H.6—7. Gr. 3°22— 
3°43. It forms with diallage some varieties of gabbro, and is abun- 
dantly associated with labradorite, or with hornblende in others. 
Under the microscope it presents a confused aggregate of crystalline 
needles and granules imbedded in an amorphous glass-like matrix. 
THe Mica Famity embraces a number of minerals now referred 
to the monoclinic system, distinguished especially by their very 
perfect basal cleavage, whereby they can be split into remarkably 
thin elastic laminz, and by a predominant splendent pearly lustre. 
They consist essentially of silicates of alumina and potash or 
magnesia, usually with some oxide of iron, but little or no lime, and 
are in some varieties distinctly hydrous. 
Muscovite (Potash-mica, Glimmer) in silvery white (also greenish 
and brownish) tables or irregular scales, capable of being split into 
thin transparent Jaminze with a pearly lustre. H. 2—3, Gr. 28— 
ol. The proportion of silica ranges between 45 and 50 per cent., 
alumina from 26 to 36, potash from 6 to 10, soda from 0 to 15, 
water from 1 to 4°7. ‘There is usually also a small percentage of 
fluorine. Abundant as an original constituent of many crystalline 
rocks (granite, &c.), and as one of the characteristic minerals of the 
erystalline schists; also in many sandstones where its small 
parallel flakes, derived like the surrounding quartz grains from 
older crystalline masses, impart a silvery or “ micaceous” lustre 
and fissility to the stone. Under the microscope thin plates of 
muscovite give bright chromatic polarization when cut parallel to 
the basal cleavage. But as the sections of the mineral displayed 
in a thin slice of any rock rarely coincide with the cleavage, but 
traverse it at various angles, they appear usually as narrow bands 
with fine parallel lines which mark the planes of cleavage.? 
The persistence of muscovite under exposure to weather is shown 
by the silvery plates of the mineral, which may be detected on a 
crumbling surface of granite or schist where most of the other 
minerals, save the quartz, have decayed; also by the frequency of 
the micaceous lamination of sandstones. 
Lepidolite (Lithia-mica), usually in scaly aggregates of a delicate 
violet colour; generally resembles muscovite, containing 49—52 per 
cent. of silica, 26°7—28°5 of alumina, about 10 of potash, 1—6 of lithia, 
and 2—8 of fluorine. Occurs in some granites and crystalline schists, 
especially in veins. 
* A valuable essay on the stages of the weathering of triclinic felspar as revealed by 
the microscope was published by G. Rose in 1867. Zeitsch. Deutsch. Geol. Ges. xix, p. 276. 
E _ the microscopic determination of the micas, see Fouqué et Michel-Lévy, op. cit. 
p- 333. 
