u 
* 
however, frequently found, with equal angles of 120°, as, 
for instance, at Vesuvius, in many granites, etc. The 
laminae present in reflected light a strong pearly-metallic 
lustre, by transmitted light the lustre is vitreous; they are 
very flexible and very elastic, a characteristic of no other 
mineral. They do not show any traces of further lamina¬ 
tion. It is otherwise where the mineral breaks into foliated 
masses (Fig. 12), where a separation into oblique rhombic 
tables cannot be mistaken. The hardness is equal to 
2*0—2-5, so that it can be scratched by the nail; by this 
circumstance mica may be essentially distinguished from 
talc and chlorite, and, besides, it has no greasy feeling to the 
touch. Before the blowpipe it melts only at the edges to 
a more or less coloured glass, and colours the flame a pale 
violet. 
The lithium micas, to which belong the silver-white 
varieties of Schlackenwald and Zinnwald, as well as the 
red and green lepidolites of Moravia, resemble the common 
mica in the form of their crystals ; they are distinguished, 
however, by their easy fusibility, and while being melted, 
they swell up and colour the flame purple-red. They 
contain always a little fluorine, and on this account always 
experience a sensible loss at a white heat. 
The constituents are, in general, silicate of alumina 
and oxide of iron, with silicate of potash or lithia; 
K Si + 4 Al (Fe) Si with which there occasionally occurs a 
little water, and in the case of lepidolite, fluorine and lithia. 
Not decomposed by acids. In the atmosphere it gradually 
exfoliates, and by exposure to water it becomes at last 
dim and friable. 
Mica forms an important constituent of granite, gneiss, 
mica-schists and many porphyries ; it is also found amongst 
the products of volcanoes, and in many of the older lavas. 
In many granites it occurs in tables from an inch to a foot 
in size ; as, for instance, at Aschaffenburg in South Norway, 
at the Urals, etc.; it is, however, generally disseminated in 
metallic-lustrous laminae, sometimes radiated, or in nodules, 
as plumose mica. The colour is either silver-white, pinch¬ 
beck brown, or black, more rarely bright yellow or rose-red, 
and is usually added to the names. The specific gravity 
varies from 2*7—3 - 0, according as it contains oxide of iron 
or of manganese, which can easily be determined before the 
blowpipe with borax and soda. The rose-red lepidolite is 
found especially at Rozna in Moravia, where it occurs in 
compact crystalline and foliated masses; the green variety 
is also found at the same place. 
The uniaxial mica (volcanic or magnesia mica), has a 
rhombic hexahedron for its primary form, and is therefore 
also known as rhombohedral or hexagonal mica. It occurs 
usually in hexagonal tables, is a little more easy to split, 
somewhat more friable, and of a more vitreous lustre than 
the former, and presents, in polarised light, a single ring 
of colours, while the other presents two. The best marked 
crystals are found at Vesuvius, sometimes of a green, and 
sometimes of a black colour; it occurs in foliated masses 
in Greenland, Labrador, Siberia; also at Bodenmais in 
Bavaria, and at several places in North America. The 
chemical constituents differ in so far as the magnesia plays 
the principal part, after the oxide of iron, among the basic 
elements. 
In the green mica of the Zillerthal, which has been 
called fuchsite, a certain amount of oxide of chrome appears 
in addition to the silicates of iron and magnesia. 
The application of mica is limited to optical experi¬ 
ments and window panes. On account of the ease with 
which it may be split, and its transparency, it is also used 
for putting up physiological and anatomical preparations 
from the vegetable and animal kingdom for microscopical 
observations. 
Fig. 14.— Chlorite, Ripidolite. 
Crystallises in right rhombic prisms of 120°, or in 
hexagonal prisms and pyramids. 
Presents a distinctly foliated structure in the direction 
of the terminal planes, slight hardness (1*0—1*5), and a 
specific gravity of 2*65—-2*8; has a slightly greasy feeling, 
is flexible but not elastic, and in appearance sometimes 
approaches mica, sometimes talc. Most frequently it is 
found in scaly foliated masses of a leek-green to a dark-green 
colour, as in the chlorite schist of frequent occurrence in the 
Alps, especially in the Zillerthal and in Piedmont. The 
variety described under the name of ripidolite by Kobell, 
which is principally found in Dauphinee and Piedmont, 
crystallises in low hexagonal double pyramids (Fig. 14), 
which sometimes appear transparent and dichroate, green 
and yellow, or hyacinth red; sometimes also it occurs 
in curved aggregations of leaves like talc, as in Fig. 15. 
Fig. 15.— Talc, Prismatic Talcose-mica. 
Crystallises likewise in rhombohedrons, sometimes in 
small hexagonal tables, which are of foliated structure and 
biaxial. It feels soft and highly greasy to the touch; 
like no other mineral, it may be torn out in thin plates, 
which are very flexible, and may be easily scratched by 
the nail. 
It occurs sometimes in worm-like curved foliated 
aggregations, which form hexagonal prisms, as in Fig. 
15. Externally, it can only with difficulty be distinguished 
from chlorite, but much more easily with the help of the 
blowpipe; for here, as it contains no alumina, but con¬ 
sists only of silicate of magnesia and water, it gives, 
when melted in fine laminae, wetted with a solution of 
cobalt, and heated, not a blue, but a rose-red colour. It 
occurs sometimes in very dense masses, almost compact, 
in fibrous-foliated, and in scaly earthy forms ; it also con¬ 
stitutes the rock known, especially in the Alps, as talcose 
slate, and pot-stone. Formula, Mg 6 Si 6 -j- 2 H. 
The pyrophyllite , or radiated talc , on the other hand, 
which from its appearance, flexibility, slight degree of 
hardness, pearly lustre, etc., has the greatest resemblance 
to talc, contains a considerable quantity of alumina, with 
very little magnesia. It is found at Beresowsk in Sibe¬ 
ria, and Salme-Chateau in Belgium. 
The scaly talc is used like graphite for the lubrication 
of machinery in order to lessen friction ; also for polishing 
alabaster, and varnishing leather; and if it is perfectly 
white, as a white paint for the face. 
Figs. 16, 17.— Granite. 
Before leaving the felspathic and micaceous minerals, 
