MINERAL COMPOSITION OF GRANITE. 17 
be colorless, bluish, opalescent or amethystine, or smoky. The quartz 
in the rock determines in some measure its shade. The vitreousness 
of the quartz greatly affects the granite. 
The next most abundant constituent of granite is mica, which is 
present in two forms — the white (rnuscovite, or potash mica), essen- 
tially a silicate of alumina with potash, soda, and ferrous oxide, and 
the black (biotite, or -magnesia mica), essentially a silicate of alumina 
with' potash, magnesia, and both ferric and ferrous oxide. Granite 
may contain one or both of these. The dimensions and number of the 
scales of black mica largely determine the shade of many granites. 
Hornblende, a constituent of many granites, although greenish 
(rarely bluish), may appear as dark as the black mica, but, unlike 
that mineral, it does not split into scales. Augite and hornblende 
resemble each other so closely when in minute particles that they can 
be distinguished only by means of the microscope. Both may occur. 
In addition to these more important minerals, others are usually 
present in minute or microscopic quantities. Some of these, kaolin, 
sericite (a potash mica or rnuscovite), chlorite, epidote, zoisite, and 
calcite, as well as paragonite (soda mica), which may possibly be 
present, are the result of chemical changes in the feldspars or the bio- 
tite or hornblende and are therefore called " secondar}v' Others — like 
zircon, apatite, titanite, rutile, tourmaline, fluorite, garnet, magnetite, 
molybdenite, ilmenite, pyrite, allanite — are regarded as original "acces- 
sory constituents." Calcite also occurs in microscopic quantity as an 
original mineral of some granites. Of these minor accessories, 
pyrite (iron disulphide) and calcite (lime carbonate) alone have 
economic significance, for these may discolor or weaken the stone 
when dissolved or oxidized on an exposed surface. Ten Maine gran- 
ites were tested for calcite by Mr. Eugene C. Sullivan at the labora- 
tory of the United States Geological Survey in May, 1906. Under 
treatment with warm dilute acetic acid they yielded percentages of 
CaO (lime), from a trace up to 0.24. They also showed from 0.011 to 
0.218 per cent of C0 2 (carbonic acid) . Seven of them showed some MgO 
(magnesia), two of them showed C0 2 in excess of amount required 
to form CaC0 3 (lime carbonate). The calculated result shows the 
presence of from 0.14 to 0.43 per cent of CaC0 3 (lime carbonate), 
and two of them show from 0.06 to 0.08 per cent of MgC0 3 (mag- 
nesia carbonate) . Mr. Sullivan writes that " the lime extracted by 
acetic acid is in nearly every case in excess of that required to form 
calcium carbonate with the carbon dioxide found. This would indi- 
cate that lime and magnesia are present in some easily soluble form, 
a See Zirkel, Petrographie, vol. 2, p. 13, and Weinschenk, Abbandl. Math.-phys. Classe 
k. Bayer. Akad., vol. 18, p. 730, PI. V. 
3495— Bull. 313—07 2 
