MICA GROUP. 
501 
Spar, in which soda is present in much larger proportion 
than lime, and of which mineral andesite or andesine, is con¬ 
sidered to be a variety. 4. Labradorite, or lime-soda-feld¬ 
spar, in which the proportions of lime and soda are the re¬ 
verse to what they are in Oligoclase. 5. Anorthite or lime- 
feldspar. The two latter feldspars are rarely if ever found 
to enter into the composition of rocks containing quartz. 
In employing such terms as potash-feldspar, etc., it must, 
however, always be borne in mind that it is only intended to 
direct attention to the predominant alkali or alkaline earth 
in the mineral, not to assert the absence of the others, which 
in most cases will be found to be present in minor quantity. 
Thus potash-feldspar (orthoclase) almost always contains a 
little soda, and often traces of lime or magnesia; and in like 
manner with the others. The terms “ glassy ” and “ com¬ 
pact” feldspars only refer to structure, and not to species or 
composition ; the student should be prepared to meet with 
any of the above feldspars in either of these conditions: the 
glassy state being apparently due to quick cooling, and the 
compact to conditions unfavorable to crystallization ; the so- 
called “compact feldspar” is also very commonly found to 
be an admixture of more than one feldspar species, and fre¬ 
quently also contains quartz and other extraneous mineral 
matter only to be detected by the microscope. 
Feldspars when arranged according to their system of crys¬ 
tallization are monodinic^ having one axis obliquely inclined; 
or tridinic^ having the three axes all obliquely inclined to 
each other. If arranged with reference to their cleavage 
they are orthodastic^ the fracture taking place always at a 
right angle; or plagiodastic^ in which the cleavages are ob¬ 
lique to one another. Orthoclase is orthoclastic and mono¬ 
clinic ; all the other feldspars are plagioclastic and triclinic. 
Minerals in Meteorites ,—That variety of the Feldspar 
Group which is called Anorthite has been shown by Ram- 
melsberg to occur in a meteoric stone, and his analysis proves 
it to be almost identical in its chemical proportions to the 
same mineral in the lavas of modern volcanoes. So also 
Bronzite (Enstatite) and Olivine have been met with in 
meteorites shown by analysis to come remarkably near to 
these minerals in ordinary rocks. 
Mica Group. —With regard to the micas, the four principal 
species (Table, p. 499) all contain potash in nearly the same 
proportion, but dilfer greatly in the proportion and nature 
of their other ins^redients. Muscovite is often called com- 
mon or potash mica; Lepidolite is characterized by contain¬ 
ing lithia in addition; Biotite contains a large amount of 
