
ROCK-FORMING MINERALS dd) 
simple (Fig. 3), so that a section cut in the right direction shows in 
polarised light only two differently tinted bands (see Plate II. 3, 4). 
When the mineral is not twinned, or when the section is cut parallel to 
the twinning plane, orthoclase felspars polarise in one uniform colour. 
The following are the more important felspars :— 
Orthoclase (monoclinic potash felspar, with high per- 
centage of silica). This mineral is usually white, grey, or 
reddish. It is not attacked by ordinary acids, but is decom- 
posed by hydrofluoric acid, and fuses before the blowpipe 
with difficulty on thin edges. As a rock-former it occurs 
most frequently as imperfect crystals, or irregular crystalline 
aggregates. In certain igneous rocks, however (as in quartz- 
porphyry), it appears as conspicuous and sometimes well- 
formed crystals disseminated among the finer grained con- 
stituents of the mass. Fine crystals of orthoclase often occur 
in drusy cavities and veins in granite, and now and again in 
fissures traversing crystalline schistose rocks. The mineral is 
an essential ingredient of many eruptive rocks (granite, 
quartz-porphyry, syenite, rhyolite, phonolite, trachyte, etc.). 
It is readily distinguished from quartz by its hardness, 
cleavage, twinning, and frequent turbidity—due to gradual 
alteration of the mineral into kaolin. 
Santdine is a glassy clear variety of orthoclase, usually 
much cracked, and often crowded with inclusions. It is the 
type of orthoclase which characterises the younger volcanic 
rocks—rhyolite, trachyte, phonolite, etc, and frequently 
assumes the form of tabular crystals (see Plate II. 3, 4). 
Adularia is another clear, transparent orthoclase, found in the irregular 
drusy cavities of some gneisses, and in fissures in schistose rocks. 
Microcline (triclinic potash felspar) has the same chemical composi- 
tion as orthoclase, from which it can hardly be distinguished without 
examination under the microscope. It is a frequent constituent of 
granite, appearing often in well-developed forms in the drusy cavities, and 
the coarsely crystalline veins associated with that rock. It occurs also in 
certain syenites and other eruptive rocks of deep-seated origin, and is 
occasionally present in gneiss. Although it thus frequently accompanies 
orthoclase proper, it has not yet been met with in rocks that contain the 
glassy variety of that mineral—sanidine. Under the microscope, micro- 
cline usually shows a polysynthetic structure, due to the presence of 
minute spindle-shaped twin lamellz, so arranged that, when the section 
is cut in a particular direction, it has in polarised light a peculiar cross- 
hatched appearance (see Plate III. 1, 2). 
