
ROCK-FORMING MINERALS 19 
coloured. It occurs most frequently in gabbros and rocks rich in 
olivine, generally in the form of aggregates, grains, and irregular masses. 
Better formed crystals have been met with in certain andesites and 
quartz-porphyries. Bronzite and Hypersthene have a chemical com- 
position similar to that of enstatite, but contain a larger percentage of 
iron. Owing to the occurrence of abundant platy inclusions, bronzite 
yields a semi-metallic lustre on broken surfaces. Hypersthene is still 
richer in inclusions, and shows reddish copper-coloured reflections due to 
the interposition of numerous minute dark brown lamellz. Bronzite 
is usually dark brown to reddish-brown, but sometimes yellowish or 
greenish ; while hypersthene is much darker—the shades ranging from 
very dark green or dark brown to greenish-black and pitch-black. The 
two minerals have much the same habitat as enstatite, occurring frequently 
in gabbros, peridotites, and serpentines, generally without crystal outlines ; 
while well-formed crystals are met with in some andesites and trachytes. 
It is by their behaviour in polarised light that geologists are able to 
distinguish between these orthorhombic pyroxenes. They are all 
pleochroic, and show a distinct change of colour when rotated on the 
microscope stage above the polariser. The pleochroism seems to 
increase with the increase of iron—the change of colour being feebler in 
enstatite and bronzite than in hypersthene. The rhombic pyroxenes 
containing little iron tend to be altered into yellowish-green fibrous 
serpentinous products termed Lasfzze. 
The two most important members of the amphibole and 
pyroxene group are undoubtedly hornblende and augite, and 
as these minerals are often hard to distinguish, it may be 
useful to add a few notes on the characters by which they 
can be recognised :— 
First, as regards habitat, the rule is that Common 
Hornblende most frequently occurs in rocks containing a 
considerable percentage of silica, and is thus often associated 
with quartz and highly silicated felspars, as orthoclase, albite, 
oligoclase. Basaltic Hornblende, on the other hand, occurs 
as an accessory ingredient chiefly in basic and intermediate 
eruptive rocks, as in many basalts, andesites, and trachytes. 
Augite is an essential constituent of basalts, and dolerites, 
and a common ingredient of some trachytes, andesites, etc. ; 
only very rarely a pale augite has been met with in granite. 
So that we may say the home of common hornblende is 
chiefly in acid plutonic rocks and crystalline schists; while 
basaltic hornblende and augite are confined mostly to eruptive 
rocks not rich in silica. 
