24 STRUCTURAL AND FIELD GEOLOGY 
It is the chief constituent of the rock serpentine. JVod/e 
Serpentine is a pure variety of a uniform colour (green or 
yellow), which takes on a fine polish, and is used as an 
ornamental stone. 
THE EPIDOTE GROUP 
The principal rock-forming member of this group is Pistazite, or 
iron-epidote—so called to distinguish it from Zozszze, or lime-epidote. 
Pistazite is a silicate of calcium, aluminium, and iron, which occurs 
crystallised in monoclinic forms, or appears in finely granular masses 
of a peculiar pistachio-green colour. The hardness is from 6 to 7, and 
the specific gravity 3-2 to 3-5. The mineral fuses with difficulty before 
the blowpipe, and is partially decomposed by hydrochloric acid. It is 
met with frequently as a constituent of schistose rocks (epidote-gneiss, 
epidote-amphibolite), and as a “contact mineral” in limestones, etc., 
which have been affected by the intrusion of igneous rock. It is a 
common alteration-product in eruptive rocks, replacing such minerals as 
hornblende, biotite, felspars, etc., and often associated with chlorite. 
Zoisite (orthorhombic) is a silicate of calcium and aluminium, met with 
not infrequently in schistose rocks and as an alteration-product of 
felspar in gabbro. [C/¢nozozszte is a monoclinic epidote containing little 
iron, and approaching to zoisite in composition. | 
THE GARNET GROUP 
Garnets are silicates of aluminium, iron, calcium, magnesium, 
chromium, and manganese, usually only two or three of these being 
abundantly present. According to the dominance of the chief con- 
stituents, we have iron-, calcium-, magnesium-, manganese-aluminium 
garnets, etc. They usually assume dodecahedral (see Plate I. 3) or 
icositetrahedral forms, and have an imperfect cleavage. The hardness is 
-6:5 to 7-5, and the specific gravity 3-4 to 4-3. The lustre is greasy or 
resinous. Common rock-forming iron-aluminium garnet is generally 
some shade of red—hyacinth to reddish-brown. It is fusible before the 
blowpipe, but is not readily decomposed by acids. In nature it alters 
chiefly to chlorite, and sometimes to serpentine, epidote, etc. Under the 
microscope its sections are rounded or many-sided ; it shows no cleavage, 
but irregular cracks which are not infrequently lined with decomposition- 
products. Enclosures often abound. Usually garnet remains dark when 
rotated between crossed nicols. It is common in many schists, is an 
essential constituent of eclogite and garnet-rocks, and occasionally occurs 
in granite and quartz-porphyry. Calcium-aluminium garnet is often 
present as a “contact mineral” in metamorphosed limestone. The clear, 
finely coloured varieties of garnet have some value as gems; amongst 
these are Almandine (iron-aluminium garnet) and Pyrvofe (magnesium- 
aluminium garnet), the former occurring in schists and granite, the latter 
in peridotites and serpentine. J/elantte,a black calcium-iron garnet, is 
met with in some trachytes, phonolites, and other volcanic rocks. 
