ROCK-FORMING MINERALS Lit 
considerable percentage of iron ; hence it is generally light or dark green 
in colour. It usually occurs as long thin columnar crystals and radiate 
aggregates. It is a common ingredient of many crystalline schists, 
where it is frequently associated with talc, chlorite, and epidote. 
In eruptive rocks (as in saussurite-gabbro) it is often met with as an 
alteration-product. 
Tremolite and actinolite sometimes assume forms so fibrous that they 
can be readily separated into thin, soft, cotton-like, or silky threads, and 
are then known as Amianthus or Asbestos. The fibres are often matted 
together so as to form felt-like substances, termed “‘ mountain-leather,” 
**mountain-cork,” etc. Most of the asbestos of commerce, however, is 
not amphibole, but fibrous serpentine (chrysotile). 
Of the monoclinic aluminous amphiboles, by far the most 
important is Hornblende. This mineral has much the same 
composition as actinolite, but contains a notable percentage 
of alumina. Two varieties are recognised—namely, Common 
Hornblende and Basaltic Hornblende. The former is dark 
leek-green to black, and occurs generally as long prismatic 
crystals, but sometimes as blade-like, fibrous, radiating 
aggregates. It is opaque in reflected light, but usually green 
in transmitted light. It is an essential constituent of many 
plutonic rocks (syenite, diorite, hornblendic granite), occurring 
now and again as an accessory ingredient in gabbro. It is 
a frequent constituent of crystalline schists (amphibole-schist, 
hornblende-gneiss). It commonly alters to chlorite or epidote, 
or may be still further broken up by weathering, and reduced 
to the condition of a ferruginous clay. 
Basaltic Hornblende is generally brownish-black to pitch- 
black, but when viewed in thin sections it usually shows a 
deep brown or reddish-brown colour. The crystals are 
commonly short, stout prisms, and are frequently well formed 
(see Plate III. 3). The mineral occurs as a macroscopic and 
microscopic ingredient of certain trachytes, andesites, and 
basalts—the larger crystals often showing corroded blackened 
borders—the result of magmatic resorption. 
The only other monoclinic amphibole that need be mentioned is 
Smaragdite—a peculiar grass-green fibrous lamellar form, approaching 
actinolite in composition, but containing a considerable percentage of 
alumina. It occurs in eclogite, where it forms parallel growths with 
omphacite—a similar green pyroxene mineral. 
The Pyroxenes have much the same chemical composi- 
tion, hardness, and specific gravity as the amphiboles, and 
B 
