
8 STRUCTURAL AND FIELD GEOLOGY 
are frequent enclosures in such minerals as rock-crystal (Plate I. 2) and 
mica. As rutile is not readily attacked by the various agents of decom- 
position, it often survives the destruction of the rocks of which it once 
formed a part, and is thus of common occurrence as grains and pebbles 
in sand and gravel. It is relatively hard and heavy ; its hardness being 
6 to 6-5, and its specific gravity 4:2 to 4:3. 
It is infusible before the blowpipe, and 
Zi ; insoluble in acids. 
Zircon (ZrSiO,) as a rock-former can 
rarely be distinguished by the naked eye. 
Itappears mostly in the form of smallbrown 
crystals (tetragonal), enclosed in other 
minerals. Although only sparingly present, 
it has a wide distribution, occurring in 
eruptive rocks of all kinds (but only rarely 
Li in the basic kinds), as well as in crystalline 
schists, especially gneiss. Larger crystals 
are found in some kinds of syenite. Like 
Cy magnetite and rutile, zircon is not readily 
decomposed, and is thus often met with in 
quartz sands which have been derived 
from the disintegration of rocks in which 
the mineral occurs as a primary con- 
stituent. The mineral is harder and heavier than rutile—the hardness being 
7.5 and the specific gravity 4:5 to 4:7. It is infusible before the blow- 
pipe, and soluble with difficulty and incompletely in heated sulphuric acid. 
Fine, clear-coloured varieties (/acynth and Jargoon) are valued as gems. 
Spinelloids.—These minerals crystallise in isometric forms, and are 
all (excepting chromite) very hard. They are not attacked by acids. 
Spinel (MgAI,O,) has a hardness of 8, and a specific gravity of 35 
to 4:1. Its cleavage is imperfect. It is infusible, and not readily 
attacked by acids. It varies in colour—red, yellow, blue, green, and 
black varieties being known. It occurs in some schists and meta- 
morphosed limestones and dolomites. [The beautiful coloured transparent 
varieties are in some request as gems—the red crystals being known as 
“spinel-ruby ” or “ balas-ruby”; the golden-yellow or orange-red as 
“rubicelle”; and the violet as “almandine-spinel.”] Pleonaste is a 
greenish-black to black magnesia-iron spinel, occurring in the ejected 
limestone-blocks of Monte Somma (Vesuvius), and met with occasionally 
in marble and in the xrenolzths or foreign rock-fragments enclosed in basalts, 
andesites, etc. Pzcotife is a dark brown to black chrome spinel, often 
present in eruptive rocks which are rich in olivine (peridotites). Chromite 
(FeCr,O,) is a dark brown to black spinelloid, of considerable commercial 
importance. Its specific gravity (4:5 to 4-8) exceeds that of spinel, but 
its hardness (5:5) is considerably less. This is the only mineral from 
which salts of chromium are obtained for the production of chrome- 
yellow and chrome-green. As a rock-constituent, it is a common and 
often abundant ingredient of olivine-rocks and serpentine. 
FIG. 2.—CRYSTAL OF RUTILE. 
A knee-shaped (geniculate) twin. 

