
ROCK-FORMING MINERALS 7 
having the composition of FeTiO;, and crystallising in rhombohedral forms. 
It has a hardness of 5 to 6, a specific gravity of 4:56 to 5-21, and the 
streak is black to reddish-brown. It is practically infusible before the 
blowpipe, and is not attacked by acids. 
This mineral occurs as massive aggregates in certain plutonic rocks, 
especially in gabbros. It is met with as an accessory ingredient in many 
eruptive rocks (granite, syenite, gabbro, basalt, dolerite, andesite), and 
also as a constituent of some crystalline schists. 
As a rock-constituent ilmenite appears under the microscope either in 
the form of rhombohedral crystals or as irregular grains and patches, 
which are often hard or impossible to distinguish from similar aggregates 
of magnetite, being like these, black, opaque, and showing a metallic 
lustre. The mineral is often altered round its margins or even throughout 
into a dull greyish-white opaque substance known as Leucoxene (see 
under 77Zanz‘?e). 
Magnetite (Fe,0,) crystallises usually in the form of octahedra or 
dodecahedra. It hasa hardness of 5-5 to 6-5, anda specific gravity of 
4-9 to 5:2. Its strong magnetism, black streak, and frequent occurrence 
in octahedra, distinguish magnetite from all other common minerals. It 
is iron-grey or black, like ilmenite, but hardly so infusible, while it is 
soluble in hydrochloric acid. Ilmenite, again, weathers with a greyish 
crust (leucoxene) ; magnetite, on the other hand, weathers brown (limonite). 
Magnetite is a widely distributed rock-former, occurring as large and 
small crystals in chlorite-schist and other foliated rocks. Now and 
again it is found associated with such rocks in the form of massive beds 
with a granular structure, throughout which chromite, ilmenite, pyrite, 
chalcopyrite, etc., are often abundantly disseminated. While common 
in acid igneous rocks, it is a still more frequent (usually microscopic) 
constituent of basic igneous rocks. Occasionally in gabbros it occurs as 
massive aggregates. It is met with also as a secondary mineral in many 
eruptive rocks—a product of the alteration of such ferromagnesian 
constituents as olivine, augite, hornblende, and biotite. Not being 
readily decomposed, magnetite often appears in alluvial sands derived 
from the disintegration of basic eruptive rocks, etc. 
Limonite (2Fe,0;+3H,O) occurs as fibrous aggregates, assuming 
nodular, stalactitic, or botryoidal forms, or as large irregular masses. 
Its hardness is 5 or thereabout, but earthy varieties are softer—the 
specific gravity =3-4 to 3:95. It is brown or yellowish-brown, and has a 
yellow-brown streak. As a rock-constituent it is always a product of 
alteration—derived from the decomposition of minerals which contain 
iron. Limonite is itself amorphous, but is often met with filling the 
moulds formerly occupied by other minerals, and thus assuming their 
crystalline form (pseudomorphs). 
Rutile (Ti02) as a rock-former occurs usually as minute dark brown or 
reddish grains, pointed prisms, and knee-shaped (Fig. 2) or heart-shaped 
twin-crystals, belonging to the tetragonal system. It is met with in 
various schistose rocks (gneiss, mica-schist, phyllite, eclogite, etc.). 
Needle-like crystals are also common in clay-slate and greywacké, and 
