|
THE MATRIX OF THE DIAMOND
|
37
|
||
|
|
|||
|
of this character, in the nepheline-dolerite of Meiches, Knop' found 25 per cent, of titanic iron.
Ilmenite is in plates with a purplish metallic glance, as in Bowley Regis basalt; and also probably in grains.
Apatite.—In
small hexagonal colourless crystals, with a high index of refraction
and a low double refraction. The crystals are unusually short, and have
a negative character. Sometimes the apatite is faintly pleochroic. It
occurs in short crystals in the ground-mass, and also in long
actinolite-like crystals, apparently as a contact mineral around
enclosures.
Epidote is
in pale yellow grains, with a high index of refraction and a weak
yellow pleochroism, and is apparently a secondary mineral due to
decomposition.
Orthite is
one of the most interesting of the non-essential minerals. It was
noticed in large rounded cleavable grains, as if a primary constituent.
It has a chestnut-brown to yellowish-brown colour in transmitted light,
and is pleochroic. The pleochroism is more marked than that of epidote,
changing from light to dark brown. It has a very high index of
refraction, so that its surface appears wrinkled or shagreened. The
double refraction is not so high as that of olivine. It has an
excellent cleavage parallel to the base, OP, and another parallel to
x> Poo . On the basal face or cleavage plane an axis appears in
converging light not quite in the centre of the field, and this axis
shows a hyperbola coloured green inside and red outside, as in epidote.2
The plane of the optic axis is diagonal to the cleavage, and the
character is negative. All these characters agree with those of
orthite, although this is a new association for that mineral. Orthite
occurs in many granitic and hornblendic rocks, as shown by Tornebohm,3 Sjogren,4 and others, and was found by Vom Rath8 in
|
|||
|
|
|||
|
|
|||
|
|
|||