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Constituent Minerals

The following minerals have been observed in the ultramafic rocks
tentatively assigned to the Trembleur intrusions in Aiken Lake map-area.

Olivine. This mineral, which comprises nearly 100 per cent of the
dunite and which is, or was originally, present in all the other ultramafic
rock types except the rare, purest pyroxenites, occurs mainly in smooth,
anhedral grains 1 to 4 mm. in diameter. In a few rocks the grains range
to 10 mm. Most specimens show a seriate texture, but at ‘east one slide
contains olivine grains of two distinct sizes, with the smaller grains
clustered into aggregates whose over-all dimensions are approximately
equal to those of the larger grains. The grain boundaries are mainly
smoothly interlocking but not sutured, although several s,ecimens of dunite
and olivine-rich peridotite were observed to contain amoeboid grains of
olivine that lie interstitial to, and poikilitically surround, crystals of
pyroxene and rounded grains of olivine of different optical orientation.
Euhedral grains are rare. The optical properties indicate a composition
ranging from 87 to 96 per cent of the Mgs.SiOy (fosterite) molecule. No
difference could be detected in either the range of composition or the average
composition of the olivines in the olivine-rich rocks as compared with those
of the olivine-poor rocks. In addition to the common alteration to serpen-
tine, which proceeds from the rim of the grains and from a network of
random fractures, the olivine has in many thin sections been partly con-
verted to iddingsite, which appears as a brown, foliated ring outlining the
original grain.

Pyroxene. The pyroxene of the ultramafic rocks varies widely in grain
size and texture. In the dunites and many of the peridotites it occurs as
rounded anhedral grains of about the same dimensions as the olivine grains.
Other peridotites, as has been described, have irregular but usually elon-
gated poikilitic pyroxene grains up to 5 inches long, which enclose olivine
and, less commonly, other pyroxene grains. Many grains have an amoeboid
habit, with irregular projections into the surrounding pyroxene and into
olivine, in such a manner as to suggest that the pyroxene replaced the
olivine. The rocks richer in pyroxene show a general tendency toward
more regular grain shape, and euhedral crystals are found in many of the
pyroxenites. Some of these crystals are more than a foot long.

Nearly all the pyroxene in these rocks is monoclinic, uniformly colour-
less to very pale green in thin section, and essentially non-pleochroic.
They are optically positive with an optic angle of 50 to 60 degrees, and a
maximum extinction angle ranging, in different slides, from 38 to 50
degrees. These clinopyroxenes are thus mainly augite, of-a variety inter-
mediate between normal augite and diopside. In general the p;roxenites,
particularly the coarser varieties, contain pyroxenes with a smaller extinc-
tion angle and larger optic angle, and thus presumably a more diopsidic
composition, than the peridotites. In some peridotites, large, irregular,
poikilitic grains of non-pleochroic pyroxene surround rounded grains of
faintly pleochroic pyroxene; this may suggest that the earlier formed
mineral is augite, and the later, interstitial and poikilitic grains are nearer
to diopside in composition.

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