18 THE MATRIX OF THE DIAMOND
In
fig. 18, the enclosed mineral is enstatite, distinguished from the
diopside by much lower double refraction and by parallel extinction.
The enstatite here encloses secondary biotite scales, and the biotite
in turn contains small hexagons of hematite.
As
is well known, olivine is almost always older than the more acid
pyroxene in any given rock. Unless we suppose that a subsequent fusion
has produced it, or that there is a secondary olivine, there would
appear to be an exception here to the useful and generally applicable
rule that the constituents of a magma crystallise out in the order of
diminishing basicity.1 According to Hussak,2
augite and hornblende occur enclosed in olivine in the
picrite-porphy-rite of Steierdorf, Banat, and the enclosure of
enstatite in olivine is often seen in meteorites. It is probable that
these are cases of contemporaneous crystallisation.
A
highly refracting rhombic mineral, resembling olivine, also occurs in
some remarkable zones which surround the bronzite in the Kimberley
rock, and which have the appearance of contact-fusion zones. These
zones, in which the olivine makes a pegmatitic or ' eozoonal'
structure, as in the chondri of meteorites, will be discussed more
fully under the description of enstatite. They may be the rudimentary
stage of the compact olivine enclosing enstatite, and point to the
contemporaneous and rapid crystallisation of olivine and enstatite—a
common occurrence in meteorites.
The significance of olivine as a characteristically igneous mineral, and as one readily produced by dry fusion, is well known.3
It is a mineral of special interest, as regards the part it plays in
connecting deep-seated terrestrial rocks with those of celestial origin.4 As the predominating constituent of the present rock, it places this clearly among the
