36 PEGMATITES ASD ASSOCIATED ROCKS OF MAINE.
nevertheless
observed at several localities. On the highest portion of Streaked
Mountain a'number of patches of schist a few square yards in area were
seen apparently entirely inclosed by pegmatite. Small schist fragments
are also inclosed by pegmatite in the Booth-bay Harbor region. W. H.
Emmons, of the United States Geological Survey, who visited Mount Mica
a year later than the writer, when the excavation had proceeded
farther, observed, a few feet below the schist hanging wall, schist
fragments which appeared to be whollv inclosed in the pegmatite. The
schistosity of these fragments made large angles with the schistosity
of the walls from which they had evidently been dislodged. The
pegmatite shows no bending of the minerals nor other changes in
character near the fragments. In the instances cited the schist
fragments appear to have been caught up while the pegmatite mass was
still partly or wholly fluid, and the density of the magma was
sullieient, at least in the Mount Mica example, to float the fragments.
TEMPERATURES OF PEGMATITE CRYSTALLIZATION
Experiments of Wright and Larsen.—Some
evidence in regard to the temperatures of the pegmatites at the time
they crystallized has been obtained from studies of quartz by Wright
and Larsen, some of the specimens being collected by the writer from
the pegmatites of Maine and other parts of New England. To quote from
their paper—a
For
* * * geologic thermometric purposes, quartz has been found by
experience to be well adapted. It is plentiful in nature and occurs in
many different kinds of rocks. SiO._> in the form of tridymite melts
at about 1,025° (centigrade); between that temperature and about 800°
tridymite is the stable phase; below about 800° quartz is the stable
phase. From evidence thus far gathered it is probable that pressure has
but slight effect on raising or lowering such an inversion point, and
that, therefore, whenever quartz appears in nature, it was formed at a
temperature below 800°.
The
studies of Wright and Larsen and of earlier observers have shown that
at about 575° C. there is a sudden change from one form of crystal
symmetry to another. Quartz developed below 575° crystallizes in what
has been called the a form (the trapezohedral-tetarto-hedral
division of the hexagonal system) and quartz developed above 575°
appears to crystallize in the ,? form (the trapezohedral-hemihe-dral
division of the same system).
Quartz
itself undergoes a reversible change at about 575°. * * * Practically
the only crystallographie change which takes place on the inversion is
a molecular rearrangement, such that the common divalent axes of the
high temperature [i form become polar in the a form, and
this fact involves certain consequences which can be used to
distinguish quartz which has been formed above 575° from quartz which
has never reached that temperature. At ordinary temperatures all quartz
is a quartz, but if at any time in its history a particular piece of quartz has passed the inversion
a Wright, F. E., and Larsen, E. S., Quartz as a geologic thermometer: Am. Jour. Set, 4th ser., vol. 28, 1909, p. 423.
