|
TEMPERATURES OF PEGMATITE CRYSTALLIZATION. 37
point
and been heated above 57-3°, it bears ever afterwards marks potentially
present which on proper treatment can be made to appear just as an
exposed photographic plate can be distinguished at once from an
unexposed plate on immersion in a proper developer, although before
development both plates may be identical in appearance."
In
addition to the change in crystal form at 575°, the quartz exhibits
changes in its coefficients of expansion, in circular polarization,
and in birefringence.
Briefly
slated, the four criteria which can be used to distinguish, at ordinary
temperatures, quartz which was formed above 575° from quartz which has
never been heated to that temperature, are: (1)
Crystal form, if crystals be available, the presence of trigonal
trapezohedrons and other evidence of tetartohedrism, irregular
development of the rhombs and the like, being indicative of the a form. (2) Character of twinning, as shown by etch figures on the basal pinacoid. In the a form,
which crystallized from solutions at comparatively low temperatures,
the twinning is usually regular and sharply marked, while in quartz
plates originally of the /? form and now a by virtue of
inversion in the solid state, the lines are usually irregular, and the
twinning patches are small and bear no relation to the outer form of
the crystal. (;{) Intergrowtlis of right and left handed quartzes are more frequent and more regular in boundary lilies in the n than in the ,3 form. (4) Plates of originally beta quartz but now n quartz
by inversion show the effect of the inversion by the shattering, which
should be most evident on large plates. Into all these criteria an
element of probability enters, and in testing
quartz plates, with this end in view, a number of plates should be
examined to strengthen the validity of the inferences drawn. 6
The
bearing of the experiments on the temperatures of crystallization of
granites and pegmatites has been briefly discussed by Wright and
Larsen, but the writer desires to amplify the discussion by a more
detailed description of its relation to those specimens with which he
is personally familiar.
No
granites from Maine were tested by Wright and Larsen, but thirteen
specimens of granites, granite gneisses, and porphyries which were,
tested from other regions show as a rule the characters of beta or high-temperature quartz, thus placing their temperature of final solidification above 570° C.
Two
specimens of rose quartz from Maine (Xos. 13 and 14, Wright and
Larsen), one of them from Paris, Oxford County, now in the collection
of the United States National Museum, were examined by Wright and
Larsen and found to show the characters of a or low-temperature
quartz. The specimens have the appearance of typical pegmatite, quartz;
and in Maine, rose quartz, so far as known, occurs only as a pegmatite
constituent.
A
specimen of rose quartz (No. 12, W. and L.) collected by the writer
from the feldspar and quartz quarry of P. II. Kinkle's Sons at Bedford,
N. Y., also showed the characteristics of ot or low-
a
Wright, F. E., and I,arson, E. S., quartz as a geologic thermometer:
'Am. Jour. Sci., 4th ser., vol. 28, pp. 423, 425. <> Idem, p. 438.
|
||
|
|
||