BRN 9-2 (uncompressed) - Flipbook - Page 82
feldspathoid group, the scapolite
group, and the zeolite group. Many of
these groups are further divided into
subgroups. Moonstone is an
orthoclase in the feldspar group.
Moonstone is known for its adularescence (or schiller/shiller). This is the
bluish cast, best seen in the center
photograph, which appears to
emanate from inside the rock. This is
an optical effect; it is a product of the
crystal structure of the mineral, not of
any particular type of inclusion.
I
I
J
If the following is a bit boring you can
just sit back and admire the sheen:
Moonstone is a sodium potassium
aluminum silicate ([Na,K]AlSi3O8) in
the feldspar group. A mineral may
have several different forms. In the
case of moonstone it may be
composed of sodium or potassium
(Na,K). Minerals are arrayed in a
taxonomic tree something like the
Linnaean system used in biology.
Moonstone is a tectosilicate mineral in
the feldspar group, alkali series,
orthoclase variety.
Tectosilicate minerals include the
quartz group (the silicas like
chalcedony, agate, amethyst, Ònormal
quartzÓ, etc.), the feldspar group
(which includes things like
plagioclase and orthoclase), the
81
In ÒPreservation of primary magmatic
features in subvolcanic pegmatites,
aplites, and granite from Rabb Park,
New MexicoÓ James D. OÕBrient
(American Mineralogist, Volume 71,
1986, pp. 608-624) describes the
moonstone and its development. He
notes that:
ÒRabb Park moonstone specimens are
well known to mineral collectors
because of their gem-quality blue
schiller, but could never be proÞtably
mined because of widespread
fracturing that has broken most of the
pegmatite crystals into pieces less
than I cm in maximum size. These
fractures are commonly conchoidal,
even in easily cleaved sanidine. They
terminate or branch at crystal
boundaries. A few are Þlled with
groundmass or heulandite, but most
are hairline cracks that lack any visible
Þlling. In places, fracture density
within single crystals varies systematically such that regularly spaced
zones of intense shattering are
separated by concentric domains of
less fractured material. These bands
deÞne growth zones at a spectacularly coarse scale of several
centimeters each in some of the
meter-scale quartz euhedra. Nested
quartz hexagons are separated from
one another by 5-10-mm thick zones
of intense shattering and more rarely,
thin selvages of pumiceous glass.
Subsequent growth bands of lessshattered quartz are individually 8-10
cm thick. Up to six such discrete
fracturing episodes are preserved in
the largest crystals. They are thought
to record episodic thermal shocks to
the growing crystals, probably caused
by periodic venting, adiabatic decompressions, and rapid temperature
changes in the throat of this
subvolcanic system.Ó
