CHUPA
PEGMATITE FIELD MOONSTONE
The term ÒmoonstoneÓ has been used to
refer to various materials, but properly refers to iridescent whitish-gray
feldspar. The sample shown below
consists of peristerite and oligoclase feldspar. Oligoclase is a sodium-rich plagioclase
feldspar. Peristerite consists of
closely-intergrown varieties of albite (= very sodium-rich plagioclase
feldspar) with slightly varying sodium and calcium contents. Light gets split along the boundaries
between the intergrown feldspars, resulting in bluish-white iridescence.
This is a portion of a single large
crystal derived from a Precambrian-aged pegmatitic granite intrusion in the
White Sea area of northwestern Russia.
Pegmatitic granites have huge crystals, formed by cooling of water-rich
magmas.
Geologic & age: Chupa Pegmatite Field, late
Mesoproterozoic, 1.75-2.10 billion years.
Locality: at or near Chupa Bay (at or near the
town of Chupa), near the western coast of the White Sea, northwestern Karelia,
far-northwestern Russia.
ÒMoonstoneÓ (iridescent
peristerite-oligoclase feldspar) (above: 10.9 cm across at its widest; below:
field of view 5.6 cm across) from the Chupa Pegmatite Field (late
Mesoproterozoic, 1.75 to 2.10 billion years old) at or near Chupa Bay/town of
Chupa, northwestern Karelia, far-northwestern Russia.
