CHUPA PEGMATITE FIELD MOONSTONE
The term “moonstone” 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.
“Moonstone” (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.