T64 Gemological Research in the 21st Century: Exploration, Geology, and Characterization of Diamonds and other Gem Minerals

Sunday, 1 November 2015
1:30 PM - 5:30 PM
Room 342

GEM POLLUCITE FROM MT. MICA PEGMATITE, OXFORD CO., MAINE

Paper number: T64-14
Program: Technical Sessions
Time: 4:55 PM – 5:10 PM

Authors

Abstract

Gem quality pollucite has recently been recovered from Mount Mica pegmatite in the Oxford pegmatite field of SW Maine. The pegmatite is famous for gem tourmaline production for 195 years. New mining by Coromoto Minerals has extended the mine down dip for over 100 meters to a depth of about 30 meters. The dike ranges in thickness from 1 to 8 meters and dips 20° to the SE. The pegmatite has a simple zonal structure consisting essentially of a wall zone and core zone. The wall zone is essentially devoid of K-feldspar. The outer portion of the pegmatite consists of quartz, muscovite, albite (An 1.8) and schorl. K-feldspar only occurs in the core zone adjacent to pockets. The pegmatite is rich in pockets ranging in size from a few cm3 to one in excess of 500 m3. Numerous intermediate to larger pockets have produced thousands of carats of gem quality tourmaline with lesser quantities of morganite and rare pollucite. Pocket density averages about one every 3 meters with larger pockets having greater spacing and small ones having less, making this a very pocket-rich pegmatite. Along the footwall side of the core zone, two relatively distinct and continuous undulating lines of garnet and schorl mark the boundary of the core zone and the footwall wall zone Miarolitic cavities are never found below the garnet line.

Overall, the mica and feldspar of the outer zone of the pegmatite are only moderately evolved. The core zone consists dominantly of quartz, muscovite, albite (cleavelandite), schorl and microcline. Scattered along the core zone are local pods of lepidolite with rare altered spodumene crystals. Adjacent to or within the lepidolite are rare pods of opaque pollucite and montebrasite. The gem-quality pollucite occurs in miarolitic cavities and is associated with crystals of cleavelandite, quartz, lepidolit and fluorapatite; and gem-quality elbaite and beryl. Gem quality pollucite is rare, but one pocket found in 2014 contained a large number of 2-3 cm size strongly etch crystals. Some of the best material has been faceted into stones up to 4.2 carats. Although, pollucite is reported from several other pegmatites in Maine, Mt. Mica is one of the few pegmatites that has produced such high quality pocket crystals of gem quality pollucite