Formation Conditions of Elbaite/Fluor-Elbaite in an Unorthodox Location in the Emmons Pegmatite, Greenwood, Oxford Co., Maine

Abstract The Emmons pegmatite on Uncle Tom Mountain, Greenwood, Oxford Co., Maine, is the most mineral species-rich pegmatite in Maine, with 227 confirmed species. Even though the Greenwood area is not a prolific elbaite-producing region, the Emmons pegmatite does produce colorful elbaite/fluor-elbaite. Two distinct zones produce elbaite and are different from those in other pegmatites in the Oxford Pegmatite Field: (1) A small amount of colored elbaite occurs in miarolitic cavities in the core and core margin zones. (2) A second zone of miarolitic cavities occurs in the lower footwall intermediate zone immediately above the upper schorl/garnet layer, a zone where cavities are generally not expected. There are two schorl-garnet layers and between them is an unusually high abundance of löllingite in lath-shaped crystals up to 10 cm in length. Arsenopyrite occurs in some areas and columbite-(Fe) is common. The elbaite is F-rich, and fluor-elbaite is abundant. Fluor-elbaite and elbaite from the Emmons pegmatite contain the highest Mn concentration of any elbaite in the Oxford Pegmatite Field, with up to 6.9 wt.% MnO. The pegmatite is Mn-rich and contains lithiophilite and many secondary Mn phosphates. Muscovite makes up approximately 8% by volume of the pegmatite and is the main site where Li and F occur. In addition, montebrasite, lithiophilite, cookeite, polylithionite–trilithionite, and spodumene are present and add to the Li amount. Fluorine is present in muscovite as well as polylithionite–trilithionite.