Thortveitite and associated Sc-bearing minerals from Ravalli County, Montana

The rare Sc mineral thofieitite, (Sc,Y12Si2O7, occurs as pm- to mm-sized crystals in fluorite-bearing granitic pegmatites and the host melagabbro within the Crystal lrtoirn6in fluorite deposit ltavalti County, Montana. Thortveitite is found as colorless and clear to smok! and translucent, suLhedral to euhedral prisms up to 3 mm in length in the massive fluorite, as mm-sized anhedra to subhedra in diopside and edenite, and as pm-sized droplet-like crystals in actinolite. Micrometric textures suggest that some thorweitite exsoived from actinolite, which contains between 1.2 and 2.9 tut.Vo Sc2O3. Associated fenoan diopside contains as much as 4.8, edenite as much as 2.1, actinolite as much as 2.0, allanite as 4gch as 0.5 and titanite as much as 0.4wt.Vo Sc2O3' respectivd. The source of the Sc is believed to be magmatic, rather than from assimilation and extraction of Sc from adjacent anr,phibolite xenoliths. Indices of refraction, determined using spindle-stage techniques , arct a7.752, p 1.780, 1 1.804' all fl.m3' T-cl = 0.052. The mineral is biaxial negative, 2Vr*. = JQo, 2V"*. = 84". The empirical formula derived on the basis of 7 atoms of o*ygen from electron-microprobJand tasei-iep analysej is (Sc1.83Y0.05Fe3*o.orRq.mLz.oo(Sir.g4 .d>r.go lvhere R represeits tvtn, Ca Mg, Ti, Na Ii, Nb, Sn, Ce, Pr, Nd, Snr, Eu, Gd, Tb, Dy, {o, Er' rm' Yb, Lu, Th, u,Ht'7'r, P' Li' Be' B' v' Cr, Co, Ni, Cu, Ba Ga Ge, As, Rb, Sr, and In. Du = 3.50, Dc = 3.50 g cm-3. The mineral contains *EE = 4.63 wt.7o and is enriched in the heaw rare-eartli elements. Cell dimensions are: a 6.5304(4), b 8.5208(4)' c 4.6806(5) A' p 102.630(7)" 'V 254.1(l) N,Z=Z,spa"egroop C2lm.'tlrcwit-cellvolumeissmallerthanthatofthorweititefrommostotherlocalitiesbecauseofitsSc enrichment. Simitartj, Oe metal--oxygen distances (2.088 to 2.199 A) are smaller than those for most natural specimens of thorweitite. Keryords: thortveitite, scandium silicate, rare-earth mineral, fluorite, actinolite, diopside, granitic pegmatite, X-ray dat& crystal-structure refinement, Crystal Mountain mine, Ravalli County' Montana