The rare Sc mineral thortveitite, (Sc,Y)2Si2O7, occurs as mum- to mm-sized crystals in fluorite-bearing granitic pegmatites and the host melagabbro within the Crystal Mountain fluorite deposit, Ravalli County, Montana. Thortveitite is found as colorless and clear to smoky and translucent, subhedral 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 mum-sized droplet-like crystals in actinolite. Micrometric textures suggest that some thortveitite exsolved from actinolite, which contains between 1.2 and 2.9 wt.% Sc2O3. Associated ferroan diopside contains as much as 4.8, edenite as much as 2.1, actinolite as much as 2.0, allanite as much as 0.5 and titanite as much as 0.4 wt.% Sc2O3. respectively. The source of the Sc is believed to be magmatic, rather than from assimilation and extraction of Sc from adjacent amphibolite xenoliths. Indices of refraction, determined using spindle-stage techniques. are: alpha 1.752, beta 1.780. gamma 1.804, all +/-0.003, gamma-alpha = 0.052. The mineral is biaxial negative, 2V(meas.) = 70-degrees, 2V(calc.) = 84-degrees. The empirical formula derived on the basis of 7 atoms of oxygen from electron-microprobe and laser-ICP analyses is (Sc1.83Y0.005Fe3+0.05R0.07)SIGMA2.00(Si1.97Al0.04)SIGMA2.00O7 where R represents Mn, Ca, Mg, Ti, Na, K, Nb, Sn, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Th, , Hf, Zr, P, Li, Be, B, V, Cr, Co, Ni, Cu, Ba, Ga, Ge, As, Rb, Sr, and In. D(M) = 3.50, D(C)- = 3.50 g cm-3. The mineral contains SIGMAREE = 4.63 wt.% and is enriched in the heavy rare-earth elements. Cell dimensions are: a 6.5304(4), b 8.5208(4), C 4.6806(5) angstrom, beta 102.630(7)degrees, V 254.1(1) angstrom3, Z = 2, space group C2/m. The unit-cell volume is smaller than that of thortveitite from most other localities because of its Sc enrichment. Similarly, the metal-oxygen distances (2.088 to 2.199 angstrom) are smaller than those for most natural specimens of thortveitite.