Spencite is a new borate-silicate of Ca and Y, (Ca,Fe) 2 (Y,La) 3 (B 3 Si (sub 4.3) Al (sub .7) )(O,OH,F,Cl) 20 from a pegmatite in Cardiff township, Haliburton County, Ontario. Found as dark reddish brown to brownish black anhedral masses. Hardness 3 1/2, specific gravity 3.05. Metamict; isotropic with n near 1.630. When the mineral is heated at 325 degrees C. the n increases to about 1.640 and the specific gravity to 3.20. At high temperatures the mineral decomposes before it recrystallizes, and X-ray diffraction data cannot be obtained. Spencite is related chemically to the minerals of the datolite group and may be isostructural with them. It is named after the Canadian mineralogist Hugh S. Spence. A re-examination of the little known mineral rowlandite from Baringer Hill, Texas, establishes it as a valid species. Composition near (Y,Fe,Ce) 3 - (SiO 4 ) 2 (F,OH). Metamict; with n 1.704. Hardness 5 1/2, specific gravity 4.39; X-ray powder data are given for material recrystallized in N at 900 degrees C. (with mean n 1.76, specific gravity 4.55).