Brockite, a new calcium thorium phosphate mineral from the Wet Mountains, Colorado, is found as massive reddish aggregates and as earthy yellow coatings in veins and altered granitic rocks. Although similar in composition to grayite, it corresponds in hexagonal symmetry to rhabdophane. Color is red-brown to pale yellow, translucent, with greasy luster. G = 3.9±0.2. In transmitted light it is moderately birefringent or opaque due to hematite stain, uniaxial (+), with parallel extinction and positive elongation. W= 1.680 ±.002, E=1.695±.002. The formula is Ca0.43Sr0.03Ba0.02Th0.41RE0.11[(PO4)0.83(CO3)0.17] ·0.9H2O; Z —3. An analysis of the rare earth content shows an unusually high proportion of Nd2O3 with respect to CeO2, La2O3, and Y2O3. The low ratio Ce/(La+Nd) of 0.58 is believed to reflect the formation of brockite under oxidizing conditions. The x-ray diffraction pattern nearly identical to that of rhabdophane; thus distinct from grayite, which is pseudohexagonal. The cell dimensions are a = 6.98 Å, c = 6.40 Å, V = 270 Å3. Upon heating, brockite acquires a monzaite-type structure, with spacings close to those of cheralite.