GROSSITE (CAAL4O7) - A RARE PHASE IN TERRESTRIAL ROCKS AND METEORITES

The colourless to white, transparent mineral grossite (CaAl4O7; Ca-dialuminate) occurs as a major phase in Ca,Al-rich inclusions (CAIs) from the meteorites Acfer 182, Acfer 059-El Djouf 001, and ALH85085. Associated minerals are perovskite, hibonite, melilite, and spinel. In addition, this mineral is found as an accessory phase in CAIs from several other meteorites. Grossite is also known from terrestrial rocks, associated with brown-millerite and mayenite (Hatrurim Formation, Israel). The empirical formula of grossite derived from microprobe analysis is (Ca1.00Fe0.01)SIGMA1.01Al3.99O7.00 or ideally CaAl4O7. Grossite is monoclinic, C2/c, with a = 12.94(1) angstrom, b = 8.910(8) angstrom, c = 5.446(4) angstrom, beta = 107.0(1)-degrees, V = 600.1(7) angstrom3, Z = 4. The calculated density is 2.88 g/cm3. The most intense lines of the powder diffraction pattern (microdiffraction) are [d in angstrom - I/I0 (hkl)] 3.515 - 100 (-311), 4.460 - 43 (020), 2.605 - 36 (-131), 2.440 - 21 (-511), 1.764 - 20 (-313), 3.609 - 13 (-220), and 2.882 -13 (130). Ca-dialuminate is optically biaxial(+), alpha = 1.6178(3), beta = 1.6184(3), gamma = 1.6516(3) (589nm), 2V(meas.) = 12(1)-degrees. The formation of meteoritic grossite is related to high-temperature processes in the early solar system. On earth, it occurs as a high-temperature mineral in argillaceous limestones.