Carmichaelite, a new hydroxyl-bearing titanate from Garnet Ridge, Arizona

Carmichaelite [MO2-x(OH)(x), where M includes Ti, Cr, Fe, Mg, and Al] is a new hydrous titanate from Garnet Ridge, an ultramafic diatreme on the Color ado Plateau in Arizona. It is named in honor of Ian S.E. Carmichael, Profes sor of Geology at the University of California, Berkeley, in recognition of his many contributions to petrology, especially his studies of Fe-Ti oxide s in volcanic rocks. The new mineral occurs as platy, cinnamon-brown inclus ions in mantle-derived pyrope crystals, which also contain inclusions of ru tile, srilankite, ilmenite, minerals of the crichtonite group, spinel, and olivine. Carmichaelite is commonly in contact with rutile and occasionally with srilankite. The average composition is TiO2 = 62.16, Cr2O3 = 18.43, Al 2O3 = 1.88, V2O3 = 0.87, Nb2O5 = 0.37, FeO = 7.61, MgO = 2.80, H2O (calc) = 5.76, total 99.89 wt%, giving an empirical formula close to Ti0.62Cr0.09Fe 0.09Mg0.06Al0.03V0.01O1.5(OH)(0.5). A cation (non-H) to (O+OH) ratio of 1:2 was confirmed by microprobe analysis of oxygen, and the presence of the hy droxyl component was supported by IR data. Carmichaelite is monoclinic, spa ce group P2(1)/c, with a = 7.706(1), b, = 4.5583(6), c = 20.187(3) Angstrom , beta = 92.334(2)degrees, V= 708.5(3) Angstrom(3), and Z = 22. The calcula ted density is 4.13 g/cm(3). The strongest diffraction lines [d(I, h k l)] from a calculated powder pattern are 2842 (100, (1) over bar 15), 3.773 (94 , 013), 2.664 (70, 213), 1.688 (54, (3) over bar 22), 1.679 (44, 226), 1.66 1 (44, (1) over bar 28), and 1.648 (34, 1.1.11). The crystal structure, whi ch has been determined from single-crystal X-ray diffraction data, consists of stacked chains of cation octahedra within layers of hexagonal closest-p acked anions. The arrangement of cation octahedra within a single chain has the same basic structural unit of five edge-sharing octahedra found in oli vine, the humite group, and leucophoenicite. Carmichaelite is inferred to have co-precipitated with its pyrope host and other. titanates in the presence of a fluid or melt phase (i.e., mantle met asomatism) at temperatures of 650-750 degrees C and pressures of 20-25 kbar . It contains the highest OW content reported for titanates of mantle origi n and offers a potential storage site for water and high field strength ele ments in the upper mantle. The new mineral is probably stabilized by high p ressure and the high concentration of Cr. Certain features of the carmichae lite structure raise the possibility of a similar high-pressure phase in th e Si-Al-O-H system.