Experimental evidence on the substitution of Ti in biotite Jtincnr
Various Ti substitutions in biotite were tested by hydrothermal syntheses of Fe and Mg biotites over the range of 600-800 'C at I 000-bars pressure and at the oxidation conditions of the methane-graphite buffer. Yields of greater than 950/o biotite were obtained in a few experiments, but complete syntheses were generally not possible. Microprobe analyses of the run products indicate that (l) the Ti-Tschermak's substitution (Ti + 2Al: R'z* + 2Si) can be the dominant Ti substitution in both Fe and Mg biotites; (2) the Ti-vacancy substitution (Ti + 16r! : 2Rr*) can occur as the dominant Ti substitution in Mg biotite and in conjunction with other substitutions in Fe biotite; and (3) the experimental data do not support, but cannot rule out, the Ti-oxy substitution (Ti + ZO': R2+ + 2OH-). Specific substitution types appear to be governed by bulk composition and by the crystal-chemical constraints of the biotite structure.
Crystal chemistry of a metamorphic biotite and its significance in water barometry
The crystal structure of a metamorphic biotite from Au Sable Forks, New York (northeast Adirondacks) has been refined to an unweighted R value of 4.2Vo. The Au Sable biotite is a 2M, mica with space group symmetry C2/c. Although the biotite equilibrated near and cooled slowly from Z: 7fi)ot5OoC, octahedral and tetrahedral cations are apparently disordered. There are few, if any, octahedral and interlayer cation site deficiencies. Average bond lengths are Ml-O 2.106, M2-O 2.086, Tl-O 1.657,T2-O 1.661, inner K-O 3.046, ourer K-O 3.289A. The tetrahedral rotation angle (a) is 5.3o. The Au Sable biotite structure is nearly identical to theigreous 2M, biotite of similar composition refned by Takeda and Ross (le7s). The Au Sable biotite coexists with magnetite (usp ,r), ilmenite (Hmr), perthite (orur), and plagioclase (Anrr). This assemblage fxes f 000"C), Io,(10-t12y, and /;r,o (lG-103) for metamorphic pressure of 7tl kbar. Metamorphism of orthogneisses in the Au Sable area of the Adirondacks took place under quite dry (p""o ( p*ua) conditions.
Chemical variation within three alkaline complexes in South Greenland
Abstract Comparison is made between three hypersolvus alkaline complexes of Precambrian age occuring in the same igneous province. Two represent the cores of deeply eroded central volcanoes; the third comprises a suite of trachytic and rhyolitic dykes. In each complex a similar suite of trachytic and comenditic magmas was intruded and the gross mineralogy and chemistry are closely comparable. However, such differences as exist are explicable in terms of variation in cooling rates, controlling the degree to which the rocks represent cumulates and the extent to which excess alkalis in liquid residua were lost to the atmosphere and wall-rocks. In the Kungnât Fjeld complex where cooling was slowest, alkali loss is inferred to have been greatest and the bulk of the constituent syenites are mildly peraluminous in consequence. In the dyke complex cooling was rapid and alkali loss less severe; the majority of the trachyte/rhyolite dyke rocks are mildly peralkalic. The central ring-complex of Tugtutoq represents an intermediate situation in relation to Kungnât Fjeld and the dyke complex.