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.
Time and Heritage
As a historian studying what can be considered a form of intellectual history, I have slowly come to adopt Michel de Certeau’s observation made at the end of the 1980s that ‘objectifying the past, for the last three centuries, has undoubtedly left unconsidered time within a discipline that has continued to use it as a taxonomic instrument’. To a certain degree, time has become commonplace for the historian who has preserved or instrumentalized it. It is not considered because it is inconceivable, but because we do not think of it or, more simply, we do not think about it. As a historian attempting to pay attention to the time I’m living in, I have thus, like many others, noticed the swift development of the category of the present until it has become obvious that the present is omnipresent. This is what I refer to here as ‘presentism’.