The high albite-monalbite and analbite-monalbite transitions Hnnnnnt Knorr, HeNs-Ulnrcn BeuneuER AND Urnrcn ScHlnrr,rnn

The temperatures Zo * and Zarpr of the di.ffusive transformation high albit+monalbite and the displacive transformation analbite-monalbite were investigated by X-ray powder meth-ods. Alkali feldspars of composition Ors,5,1s,2s,36 were prepared by hydrothermal synthesis at temperatures T"q"1 between 1060 and 700"C. Run durations were sufficiently long (3-:12 days) to allow the samples to reach Al,Si equilibrium distributions. Io,s.can be represented by the equation 4-['C]:978- 19.2.Ot, whereas 4i"pr also depends on Q"o, le. the degree of Al,Siorder,accordingtoZu"o, [ 'C] :715-18.9.Or-0.221.Of+0.269-T"q, i tAsacon-sequence Or*o,, the composition of the triclinic,/monoclinic inversion at room temperature, also changes with?io"o: Orui,pr[mol7o] : 27.5 + 0.00842. T.q",r. Distinction between topochemically triclinic and monoclinic symmetry is possible on the basis of the temperature variation of lattice angles c, y and simple line separations; e.g. cos2a of high albite smoothly approaches but never reaches zero with increasing temperature, whereas cos2a of analbite is consistent with a straight-line extrapolation to zero. At room temperature an interval of uncertain distinction extends from 7: 90.18 to 90.25o and from Al3l: 1.95 to 2.00 I"201 n high albite and analbite. Structural explanations are given (l) for the variations of c and y with Al,Si order in equi-librated albites, (2) for the variations ofa and y due to short-term heatlng ofalbite and anal-bite, (3) for the dependence of Tuoo, and Or*o, on Z.q,u, (4) for the result ft*o,> [' ' in Na-feldspar. Tooor b thought to react sensitively on structural disturbances which may be caused by thermal and mechanical history, chemical impurities, and other factors. If?di"er is found to be lower than 7i-, this is ascribed to such defects