2006 Philadelphia Annual Meeting (22–25 October 2006)

Paper No. 17
Presentation Time: 1:30 PM-5:30 PM


GUNDERSON, Katie M.1, LONDON, David2 and MORGAN VI, George B.2, (1)School of Geology & Geophysics, University of Oklahoma, 100 East Boyd Street Room 810 SEC, Norman, OK 73019, (2)School of Geology and Geophysics, Univ of Oklahoma, 100 East Boyd Street, SEC 810, Norman, OK 73019, katie_gunderson@ou.edu

The current understanding of Al-Si ordering in K-feldspar (Kfs) is that regardless of growth conditions or medium, all Kfs crystallizes initially as disordered monoclinic sanidine or orthoclase. Triclinic microcline, in which Al is ordered into one of four non-equivalent T sites, is believed to form only via recrystallization of the original monoclinic Kfs. The degree of Al-Si ordering, therefore, has been used to assess the thermal history of Kfs and the extent of its recrystallization after growth. This application rests on the assumption that ordered triclinic Kfs cannot be grown directly from melt (or aqueous vapor) under any circumstances, and it is important to verify this fact as far as possible. Toward this end, we have synthesized Kfs crystals from hydrous KAlSi3O8 – B2O3 melts supercooled to 550º C at 200 MPa. The euhedral Kfs crystals, which precipitate via the dissolution of melt or glass through aqueous vapor, are ~ 0.5 mm in size and commonly exhibit Manebach or Carlsbad twinning. The Kfs contains up to 1 wt% of B2O3, which probably substitutes for Al in the structure. Polished crystal sections exhibit sector-zoned blue cathodoluminescence, which we attribute to growth-related defects. Our working hypothesis was that fluxing components might facilitate ordering during growth, but single-crystal X-ray diffraction and unit cell parameters show that all of the feldspars are monoclinic sanidine. The triclinicity, Δ (fraction of Al-Si ordering from 0 to 1) of Kfs was determined using the algorithm Δ = 12.5[d(131) – d(1-31)]. Triclinicity ranged from Δ = 0.01 to 0.03, indicating that the feldspars are less than 3% ordered. There was no significant difference in Al-Si ordering or in unit cell parameters based on experimental durations from 25 to 800 hrs. This highly disordered state is consistent with the only known report of a low-temperature, natural K-feldspar (orthoclase) that contains ~ 1 wt% B2O3.