AL-SI ORDERING IN K-FELDSPAR SYNTHESIZED FROM HYDROUS BOROSILICATE MELT

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.