Rate of Al-Si ordering in sanidines from an ignimbrite cooling unit

Scott, R.L., Bachinski, S.W., Nesbitt, R.W. and Scott, M.R. (1971) Rate of Al-Si ordering in sanidines from an ignimbrite cooling unit. American Mineralogist, 56, 1208-1221.

Abstract

X-ray diffraction studies of monoclinic alkali feldspars [016 (AbfAn)27] from a 300-meter-thick ignimbrite indicate that those feldspars in the central portion of the unit are significantly more ordered than those near the margins. The variations in degree of order are indicative of structural states that cover most of the more disordered half of the range between high sanidine and orthoclase. Further Al-Si ordering is believed to have become ineffective at temperatures below approximately 500 ºC, based upon the lack of significant K-Na unmixing The length of time required for feldspars to order to their observed state can be estimated by calculation of the time required for the ignimbrite to cool from 800 ºC, the approximate emplacement temperature, to 500 ºC; this period was found to be a few hundred to 650 years, depending upon the cooling effects of rain water from above and ground water from below.

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