Hollow sanidine as much as 100μm long enclosed in micropoikilitic quartz in Precambrian Rose dome granite, Woodson County, Kansas, probably grew from a supersaturated melt. The melt formed by the addition of heat and water to granite xenoliths from a Late Cretaceous mica-peridotite host. The granitic melt probably became supersaturated in sanidine due to a substantial loss of water from both the peridotite and granite melts upon emplacement. Universal-stage 2VX measurements in the range 37-44° are consistent with a high-sanidine structure. Microprobe analyses show that the sanidine contains relatively high concentrations of Fe, Mg, and Si, and low Al. In contrast, relict K-feldspar has a more normal composition: MgO below detection limits and 0.04 wt.% FeO. The hollow sanidines, which contain more than three silica cations per eight oxygens, are, to the best of our knowledge, the most siliceous alkali feldspars ever analyzed.

Hollow calcic plagioclase has been reported from lunar maria and glassy mid-ocean ridge basalts, but, as far as we know, naturally occurring hollow alkali feldspar has not been reported previously. The morphology of both the hollow sanidine and hollow plagioclase is indicative of rapid growth from supersaturated melts. Unlike the lunar basalts and mid-ocean ridge basalts, loss of volatiles rather than heat probably induced supersaturation of the granitic melt. In contrast to the high Fe and Mg in hollow plagioclase, Fe and Mg in the hollow sanidine cannot be attributed to a Ca(Fe,Mg)Si3O8 component. To rationalize the sanidine analyses a K2 (Fe,Mg)Si7O16 feldspar component is proposed. If the proposed component is written in a more usual formulation based on 8 oxygens, it must be expressed as the sum of [K(Mg,Fe2+)Si3O8]- and [KSi4O8]+. Charge balance of the component requires (K,Na)+ occupancy of an octahedral site within a [Si4O8]0 structural unit to compensate for the excess negative charge left unbalanced by (K,Na)+ occupancy of an octahedral site within a [(Fe,Mg)Si3O8]2- unit.

The sanidine/granitic melt partition coefficient for Mg is estimated to be locally greater than five. The ratio of Mg in the sanidine to Mg in the mica peridotite is about 0.022. This value is close to previously reported feldspar/liquid Mg partition coefficients and implies that mica peridotite and granite melts achieved at least local metastable equilibrium.

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