THE ELUSIVE (◊P)(M+SI)-1 SUBSTITUTION IN POTASSIC ALKALI FELDSPAR

Although it is well known that feldspars comprise >50 vol% of most granitic rocks, it is less well known that feldspars may represent the dominant mineralogical host for phosphorus in the rocks. This is especially true in highly-fractionated, Ca-poor, S-type granites, for which P is usually accommodated in feldspar via the (AlP)Si_-2 (berlinite) substitution that becomes more effective with increasing aluminosity of magma. Morgan and London (2005: Contr. Miner. Petrol., 150, 456-471) observed that synthetic potassic alkali feldspars (Kfs) produced in strongly undercooled metaluminous granitic liquids may contain significant amounts of P accommodated by a (◊P)(M⁺Si)_-1 substitution (M⁺ = Na, K, H₃O). The latter substitution was not previously identified in natural feldspars, and was intuited to result in the experiments from the presence of alkaline, P-enriched boundary layers adjacent to crystals produced by very rapid feldspar growth. This suggests that the (◊P)(M⁺Si)_-1 substitution is likely to occur only in small, P-enriched granitic bodies that undergo significant liquidus undercooling. On this basis, Morgan and London (2005) re-evaluated previous data from feldspars in peraluminous granites and rhyolites and identified two occurrences in which P appeared to be accommodated in Kfs by the (◊P)(M⁺Si)_-1 substitution: the marginal BPF3 facies of the Beauvior granite, Massif Central, France, and pegmatites marginal to Li-mica granites of Cornwall district, England. We have re-investigated those feldspar compositions by electron microprobe using a more complete analytical routine that includes minor/trace components (e.g., Rb and Cs) that were not analyzed previously. The results show that all P in these feldspars is explained by the berlinite substitution and, hence, observation of (◊P)(M⁺Si)_-1 in natural feldspars remains elusive. Its absence in the analyzed feldspars may stem from very aluminous magmatic compositions, suggesting this substitution may be more likely to occur in metaluminous to alkaline systems. It is also probable, however, that non-ideality associated with this substitution makes it energetically prone to obliteration by subsolidus recrystallization that may involve little or no change in feldspar morphology.