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Formula: KNa3(Al4Si12)O32
Tectosilicate (framework silicate)
Crystal system: Hexagonal
Environments
Metamorphic igneous environments
Localities
At the type locality, Pfaffenberg hill, Waldheim, Mittelsachsen, Saxony, Germany, pfaffenbergite is found in
nanogranitoids included in
garnets from eclogites.
Pfaffenbergite is isostructural with kokchetavite and
wodegongjieite, both sheet silicates with
feldspar chemical composition, and pfaffenbergite corresponds chemically
to a K-Na feldspar. It is suggested that it formed as a result of melt
crystallisation during cooling, after entrapment within metamorphic garnets. The
pressure-temperature conditions of formation of pfaffenbergite must have been below the P-T conditions of
entrapment of the inclusions, namely 1000 to 1050°C and 2.2 to 4.5 GPa.
Pfaffenbergite is interpreted as a metastable phase crystallising rapidly in a silicate melt enclosed in a small
pore under non-equilibrium conditions. The increasing number of findings of metastable phases in the last years suggests
that these minerals are more common than expected, and it is even possible they may represent rock-forming minerals in
natural rocks that experienced rapid cooling/rapid crystallisation, for instance,
lavas and ignimbrites, along
with experimental products involving silicate melts
(AM 111.1.60–72).
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