Chemical Composition of Mn- and Cl-Rich Apatites from the Szklary Pegmatite, Central Sudetes, SW Poland: Taxonomic and Genetic Implications

摘要

Although calcium phosphates of the apatite group ( apatites) with elevated contents of Mn are common accessory minerals in geochemically evolved granitic pegmatites, their Mn-dominant analogues are poorly studied. Pieczkaite, M 1 Mn 2 M 2 Mn 3 (PO 4 ) 3 X Cl, is an exceptionally rare Mn analogue of chlorapatite known so far from only two occurrences in the world, i.e., granitic pegmatites at Cross Lake, Manitoba, Canada and Szklary, Sudetes, SW Poland. In this study, we present the data on the compositional variation and microtextural relationships of various apatites highly enriched in Mn and Cl from Szklary, with the main focus on compositions approaching or attaining the stoichiometry of pieczkaite ( pieczkaite-like apatites ). The main goal of this study is to analyze their taxonomical position as well as discuss a possible mode of origin. The results show that pieczkaite- like apatites represent the Mn-rich sector of the solid solution M 1 (Mn,Ca) 2 M 2 (Mn,Ca) 3 (PO 4 ) 3 X (Cl,OH). In the case of cation-disordered structure, all these compositions represent extremely Mn-rich hydroxylapatite or pieczkaite. However, for cation-ordered structure, there are also intermediate compositions for which the existence of two hypothetical end-member species can be postulated: M 1 Ca 2 M 2 Mn 3 (PO 4 ) 3 X Cl and M 1 Mn 2 M 2 Ca 3 (PO 4 ) 3 X OH. In contrast to hydroxylapatite and pieczkaite, that are members of the apatite-group, the two hypothetical species would classify into the hedyphane group within the apatite supergroup. The pieczkaite- like apatites are followed by highly Mn-enriched fluor- and hydroxylapatites in the crystallization sequence. Mn-poor chlorapatites, on the other hand, document local contamination by the serpentinite wall rocks. We propose that pieczkaite- like apatites in the Szklary pegmatite formed from small-volume droplets of P-rich melt that unmixed from the LCT-type (Li–Cs–Ta) pegmatite-forming melt with high degree of Mn-Fe fractionation. The LCT melt became locally enriched in Cl through in situ contamination by wall rock serpentinites.