Abstract
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, M1Mn2M2Mn3(PO4)3XCl, 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 M1(Mn,Ca)2M2(Mn,Ca)3(PO4)3X(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: M1Ca2M2Mn3(PO4)3XCl and M1Mn2M2Ca3(PO4)3XOH. 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.
Highlights
The apatite supergroup includes minerals with the generic formula IX M12 VII–IX M23 (TO4 )3 X.The two distinct M sites may incorporate a wide range of cations—Ca2+, Pb2+, Ba2+, Sr2+, Mn2+, Na+, Ln3+, Y3+, Bi3+ ; while T = P5+, As5+, V5+, Si4+, S6+, B3+ ; and X = F−, OH−, Cl−, and O2−
Minerals 2018, 8, 350 dominated by the same cation; T = P, V or As), hedyphane group (M sites dominated by different cations; T = P, As or S), belovite group (M1 site split into distinct M1 and M1’ sites dominated by different cations; M2 = Sr; T = P), britholite group
Apatites from Szklary that approach or attain the stoichiometry of pieczkaite (Types 1,2 apatites of this study) have compositions defined by two independent substitutions of Mn for Ca and Cl for OH
Summary
The apatite supergroup includes minerals with the generic formula IX M12 VII–IX M23 (TO4 ) X. The most common among the apatite-supergroup minerals are the calcium phosphates, with the general formula Ca2 Ca3 (PO4 ) (F,Cl,OH), a group that includes three distinct species of fluor-, chlor-, and hydroxylapatites with anion solid solutions between them [2,3]. In this contribution, the term apatites will be used to denote such phosphates with unlimited degrees of Mn↔Ca and. Species names will be applied when referring to specific compositions
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