Publication Date:
2018
Description:
〈div data-abstract-type="normal"〉〈p〉Dargaite, ideally BaCa〈span〉12〈/span〉(SiO〈span〉4〈/span〉)〈span〉4〈/span〉(SO〈span〉4〈/span〉)〈span〉2〈/span〉O〈span〉3〈/span〉, is an additional member of the arctite group belonging to minerals with a modular intercalated antiperovskite structure derived from hatrurite. The holotype specimen was found at a small outcrop of larnite pseudoconglomerates in the Judean Mts, West Bank, Palestinian Autonomy. Larnite, fluorellestadite–fluorapatite, brownmillerite, fluormayenite–fluorkyuygenite and ye'elimite are the main minerals of the holotype specimen; ternesite, shulamitite and periclase are noted rarely. Dargaite, nabimusaite and gazeevite occur in linear zones with higher porosity within larnite rocks. Pores are filled with ettringite and Ca-hydrosilicates, less commonly with gibbsite, brucite, baryte, katoite and calciolangbeinite. Dargaite is colourless, transparent with a white streak and has a vitreous lustre. It exhibits pronounced parting and imperfect cleavage along (001). Mohs’ hardness is ~4.5–5.5. The empirical formula is (Ba〈span〉0.72〈/span〉K〈span〉0.24〈/span〉Na〈span〉0.04〈/span〉)〈span〉Σ1〈/span〉(Ca〈span〉11.95〈/span〉Mg〈span〉0.04〈/span〉Na〈span〉0.01〈/span〉)〈span〉Σ12〈/span〉([SiO〈span〉4〈/span〉]〈span〉0.91〈/span〉 [PO〈span〉4〈/span〉]〈span〉0.05〈/span〉[AlO〈span〉4〈/span〉]〈span〉0.03〈/span〉[Ti〈span〉4+〈/span〉O〈span〉4〈/span〉]〈span〉0.01〈/span〉)〈span〉Σ4〈/span〉([SO〈span〉4〈/span〉]〈span〉0.84〈/span〉[PO〈span〉4〈/span〉]〈span〉0.14〈/span〉[CO〈span〉3〈/span〉]〈span〉0.02〈/span〉)〈span〉Σ2〈/span〉(O〈span〉2.54〈/span〉F〈span〉0.46〈/span〉)〈span〉Σ3〈/span〉. Dargaite is trigonal 〈span〉R〈/span〉〈span〉〈span〉〈img data-mimesubtype="gif" data-type="simple" src="http://static.cambridge.org/resource/id/urn:cambridge.org:id:binary:20190313104343836-0311:S0026461X18001019:S0026461X18001019_inline1.gif"〉
〈span data-mathjax-type="texmath"〉
〈/span〉
〈/span〉〈/span〉〈span〉m〈/span〉, the unit-cell parameters are: 〈span〉a〈/span〉 = 7.1874(4) Å, 〈span〉c〈/span〉 = 41.292(3) Å, 〈span〉V〈/span〉 = 1847.32(19) Å〈span〉3〈/span〉 and 〈span〉Z〈/span〉 = 3. The crystal structure of dargaite was refined from X-ray single-crystal data to 〈span〉R〈/span〉〈span〉1〈/span〉 = 3.79%. The calculated density is 3.235 g cm〈span〉–3〈/span〉. The following main Raman bands are distinguished on the holotype dargaite (cm〈span〉–1〈/span〉): 122, 263, 323, 464, 523, 563, 641 and 644, 829 and 869, 947, 991 and 1116. The formation conditions of dargaite are linked to the local occurrence of pyrometamorphic by-products (gases, fluids and melts) transforming earlier mineral associations at ~900°C.〈/p〉〈/div〉
Print ISSN:
0026-461X
Electronic ISSN:
1471-8022
Topics:
Geosciences
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