A CO3-bearing member of the hydroxylapatite–hydroxylellestadite series occurs as a Si-rich part of a hydroxylapatite–hydroxylellestadite series mineral (hydroxylapatitess) in a skarn xenolith from Tadano, Fukushima Prefecture, Japan. Hydroxylapatitess is composed of Si-poor and Si-rich parts. The Si-poor part is F-rich hydroxylapatite. The infrared spectrum of the Si-rich part demonstrates the presence of B-type CO3. A representative analysis of the Si-rich part yielded the empirical formula Ca4.989[(PO4)1.315(SiO4)0.848(SO4)0.368(CO2.943)0.480]Σ3.011(OH0.629Cl0.264F0.107)Σ1.000 on the basis of 8 cations, assuming Si = S + C. There is a very strong inverse correlation between Si and P in both the Si-poor and Si-rich parts. These data indicate that the substitution mechanism (SO4,CO3)2– + (SiO4)4– = 2(PO4)3– probably occurs in hydroxylapatitess. Therefore, it was concluded that the Si-rich part corresponds to a CO3-bearing hydroxylapatite–hydroxylellestadite series mineral with CO3 partially replacing SO4. Most of the Si-rich part shows CO3 > SO4 and its composition approaches Ca5(SiO4)1.5(CO3,SO4)1.5(OH,Cl,F).

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