Original paper
The calcioferrite group approved and kingsmountite redefined
Grey, Ian E.; Kampf, Anthony R.; Smith, Jason B.; Williams, Tim; MacRae, Colin M.
European Journal of Mineralogy Volume 31 Number 5-6 (2019), p. 1007 - 1014
17 references
published: Dec 20, 2019
published online: Jun 27, 2019
manuscript accepted: May 29, 2019
manuscript revision received: May 22, 2019
manuscript received: Apr 26, 2019
DOI: 10.1127/ejm/2019/0031-2872
Open Access (paper may be downloaded free of charge)
Abstract
The calcioferrite group has been formally approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (proposal 19-B). It comprises four minerals with C-centred monoclinic cells and general formula Ca4 A 2+ B 3+ 4(PO4)6(OH)4·12H2O, where A and B = Mg and Fe for calcioferrite, Mg and Al for montgomeryite, Mn and Fe for zodacite and Mn and Al for fanfaniite, together with the triclinic mineral kingsmountite, Ca3Mn2+Fe2+Al4(PO4)6(OH)4·12H2O. The minerals with B = Fe form the calcioferrite subgroup and those with B = Al form the montgomeryite subgroup. The triclinic member was recently approved as the mineral “aniyunwiyaite” (IMA2018-054). New measurements on the holotype specimen of kingsmountite, however, show that it has the same crystallographic and chemical properties as “aniyunwiyaite” and, consequently, “aniyunwiyaite” has been discredited as being kingsmountite. Kingsmountite is triclinic, P 1 ¯ , with a = 20.067(6) Å, b = 13.197(4) Å, c = 6.255(3) Å, α = 89.35(2)°, β = 91.21(2)°, γ = 112.20(2)°, V = 1533.4(10) Å3 and Z = 2. The structure was solved and refined to R obs = 0.059 for 4351 reflections with I > 3σ(I). The crystal structure is a superstructure of the C-centred monoclinic montgomeryite structure, having a doubled a m cell parameter. The superstructure results from ordering of octahedrally coordinated Mn in one of four independent 8-coordinated Ca sites, and ordering of A site cations, which in the Ca4 AB 4(PO4)6(OH)4·12H2O structures are statistically distributed over half-occupied sites.
Keywords
calcioferrite group • montgomeryite subgroup • kingsmountite • crystal structure • “aniyunwiyaite” discredited