Minasgeraisite-(Y) discredited as an ordered intermediate between datolite and hingganite-(Y)

Abstract

AbstractMinasgeraisite-(Y) is discredited on the basis of it being an ordered intermediate between datolite and hingganite-(Y) (IMA-CNMNC Proposal 23-F). An idealised formula is (Ca2Y2)□2(Be2B2)Si4O16(OH)4, which corresponds to Ca2□B2Si2O8(OH)2 (datolite) + Y2□Be2Si2O8(OH)2 (hingganite-(Y)). The type material is rich in Bi, the Bi-richest portion yet discovered from the type locality is shown to be an intermediate member between datolite, hingganite-(Y) and a hypothetical end-member phase yet to be found of composition Bi2□Be2Si2O8(OH)2. Minasgeraisite-(Y) has a different space group to datolite and hingganite-(Y). This lowering of symmetry to an acentric triclinic system is caused by different element occupancies on the A site of the gadolinite supergroup structure, which for minasgeraisite-(Y) becomes four individual sites. Such an order–disorder of elements is not considered as species-defining criteria despite the change in space group. Therefore, minasgeraisite-(Y) is discredited.