The astrophyllite supergroup: nomenclature and classification

Here we report a nomenclature and classification for the astrophyllite-supergroup minerals. The HOH block is the main structural unit in all astrophyllite-supergroup structures; it consists of three H-O-H sheets where the T4O12 astrophyllite ribbons occur in the H sheets. In each structure, HOH blocks alternate with I (Intermediate) blocks along [001]. The twelve minerals of the astrophyllite supergroup are divided into three groups based on (1) the type of self-linkage of HOH blocks, i.e. (a) HOH blocks link directly where they share common vertices of D octahedra, or (b) HOH blocks do not link directly; and (2) the dominant cation of the O sheet (the C group: C-7 apfu). In the astrophyllite group (HOH blocks connect via D-X-D(P)-D bridges, Fe2+ is dominant at C-7), there are six minerals: astrophyllite, niobophyllite, zircophyllite, tarbagataite, nalivkinite and bulgakite. In the kupletskite group (HOH blocks connect via D-X-D(P)-D bridges, Mn2+ is dominant at C-7), there are three minerals: kupletskite, niobokupletskite and kupletskite-(Cs). In the devitoite group (HOH blocks do not connect via D-X-D(P)-D bridges), there are three minerals: devitoite, sveinbergeite and lobanovite. The general formula for the astrophyllite-supergroup minerals is of the form A(2p)B(r)C(7)D(2)(T4O12)(2)(IXD2XA4XDnWA2)-X-O-X-O-W-P, where C [cations at the M(1-4) sites in the O sheet] = Fe2+, Mn, Na, Mg, Zn, Fe3+, Ca, Zr, Li; D (cations in the H sheets) = Ti-[6,Ti-5], Nb, Zr, Sn4+, Fe-[5](3+), Mg, Al; T = Si, minor Al; A(2p)B(r)IW(A2) (I block) where p = 1,2; r = 1,2; A = K, Cs, Ba, H2O, Li, Rb, Pb2+, Na,square; B = Na, Ca, Ba, H2O,square; I represents the composition of the central part of the I block, excluding peripheral layers of the form A(2p)B(r)W(A2), e.g. (PO4)(2)(CO3) ( devitoite); X-D(O)= O; X-A(O) = OH, F; X-D(P) = F,O,OH,H2O,square, where n = 0, 1, 2 for (X-D(P))(n); W-A = H2O,square.