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 T (sub 4) O (sub 12) 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 (sub 7) apfu). In the astrophyllite group (HOH blocks connect via D-X (super p) (sub D) -D bridges, Fe (super 2+) is dominant at C (sub 7) ), there are six minerals: astrophyllite, niobophyllite, zircophyllite, tarbagataite, nalivkinite and bulgakite. In the kupletskite group (HOH blocks connect via D-X (super p) (sub D) -D bridges, Mn (super 2+) is dominant at C (sub 7) ), there are three minerals: kupletskite, niobokupletskite and kupletskite-(Cs). In the devitoite group (HOH blocks do not connect via D-X (super p) (sub D) -D bridges), there are three minerals: devitoite, sveinbergeite and lobanovite. The general formula for the astrophyllite-supergroup minerals is of the form A (sub 2p) B (sub r) C (sub 7) D (sub 2) (T (sub 4) O (sub 12) ) (sub 2) IX (super O) (sub D2) X (super O) (sub A4) X (super p) (sub Dn) W (sub A2) , where C [cations at the M(1-4) sites in the O sheet] = Fe (super 2+) , Mn, Na, Mg, Zn, Fe (super 3+) , Ca, Zr, Li; D (cations in the H sheets) = (super [6,5]) Ti, Nb, Zr, Sn (super 4+) , (super [5]) Fe (super 3+) , Mg, Al; T = Si, minor Al; A (sub 2p) B (sub r) IW (sub A2) (I block) where p = 1,2; r = 1,2; A = K, Cs, Ba, H (sub 2) O, Li, Rb, Pb (super 2+) , Na, ; B = Na, Ca, Ba, H (sub 2) O, ; I represents the composition of the central part of the I block, excluding peripheral layers of the form A (sub 2p) B (sub r) W (sub A2) , e.g. (PO (sub 4) ) (sub 2) (CO (sub 3) ) (devitoite); X (super O) (sub D) = O; X (super O) (sub A) = OH, F; X (super p) (sub D) = F, O, OH, H (sub 2) O, , where n = 0, 1, 2 for (X (super p) (sub D) ) (sub n) ; W (sub A) = H (sub 2) O, .