INSIGHTS INTO ASTROPHYLLITE-GROUP MINERALS. II. CRYSTAL CHEMISTRY

Abstract

The crystal chemistry of 20 specimens of astrophyllite-group minerals from seven localities has been studied using a combi-nation of single-crystal X-ray analysis of the structure and electron-microprobe analyses. Members of the astrophyllite group are triclinic (P ¯ 1) and monoclinic (C2/c or A2) heterophyllosilicates with the general formula A 2 BC 7 D 2 T 8 O 26 (OH) 4 X 0–1, where A = [10]–[13] K, Rb, Cs, Na, H 3 O + , H 2 O, or ; B = [10] Na or Ca; C = [6] Mn, Fe 2+ , Fe 3+ , Na, Mg, or Zn; D = [6] Ti, Nb, or Zr; T = Si and Al, and X = F, OH, O, or . The structure consists of two composite sheets, a heterogeneous (H) sheet and a sheet of octahedra (O), stacked along [001] in a 2:1 ratio. These 2:1 composite layers are bonded via a shared apical D 6 anion through an interlayer containing an ordered array of K and Na, resulting in a layered HOH structure reminiscent of the TOT structure in 2:1 phyllosilicates. There are four crystallographically distinct MO 6 octahedra in the O sheet; they host Mn, Fe 2+ , Fe 3+ , Na, Mg and Zn. The M(1) octahedron, the largest and least distorted within the O sheet, is host predominantly to Mn and Fe 2+ , with significant Na (up to 0.274 apfu). The M(2) octahedron is highly distorted and contains predominantly Mn and Fe 2+ , whereas the M(3) and M(4) octahedra are the smallest and are occupied by Mn, Fe 2+ , Fe 3+ , Mg and Zn. The H sheet consists of open-branched zweier [100] single [Si 4 O 12 ] 8– chains cross-linked by corner-sharing D 6 octahedra in triclinic species and kupletskite-Ma2b2c, and DO 5 polyhedra in magnesium astrophyllite. Distortion of the D 6 octahedron is minimized by (Nb,Zr) ⇔ Ti substitution, resulting in displacement of the cation toward the center of the octahedron. The composition of the [Si 4 O 12 ] 8– chains does not vary; there is essentially no Al, and Mössbauer spectroscopy did not indicate the presence of [4] Fe 3+ . Lateral fit of the O and H sheets is accomplished primarily by: (1) flattening of O-sheet octahedra, (2) tilting of the H-sheet tetrahedra out of the (001) plane, and (3) elongation or thickening of the tetrahedra along [001].