Natroapophyllite, a new orthorhombic sodium analog of apophyllite I. Description, occurrence, and nomenclature

NatroapophyUite NaCa₄Si₈O₂₀F · 8H₂O, a sodium analog of apophyllite, has been found in white skarn at the Sampo Mine, Okayama, Japan. Electron microprobe analysis of the mineral gives SiO₂ 52.79, CaO 25.41, Na₂O 3.05, K₂O 0.33, F 2.27, H₂O (by difference) 17.11 weight percent; (−O≡F₂ = 0.96). Optical and crystallographic parameters are: α = 1.536(2), β = 1.538(2), γ = 1.544(2), 2V(meas.) = 32(1)°, r < v; a = 8.875(4), b = 8.881(6), c = 15.79(1)Å, space group Pnnm, Z = 2; D obs. = 2.50, D calc. = 2.30 g/cm³.

The crystal structure, refined (R = 0.056) by least-squares methods from 563 diffuse X-ray reflections collected with an automatic single-crystal diffractometer, has rings of SiO₄ tetrahedra. Ca- and Na-polyhedra are more distorted in natroapophyllite compared with apophyllite because of the substitution of Na for K. TGA shows discontinuous water loss of 16 weight percent, similar to apophyllite. DTA and IR data suggest that natroapophyllite has hydrogen atoms in three different structural environments.

A pseudo solid-solution series exists between natroapophyllite and fluorapophyllite which bridges the orthorhombic–tetragonal symmetry change. Crystals are shown which display continuous and discontinuous zoning.