Ferriprehnite, Ca₂Fe³⁺(AlSi₃)O₁₀(OH)₂, an Fe³⁺ analogue of prehnite, from Kouragahana, Shimane Peninsula, Japan

Abstract

Ferriprehnite (IMA2020–057), ideal formula Ca₂Fe³⁺(AlSi₃)O₁₀(OH)₂, is a new mineral that was found as secondary mineral in druses developed in the hydrothermal altered dolerite from Kouragahana, Shimane Peninsula, Japan. Ferriprehnite is an Fe analogue of prehnite. The crystals consisting of ferriprehnite and prehnite occur as a radial aggregate. The tabular crystals are up to 300 µm long, 100 µm wide, and 50 µm thick. Ferriprehnite is colorless to pale green with white streak and vitreous luster. It has a Mohs hardness of 6½. Its cleavage is good on {100}. The calculated density is 2.97 g/cm³. The empirical formula of ferriprehnite on the basis of 10O + 2OH using the result obtained by electron microprobe analysis is Ca_1.99(Fe³⁺_0.66Al_0.34)_Σ1.00(Al_1.02Si_2.98)_Σ4.00O₁₀(OH)₂. Structure refinement converged to R1 = 4.64%. Its space group is orthorhombic Pma2 with unit–cell parameters a = 18.6149(10) Å, b = 5.4882(3) Å, c = 4.6735(3) Å, and V = 477.46(1) ų. The determined site occupancy at the octahedral M site is Fe_0.637(9)Al_0.363 indicating that the M site is predominantly occupied by Fe. <M–O> increases with increasing Fe content leading to isotropic expansion of MO₆ octahedra. The a– and c– dimensions of ferriprehnite are longer than those of prehnite due to Fe substitution for Al at the M site.