Occurrence and new data of dellaite from the Akagane mine, Japan

Abstract

A new occurrence of dellaite, a very rare mineral, at the Akagane mine, Japan, was confirmed and its chemical composition and XRD data for the natural material are reported for the first time in this paper. The mineral occurs in the high-temperature skarn of bicchulite and vesuvianite as a veinlet or interstitial constituent. Results of electron microprobe analysis indicated that the chemical composition of dellaite is almost identical to Ca₆Si₃O₁₁(OH)₂, and only a small amount of chlorine substitutes (OH). The six strongest lines in the powder XRD pattern are as follows: 3.07 (53) 0-22, 2.99 (38) 022, 2.86 (44) -2-11, 2.82 (100) 20-3, 2.55 (51) 005, and 2.29 (38) 030. The lattice parameters refined from the powder XRD data are as follows: a = 6.815(4), b = 6.937(3), and c = 12.890(6) Å; α = 90.71(4), β = 97.68(4), γ = 98.20(4)°, and V = 597.4(5) ų. The indices are α = 1.655 and γ = 1.664, and optically biaxial; negative with 2V = large (>70°). Dellaite appears to be a later-stage hydration product of pre-existing skarn mineral(s) under a silica-deficient environment.