Vimsite from the Fuka mine, Okayama Prefecture, Japan

Abstract

Vimsite was found as a mass or a veinlet in crystalline limestone associated with gehlenite–spurrite skarns at the Fuka mine, Okayama Prefecture, Japan. Vimsite occurs as colorless to white aggregates of anhedral or prismatic transparent crystals up to 1 mm in length in association with shimazakiite, sibirskite, priceite, uralborite, calciborite, kurchatovite, and calcite. It is also formed as a 0.5 mm wide veinlet. An electron microprobe analysis of vimsite gave an empirical formula (Ca_0.993Mg_0.009Fe_0.003Mn_0.001)_Σ1.006B_1.996O₂(OH)₄ based on O = 6. The unit cell parameters are a = 10.021(3), b = 9.566(4), c = 4.447(2) Å, β = 91.231 (9)°. The calculated density is 2.523 g cm⁻³. The Vickers microhardness is 186–206 kg mm⁻² (25 g load). It is likely that vimsite from the Fuka mine was formed by subsequent hydrothermal alteration, as with uralborite and priceite, of sibirskite produced by the hydrothermal alteration of shimazakiite.