Johnkoivulaite, Cs(Be₂B)Mg₂Si₆O₁₈, a new mineral of the beryl group from the gem deposits of Mogok, Myanmar

Abstract

A new mineral of the beryl group, johnkoivulaite, Cs(Be₂B)Mg₂Si₆O₁₈, was recovered from the gem gravels in the Pein Pyit area of the Mogok region in Myanmar. Thus far, only a single crystal has been identified. It has dimensions of about 5.8 × 5.7 × 5.5 mm. This specimen has an irregular shape but still has discernible crystal form with geometric growth patterns observed on the crystal faces. The crystal of johnkoivulaite is grayish-violet in color and strongly pleochroic, going from nearly colorless with E┴c to dark bluish-violet with E||c. Johnkoivulaite has a Mohs hardness of about 7½ and a measured density of 3.01(10) g/cm³. It is uniaxial (–) with ω = 1.607(1) and ε = 1.605(1) (white light). Electron microprobe analyses gave the empirical formula of (Cs_(0.85)K_(0.10)Na_(0.01))(Be_(1.88)B_(1.12))(Mg_(1.66)Fe_(0.27)Mn_(0.01)Al_(0.05)) (Si_(5.98))O₁₈ with Be calculated by stoichiometry and confirmed by LA-ICP-MS measurements. Johnkoivulaite is hexagonal, P6/mmc (no. 192) with a = 9.469(2), c = 9.033(2) Å, V = 701.5(3) ų, and Z = 2. Johnkoivulaite is isostructural with beryl and exhibits partial substitution of B for Be at the distorted tetrahedral site, Mg for Al at the octahedral site, and Cs in the channel sites within the stacked Si₆O₁₈ rings. This substitution can be written as (CsMg₂B)(☐Al₂Be)₋₁. Johnkoivulaite, the seventh member of the beryl group, is named in honor of gemologist John Koivula in recognition of his contributions to mineralogy and gemology.