No abstract

A new mineral of the beryl group, johnkoivulaite, Cs(Be 2 B)Mg 2 Si 6 O 18 , was recovered from the gem gravels of the Pein Pyit area of the Mogok region in Myanmar. Thus far, only a single crystal has been identified. It has dimensions 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 cm -3 . 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 )O 18 with Be calculated by stoichiometry and confirmed by LA-ICP-MS measurements. Johnkoivulaite is hexagonal, P6/mmc (#192) with a = 9.469(2), c = 9.033(2) Å, V = 701.5(3) Å 3 , 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 Si 6 O 18 rings. This substitution could be written as (CsMg 2 B)(□Al 2 Be) -1 . Johnkoivulaite, the 7 th member of the beryl group, is named in honor of gemologist John Koivula in recognition of his contributions to mineralogy and gemology.