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


Aaron C. Palke1, Lawrence M. Henling2, Chi Ma3George R. Rossman3, Ziyin Sun1, Nathan Renfro1, Anthony R. Kampf4, 
Kyaw Thu
5, Nay Myo6, Patcharee Wongrawang7, Vararut Weeramonkhonlert7  

 
1 Gemological Instutute of America
  Carlsbad, CA  92008, USA  
2 Beckman Institute
   Pasadena, CA  91125, USA 
   
Division of Geological and Planetary Sciences  
California Institute of Technology
Pasadena, CA  91125-2500, USA
4Mineral Sciences Department
Natural History Museum of Los Angeles County 
Los Angeles, CA  90007,  USA
5 S Gemmological Institute
Mogok 11201, Myanmar
6 Greatland Gems and Jewelry
Mogok 11101, Myanmar

Gemological Institute of America
Bangkok 10500,  Thailand


ABSTRACT

A new mineral of the beryl group, johnkoivulaite, Cs(Be2B)Mg2Si6O18, 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 5.85.75.5 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 7 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 (Cs0.85K0.10Na0.01)(Be1.88B1.12)(Mg1.66Fe0.27Mn0.01Al0.05)(Si5.98)O18 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) , V = 701.5(3) 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 Si6O18 rings. This substitution could be written as (CsMg2B)(Al2Be)-1. Johnkoivulaite, the 7th member of the beryl group, is named in honor of gemologist John Koivula in recognition of his contributions to mineralogy and gemology.