Synthetic tourmaline (olenite) with excess boron replacing silicon in the tetrahedral site: I. Synthesis conditions, chemical and spectroscopic evidence
Schreyer, Werner; Wodara, Ulrich; Marler, Bernd; Van Aken, Peter A.; Seifert, Friedrich; Robert, Jean-Louis
European Journal of Mineralogy Volume 12 Number 3 (2000), p. 529 - 541
published: May 31, 2000
manuscript accepted: Jan 11, 2000
manuscript received: Oct 23, 1999
ArtNo. ESP147051203002, Price: 29.00 €
Abstract One selected composition within the system Na2O-Al2O3-B2O3-SiO2-H2O (NABSH) was studied with the aim of synthesizing the new tourmaline end member olenite with the theoretical formula NaAl3Al6 [Si6O18] (BO3)3O3(OH). The starting material consisted of a gel with the anhydrous composition 0.625Na2O·4.5Al2O3·6SiO2, but with 100 % excess B2O3 added over that of the above formula, and a surplus of water to aid crystallization. Run conditions ranged from 4 to 50 kbar, 400-900°C, but a tourmaline phase could only be obtained at or above 10 kbar, with yields between 80 and 100 % when ignoring amorphous quench products from the coexisting fluids. The synthetic olenites exhibit much smaller cell parameters than those reported for natural olenites, and indeed the smallest ones ever measured for any tourmaline phase. Analytical data on an olenite prepared at 25 kbar, 600°C, show excess boron and water relative to the theoretical formula, coupled with deficiencies in Si, Al, and Na. Spectroscopic investigations (MAS NMR, EELS, IR) prove - directly or indirectly - that boron occurs not only in trigonal coordination, but is also located in the tetrahedral ring site. Thus, a provisional structural formula is derived as (Na0.65□0.35) (Al2.72□0.28) (Al5.42Si0.58) [Si3.73B2.27O18] (BO3)3 (OH)3.87 O0.13. In this synthetic olenite the OH-content is close to the maximum of 4.0 p.f.u.; boron replaces tetrahedral Si according to BHSi-1, with this substitution cancelling the proton deficiency of the theoretical olenite formula. Because the sum (B+Si) exceeds 9.0, some Si seems to replace octahedral Al. Nevertheless, octahedral vacancies remain. There are indications that the above tourmaline composition is not unique for the system studied, but that a range of olenite solid solutions exists as a function of starting material and run conditions, possibly extending to the ideal olenite formula. Excess-boron tourmalines are probably confined to very Al-rich (or M3+-rich) compositions which - for stoichiometric reasons - should have proton deficiencies, but these may be compensated by the BHSi-1 substitution.