Stoppaniite, a new member of the beryl group: crystal structure and crystal-chemical implications
Ferraris, Giovanni; Prencipe, Mauro; Rossi, Paolo
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 491 - 496
published: Jun 22, 1998
manuscript accepted: Jan 2, 1998
manuscript received: Aug 20, 1997
ArtNo. ESP147051003009, Price: 29.00 €
Abstract Stoppaniite, (Fe,Al,Mg)4[Be6Si12O36]·(H2O)2(Na,□)2, crystallizes in space group P6/mcc with a - 9.397(1), c - 9.202(2) Å, Z = 1, Dcalc = 2.817 g/cm3. An anisotropic least-squares refinement based on 367 independent reflections, collected by single crystal X-ray diffractometry (MoKa), converged to R = 0.049. In the  channels of the beryl-type structure the (0,0,1/4) and (0,0,0) positions are occupied by a water molecule and 50 % Na, respectively. About 0.5 excess Be p.f.u. substitutes mainly for Si and, in minor quantity, for octahedral cations. Bond length and valence analyses show that Fe is (mostly) bivalent and some OH substitutes for oxygen. A review of published chemical data for the relevant minerals shows that the beryl group can be defined with the general chemical formula (M3+,M2+)4[Be6Si12O36-z(OH)z]-(H2O)yAx, where members with dominant M = Al (beryl), Sc (bazzite) and Fe (stoppaniite) are known. Cation A (x presumably not larger than 1) is mainly Na, but also Cs; in the latter case the crystallographic positions of water (y ≤ 2) and A are inverted. Limited substitutions for Be (mainly Li) and, to a minor extent, for Si (mainly Be) are reported.