Clinoholmquistite discredited : the new amphibole end-member fluoro-sodic-pedrizite

Re-examination of holotype "clinoholmquistite", ideally A□ BLi2C(Mg 3Al2) TSi8 O 22X(OH)2 (Ginzburg 1965) from the Tastyg spodumene deposit, Tuva, Siberia, Russia by EMP and SIMS analysis and structure refinement shows that the sample consists of a mixture of two distinct amphibole compositions, tremolite and a new amphibole end-member, fluoro-sodic-pedrizite, ideally ANa BLi2C(Mg2Al 2Li) TSi8 O22XF2 (IMA-CNMMN 2004-002). Fluoro-sodic-pedrizite from Tastyg has the following crystal-chemical formula and unit-cell parameters : A(Na0.64K 0.01) B(Li1.93Ca 0.04Na0.03) MI(Mg 1.69Fe0.312+) M2(Al1.98Cr0.01Zn0.01) M3(Li0.64Fe0.212+Mg 0.13Mn0.02) TI(Si 3.96Al0.04) T2Si4 O 22X(F1.10OH0.90), a = 9.368(8), b = 17.616(10), and c = 5.271(4) Å, β = 102.38(4 )°, V = 849.6 Å3, Z = 2. The structure has been refined to Robs = 2.3% (I > 3σI) and R all = 3.8%. Refined site-scattering values and site-geometries were used, together with EMP and SIMS results, to obtain site populations. Fluoro-sodic-pedrizite is the first amphibole end-member with dominant CLi found in Fe-poor geologic environments. The coexisting tremolite contains only 0.002 wt% Li 2O and 0.06 wt% B2O3, probably ordered at the T1 site. Crystal-chemical arguments, as well as preliminary experimental work, suggest clinoholmquistite is unstable.