Abstract

Re-examination of holotype “clinoholmquistite”, ideally ABLi2C(Mg3Al2) TSi8 O22X(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(Mg2Al2Li) TSi8 O22XF2 (IMA-CNMMN 2004-002). Fluoro-sodic-pedrizite from Tastyg has the following crystal-chemical formula and unit-cell parameters: A(Na0.64K0.01) B (Li1.93Ca0.04Na0.03) M1 (Mg1.69Fe2+0.31) M2 (Al1.98Cr0.01Zn0.01) M3 (Li0.64Fe2+0.21Mg0.13Mn0.02) T1 (Si3.96Al0.04) T2 Si4 O22X (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 Rall = 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% Li2O and 0.06 wt% B2O3, probably ordered at the T1 site. Crystal-chemical arguments, as well as preliminary experimental work, suggest clinoholmquistite is unstable.

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