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

CNR - Istituto di Geoscienze e Georisorse, Via Ferrata 1, I-27100 Pavia, Italy

Correspondence: ^* E-mail: oberti{at}crystal.unipv.it

Re-examination of holotype "clinoholmquistite", ideally ^ABLi₂ ^C(Mg₃Al₂) ^TSi₈ O₂₂ ^X(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 ^BLi₂ ^C(Mg₂Al₂Li) ^TSi₈ O₂₂ ^XF₂ (IMA-CNMMN 2004-002). Fluoro-sodic-pedrizite from Tastyg has the following crystal-chemical formula and unit-cell parameters: ^A(Na_0.64K_0.01) ^B (Li_1.93Ca_0.04Na_0.03) ^M1 (Mg_1.69Fe²⁺_0.31) ^M2 (Al_1.98Cr_0.01Zn_0.01) ^M3 (Li_0.64Fe²⁺_0.21Mg_0.13Mn_0.02) ^T1 (Si_3.96Al_0.04) ^T2 Si₄ O₂₂ ^X (F_1.10OH_0.90), a = 9.368(8), b = 17.616(10), and c = 5.271(4) Å, ß = 102.38(4)°, V = 849.6 ų, Z = 2. The structure has been refined to R_obs = 2.3% (I > 3_I) 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% 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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