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Fluoronyböite from Jianchang (Su-Lu, China) and nyböite from Nybö (Nordfjord, Norway): a petrological and crystal-chemical comparison of these two high-pressure amphiboles

¹ CNR-Istituto di Geoscienze e Georisorse, sezione di Pavia, I-27100 Pavia, Italy
² Centro Grandi Strumenti, Università degli Studi di Pavia, I-27100 Pavia, Italy
³ Muséum National d’Histoire Naturelle, Bâtiment Minéralogie, F-75005 Paris, France

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

Fluoronyböite, ideally NaNa₂(Al₂Mg₃)(Si₇Al)O₂₂F₂, has been found in the Jianchang eclogite pod, Su-Lu coesite-eclogite province, China. It has been approved as a new mineral by the IMA. Single-crystal structure refinement and electron microprobe analysis were used for characterization: C2/m, with a = 9.666(4), b = 17.799(6), c = 5.311(2) Å, ß = 104.10(3)°, V = 886.2(8) ų, Z = 2, formula: ^A(Na_0.78K_0.06) _0.84 ^B(Na_1.53Ca_0.47) _2.00 ^C(Fe²⁺_0.89Mg_2.55Mn_0.01Zn_0.01Fe_0.32³⁺Al_1.21Ti_0.01) _5.00 T(Si_7.14A_l0.86) _8.00O₂₂^X(OH_0.84F_1.16) _2.00.

Fluoronyböite formed during UHPM conditions, and is preserved in the retrograded kyanite-bearing eclogite sample DJ102 together with clinopyroxene (Jd₇₀Ae₂₀Di₁₀), garnet (Alm₆₀Prp₂₁Grs₁₇Sps₀₂), and rutile. Lower-pressure minerals are also present (fluoro-alumino-magnesiotaramite, apatite, paragonite), and symplectitic rims were also developed around clinopyroxene crystals. Cation ordering and the structural and physical properties of fluoronyböite are reported and discussed with reference to those of F-free nyböite from the type locality at Nybö in Norway, for which some as yet unpublished mineral data are also reported. Relations between composition and petrogenetic conditions of these rare high-pressure amphiboles are discussed.

KEYWORDS: fluoronyböite, nyböite, amphibole, structure refinement, UHPM metamorphism

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