Fluoronyböite from Jianchang (Su-Lu, China) and nyböite from Nybö (Nordfjord, Norway): description and comparison of two high-pressure amphibole end-members.

Fluoronybo¨ ite, 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)AÊ , b = 104.10(3)º, V = 886.2(8)AÊ 3, Z = 2, formula: A(Na0.78K0.06)S0.84 B(Na1.53Ca0.47)S 2.00 C(Fe2+ 0.89Mg2.55Mn0.01Zn0.01Fe3+ 0.32Al1.21Ti0.01)S 5.00 T(Si7.14Al0.86)S8.00O22 X(OH0.84F1.16)S2.00. Fluoronybo¨ ite 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 fluoronybo¨ ite are reported and discussed with reference to those of F-free nybo¨ ite from the type locality at Nybo¨ in Norway, for which some as yet unpublished mineral data are also reported. Relations between composition and petrogenetic conditions of these rare highpressure amphiboles are discussed.