Charoite

Abstract: Charoite, ideally (K,Sr,Ba,Mn) 15–16 (Ca,Na) 32 [(Si 70 (O,OH) 180 )](OH,F) 4 · n H 2 O, is a rock-forming mineral from the Murun massif in Yakutia, Sakha Republic, Siberia, Russia, where it occurs in a unique alkaline intrusion. Charoite occurs as four different polytypes, which are commonly intergrown in nanocrystalline fibres. We report the structure of charoite-96 ( a = 32.11(6), b = 19.77(4), c = 7.23(1) A, β = 95.85(9)°, V = 4565(24) A 3 , space group P 2 1 / m ), which was solved ab initio by direct methods on the basis of 2676 unique electron diffraction reflections collected by automated diffraction tomography and refined to R 1 / wR 2 = 0.34/0.37. The structure of charoite-96 is related to that of the charoite-90, which was also solved recently. Both structures are composed of three different types of dreier silicate chains running along [001] and separated by ribbons of edge-sharing Ca- and Na-centred octahedra. In the structure of charoite-96, adjacent blocks formed by three different silicate chains and stacked along the x axis, are shifted by a translation of ½ c . The shifts involve a hybrid dreier quadruple chain, [Si 17 O 43 ] 18− and a double dreier chain, [Si 6 O 17 ] 10− . In charoite-90 adjacent blocks are stacked without shifts.

Abstract: Сharoitite consists of gem-quality mineral charoite and subordinate quartz, aegirine, K-feldspar, tinaksite, canasite, and some other minerals. This rock type is known only from one locality in the world associated with the Early Cretaceous (131.3 ± 2.4 Ma, K–Ar age) Malyy Murun syenite massif, Siberia, Russia. Although charoitite mineralogy is well known, there is disagreement whether it reflects metasomatic or magmatic activity. In order to understand when the charoitites formed we attempted to date it by 40Ar/39Ar incremental step-heating and laser ablation techniques. Our results show that the fibrous structure of water-bearing charoite does not retain radiogenic argon. Laser ablation 40Ar/39Ar for K-feldspar and tinaksite from the charoitite yielded several age clusters even from the same mineral grain. The oldest cluster of 134.1 ± 2.9 Ma for the K-feldspar agrees with the age of the Malyy Murun syenites. The youngest age of 113.3 ± 3.4 Ma for charoitite K-feldspar overlaps with the youngest of publ...