Crystal Structure and Microtwinning of the New Mineral Clinobarylite BaBe 2 Si 2 O 7

Abstract

Beryllium is a rare lithophilic element, which is most typical of acidic and alkaline rocks. The number of minerals of this element in the Khibiny–Lovozero complex is rather large. A total of 16 beryllium minerals have already been found in this complex. We discovered a new beryllium barium silicate in pegmatite veins at Yukspor Mountain of the Khibiny massif in the form of colorless, transparent, flattened prismatic crystals (which are often split) up to 2 cm in length. The crystals show perfect cleavage along the (100) plane and less perfect cleavage along the (001) and (101) planes. The mineral is optically biaxial and positive, with N g = 1.705(5); N p = 1.698(3), N m = 1.700(3) , and 2 V = 70(10)° . The chemical composition (wt %), which was determined by local X-ray spectral analysis (the Be content was determined by atomic emission spectral analysis), is as follows: BaO, 47.66; SiO 2 , 36.38 ; BeO, 14.90. The empirical formula (for 7 O atoms) can be written as Ba 1.03 Be 1.97 Si 2.00 O 7.00 . The new mineral is a monoclinic polymorph of orthorhombic barylite BaBe 2 Si 2 O 7 and was recognized as a new mineral type under the name “clinobarylite” by the Commission on New Minerals and Mineral Names of the International Mineralogical Association. Both modifications of BaBe 2 Si 2 O 7 were discovered in differentiates of highly alkaline rocks.