Geological Museum, Copenhagen

• Main Authors: Ole V. Petersen, Space Group, Belongs Morpho
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Subjects: Greenland; Triplets
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.601.3505; http://www.minersoc.org/pages/Archive-MM/Volume_42/42-322-251.pdf

logically to the crystal class 7~2m. The paper describes pseudotrigonal contact riplets recently found as freely developed individuals on two specimens donated to the Geological Museum, Copenhagen. The contact triplets, described as a symmetric twin, have the composition planes, which are also the twin planes, (io0 and (o I i); the pseudo-threefold axis, along which the crystals are pronouncedly elongate, is [II I]. The calculated re-entrant angles on the two sides of the pseudotrigonal prism, the protruding angle on the third side, and the 'central misfit', expressed as the geometrical ngles, are I78 ~ 4I', I8I ~ 2o', and 2 ~ 39 ' respectively. Tetragonal crystals forming pseudotrigonal twins are rare and it is emphasized that the pseudotrigonal habit has become apparent only because: the faces developed on individual I and on individuals II and III are not the same; and the proportion between the lengths of the traces of the faces (io i) individual I and (oi I) individuals II and III in a plane perpendicular tothe pseudo-threefold axis and the traces in the same plane of all other faces constituting the pseudo-threefold prism are 2: i. TUGTUPITE (NasAlzBe2SisO24(CI, S)2), dis-covered in i957 by Professor H. Sorensen at Tugtup agtak6rfia on the north coast of the Tunugdliarfik Fjord, South Greenland, was de-scribed in the reports of the International Geo-