“Anorthoclase”, an intermediate member of the high-albite–sanidine alkali feldspar solid solution, is a major component in trachytic and phonolitic lavas. Mount Erebus volcano is well known for its anorthoclase megacrysts which show a range of twin morphologies. Thanks to a set of more than 400 specimens collected by volcanologist Haroun Tazieff, we have revisited and improved the knowledge about the twins from this locality, and documented the crystals’ shapes, sizes and frequencies. The status of the X Carlsbad B twin is revised. Several twins, not yet mentioned from this locality, are identified. Two new types of twins are described and referred to as Erebus and Tazieff twins. The spectacular Erebus twin associates two Carlsbad B twins or one Carlsbad B twin and one single crystal; it is a normal twin with (02) as composition plane. The 52° V shaped Tazieff twin is the first example of the corresponding parallel twin of the Manebach twin, with the  axis and a composition plane close to (02).
Research Article|October 12, 2018
Twinning in “anorthoclase” megacrysts from phonolitic eruptions, Erebus volcano, Antarctica
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Jean-Claude Boulliard, Eloïse Gaillou; Twinning in “anorthoclase” megacrysts from phonolitic eruptions, Erebus volcano, Antarctica. European Journal of Mineralogy doi: https://doi.org/10.1127/ejm/2019/0031-2804
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