EPIDIDYMITE TWINS - CONTRIBUTION TO THE MINERALOGY OF ILIMAUSSAQ, NO-99

FLINK's (1893) first description of epididymite from the type locality , the Narssarssuk Pegmatite, South Greenland, mentioned the presence o f pseudohexagonal trillings. Subsequent finds and descriptions of epid idymite from several localities including the Langesund area, Norway; Khibiny and Lovozero, Kola Peninsula, Russia; Mont Saint-Hilaire, Queb ec, Canada, and Zomba-Malosa, Malawi, have documented that the formati on of pseudohexagonal trillings is a characteristic feature of this sp ecies. All the trillings described so far are habitually very similar, forming stubby, distinctly pseudohexagonal prisms, not uncommonly wit h each individual orientation represented by a system of parallel crys tals rather than a single one. Of the two habitually different types o f twins described in the present paper, the second one is particularly interesting. These trillings, found in material from a small vein at the summit of Mt. Nakalak in the Ilimaussaq alkaline complex, South Gr eenland, are twinned according to the same law as the previously descr ibed twins, i.e. twin planes = {110}, but are habitually so different that their recognition as epididymite trillings is by no means certain . They appear as pseudohexagonal (mono)pyramids, about 1 mm in size; t he single individuals of the trillings are dominated by orthorhombic p yramids {311} at the bottom and {312} at the top part; alternating gro wth of the two forms along the pseudo six-fold axis can also be observ ed. These twins are characterized by their internal, highly irregular inter-individual faces. The misfit between the individuals not in twin position calculates at 0.52 degrees; the reentrant angles of four fac es, two and two opposite each other, of the pseudohexagonal pyramid, c alculates at 0.33 degrees for the form {311} and 0.30 degrees for the form {312}; the protrudent angle of the remaining two faces of these f orms calculates at 0.17 degrees and 0.15 degrees, respectively.