Ov. Petersen et al., EPIDIDYMITE TWINS - CONTRIBUTION TO THE MINERALOGY OF ILIMAUSSAQ, NO-99, Neues Jahrbuch fur Mineralogie Monatshefte, (5), 1997, pp. 221-228
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.