COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 53
is responsible for all the material obtained from southeastern Finland (around
and close to rapakivi areas), and also for the optical data. Hytönen is mainly
responsible for the triclinicity determinations (and separation) of potash
feldspars and, together with Vorma, for the mineralogical and crystallographic
considerations.
EARLIER VIEWS ON THE PRESENCE OF ORTHOCLASE IN FINNISH ACID
PL UTONIC ROCKS
In earlier geological literature on the Finnish basement, there are some
scattered notes on the orthoclase occurring in the oldest Finnish rocks, in
which the microcline was always taken as the predominant form of potash
feldspar. Seldom, however, is any explanation given on what basis the
potash feldspar was considered as being orthoclase. It seems that mostly
the extinction angle y 1\ ..!.. (010) was used for this purpose, but often the
lack of typical cross-hatching was probably taken as proof of the presence
of orthoclase instead of microcline. In only a few cases was the optical
axial angle determined for the potash feldspar occurring in the Precambrian
basement of Finland.
In the following, a review of the Finnish literature discussing orthoclase
will be made, excluding, however, rapakivi, which has always been considered
as containing considerably orthoclase. In this review, the less dependable
data will also be included, because l'ecently some of them have been
substantiated by X-ray methods.
In his classical work on the Archean formation of the Tampere area,
Sederholm (1899) mentioned orthoclase in the following connections: 1. in
even-grained granitic pebbles of the conglomerate of the Tampere area the
occurrence of orthoclase with patches of micl'ocline (op. eit. pp. 26-27), as
weIl as, in conglomerates with pebbles of coarse-grained granite (p. 32), in
orthoclase porphyrite (p. 39) and in da ci te (p. 41); 2. in porphyrite rich in
orthoclase (p. 67); 3. in slightly altered phyllite on the shore of Näsijärvi
(p. 87); 4. in porphyritic granite at Lavia, as weIl as in other similar post­
Bothnian porphyritic granites of the area (p. 107) and in the even-grained
granites grading into the former (p. 1l0-11l, 115).
In southeastern Finland, in the area covered by the map sheet of Mikkeli,
Frosterus (1902 a) claims that the post-Bothnian porphyritic granites are
likewise orthoclase-bearing (op. eit. p. 42-44). Especial mention is made of
the porphyritic granite at Petäjävesi, Central Finland, which is rich in
orthoclase and, in the investigator's opinion, comparable with rapakivi
granites. This occurrence has been recently re-investigated by Rouhunkoski
(1957), who could prove the ample occurrence of orthoclase in this rock.