COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

52 Bulletin de la Commission geologique de Finlande N: 0 212.
INTRODUCTION
One of us has in many connections (Marmo 1956, 1963) stressed the
importance of the behaviour and modification of potash feldspar in granitic
rocks as providing more than merely a hint as to the origin and evolution
of such rocks.
During the geological mapping of certain parts of southern Finland by
Matisto (1962), and southeastern Finland by Vorma, as weH as somewhat
later of Central Finland by Marmo, greatly varying triclinicities were observed
among the potash feldspars of the porphyroblastic granodiorites and
granites in the areas. These observations were at variance with the earlier
views on the Precambrian potash feldspars, but still, with respect to the
granite problem, of utmost interest:
- In the 1950's, the opinion steadily grew stronger that the microcline
(the prevailing potash feldspar contained in Precambrian rocks) was a
product of the microclinization of orthoclase; and Laves (1950) contended
on crystaHographic grounds that if microcline is cross-hatched, it must
have definitely had a monoclinic ancestry. On petrological grounds, Marmo
(1956 and 1959) objected, claiming also the possibility of a cross-twinned
microcline having a triclinic growth, while admitting, however, that the
very initial stage in the growth of the crystals must have been monoclinic.
- Microcline has never been synthetized in laboratories by direct
crystallization, but obtained only by areplacement of albite (Laves 1951,
Wyart and Sabatier 1956)). Instead monoclinic potash feldspar has always
been produced.
- According to recent views (Goldsmith and Laves 1954), microcline
cannot exist above 500 0
ein the presence of water, but will undergo monoclinic
modification. On the other hand, a monoclinic potash feldspar may be
formed at very low temperatures as weH. Therefore, Marmo (1959) suggested
that if the temperature of crystaHization (in the presence of water) is sufficiently
low (probably below 500 0
C) and the accumulation of materials
(growth of the mineral) is sluggish, triclinic potash feldspar will result, as it
probably has in the granitized rocks exlusively containing microcline. If
the temperature is raised or the crystallization takes place more rapidly, a
potash feldspar of inferior triclinicity will be produced, as in the case, for
instance, of many younger Alpine granites of marked intrusive character.
If aH the suppositions referred to are correct or nearly so, then a thorough
study of the potash feldspars of salic plutonic rocks would have a very
important bearing on the genetical problems concerning granitic rocks in
general. This is the reason why the present study was begun. The senior
author (Marmo) is mainly responsible for the petrological part of this work;
which is restricted, however, to the occurrences in Central Finland. Vorma