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5 ON THE OCCURRENCE OF POTASH FELDSPARS OF INFERIOR TRICLINICITY WITHIN THE PRECAMBRIAN ROCKS IN FINLAND 1) BY VLADI MARMO, KAI HYTÖNEN and ATSO VORMA Geological Survey 01 Finland, Otaniemi ABSTRACT One hundred forty-seven feldspars extracted from porphyroblasts of Precambrian granites, granodiorites and quartz diorites were investigated by X-ray for triclinicity. Some of them were examined also optically. The presence of monoclinic potash feldspars, as weH as of others of inferior triclinicity, was indicated to a much larger extent than had been earlier supposed with respect to Finnish Precambrian rocks; but the occurrence of such potash feldspars seems nevertheless to be restricted to particular, comparatively weH defined areas. This study should still be considered as being of a preliminary character. CONTENTS INTRODUCTION EARLIER VIEWS ON THE PRESENCE OF ORTHOCLASE IN FINNISH ACID PLUTONIC ROCKS NEW DISCOVERIES OF ORTHOCLASE IN FINNISH PRECAMBRIAN ROCKS .... CENTRAL FINLAND ..•...• .••• • ... ..•.... . . ..... .... •. .•............. . SOUTHEAST FINLAND OTHER PARTS OF FINLAND •.•............. . . .. ... . .................... OPTICAL STUDY ......................................................... . A REVIEW ON THE OPTICAL PROPERTIES OF POTASH FELDSPAR .... ..•.. •.. RESULTS OF THE OPTICAL DETERMINATIONS OF THIS STUDY .. •. . .•..•••.. CONCLUSIONS REFERENCES Page 52 53 56 56 66 70 72 72 73 75 77 1) Received August 18, 1963.
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
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