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Suomen Geologinc:l Seura. N: 0 35. Geologiska Sällskapet i Finland. 73
nature of the submicroscopic twinning (balanced or unbalanced), the extinction
angle y /\ ...L (010) varies from 0° to 18° for complete order in Al/Si
in potash feldspar; at the same time 2Va remains almost constant (ca. 84°).
The optical orientation and the values of the optical axial angle are the
most sensible criteria for the evalution of the degree of Al/Si order of potash
feldspars. During sanidinization, 2Va of microcline changes from 84° (optical
axial plane nearly perpendicular to (010)) to 0°. Then the axial angle again
opens to 60°, the axial plane becoming parallel to (010).
In the light of the afore-mentioned statements, it is possible to determine
with the aid of 2Va the structural state of potash feldspars provided
that the composition of the potash feldspar is approximately known. In this
connection it is appropriate to cite the statement of Marfunin (1961): »There
are two types of order disorder in potash feldspar: 1) disorder without
deviation from monoclinic symmetry (submicroscopically and/or sub-x-ray
twinned; balanced twinning) and 2) disorder with change of symmetry into
a triclinic one (single crystal). In addition there are all intermediate states
between disorder and order of both types (unbalanced twinning). Every
state of substitution disorder may be sub-X-ray twinned.» Thus the triclini
city obtained from the X-ray powder pattern value of Goldsmith and
Laves (1954) would not be sufficient for the determination of the structural
state of the sub-X-ray twinned potash feldspar.
RESULTS OF THE OPTICAL DETERMINATIONS OF THIS STUDY
Twenty of the potash feldspars, which are listed in Tables 2, 4, 5 and 6,
were selected for optical study. All are supposed to be representatives of
synkinematic porphyroblastic granodiorites or to belong genetically together
with them. In addition, the specimen of the only late kinematic potash
granite of southern Finland containing orthoclase found to date is included
(Härme 1962). According to the X-ray powder diagram, the potash feldspar
in the specimens selected is mainly orthoclase. In addition, however, in sets
of potash feldspar with varying triclinicities were found in these specimens.
Under the microscope, all the potash feldspars studied contain areas
without cross-hatching. Such areas change gradually into areas with crosshatched
microcline. The optical investigation reveals that the areas with
cross-hatching are much more frequent than could be expected from the
triclinicity values obtained by the X-ray powder study. On the other
hand, many insets without cross-hatching were found that possess the
optics of maximum microcline.
With the U -stage, only those areas of potash feldspar were studied that
do not have any cross-hatching. The optical axial angle 2Va and the extinction
angle (y /\ ...L (010)) for the potash feldspar in synkinematic grano-
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