COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

72 Bulletin de la Commission geologique de Finlande :0 212.
Actually, in Table 10, only the following occurrences of orthoclase are
those not belonging to the areas either described above or already earlier
known to contain orthoclase (rapakivi): Suvasvesi, Heinävesi, Kuusjärvi,
Ilomantsi, Kuopio, Kaavi. They all are situated in eastern Finland. Thus,
such an »orthoclase areal) may weIl be expected to exist there.
OPTICAL STUDY
A REVIEW ON THE OPTICAL PROPERTIES OF POTASH FELDSPAR
The optical properties of potash feldspars are dependent on the composition,
submicroscopically exsolved phases, submicroscopic twinning, and the
degree of the Al/Si order.
The influence of chemical composition on optical properties has been
treated by Winche1l1925 (K-Na feldspars and K-Ba feldspars), Spencer 1937
and Tuttle 1952 (K-Na feldspars), and Oftedahl 1948 (K-Na-Ca feldspars).
Hewlett (1959) studied the influence of Ca, Ba, Sr, and Fe on the optical
properties of potash feldspar , emphasizing that the substitution of Na for
K causes such a small change in the index of refraction in the range Or100
to Or 60 that its effects normally are masked by the larger change caused
by the substitution of minor amounts of Ca, Sr, Ba and Fe. Microclinemicroperthite
usually contains in natural occurrences less than 5 wt. %
NaAISi 3
0 s in solid solution in the potash feldspar phase (Goldsmith and
Laves 1961). The influence of this substitution on 2V of microcline can be
omitted. In orthoclase-microperthites the corresponding NaAISi 3
0 s content
is of the magnitude of 8-10 wt. % (Goldsmith and Laves, op. cit.).
Tuttle's diagram (1952) shows the axial angle to change a few degrees over
the composition range Or100-0rgo'
In the light of present knowledge, the influence of the cryptoperthitic
exsolutions on tbe optical properties of potash feldspar is small. During
homogenization of cryptoperthites there occur very small changes in the
indices of refraction, and the changes in 2V are almost negligible, occasionally
possibly some few degrees.
Marfunin (1961, 1962) has shown that submicroscopically twinned triclinic
potash feldspar has an axial angle smaller by 2 to 3 degrees than the
single crystal potash feldspar with the same degree of order in Al/Si. If the
twinning is balanced, the average structure has the axial plane perpendicular
to (010), and the optical orientation is monoclinic. Depending upon the