Acta naturalia Islandica - 01.07.1964, Page 12

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SIGURDUR STEINTHÓRSSON
A four-axes Universal stage was employed to determine the angle 2H of
olivines, pyroxenes and few plagioclase crystals. The maximum extinction angle
of pyroxene, and the parallel (max.) extinction angles of plagioclase were
also determined by this means. It was attempted to use the methods of Köhler
and Reinhard to determine the An-content of the plagioclase, but too' much
zoning rendered the methods impracticable. The cores of the phenocrysts, how-
ever, could be determined in this way giving (a) the composition, (b) the
nature (high temperature) and (c) the twin law. For the groundmass plagio-
clase the zone method of Rittmann was employed in conjunction with refractive
index. The rock was ground down, the feldspars separated in heavy liquid
(bromoform) and maximum and minimum R.I. found in sodium light by the
immersion method and a Leitz-Jelley refractometer. The accuracy is estimated
as ± 0.003, but might be less in certain cases because of the possiblity that
the actual maximum or minimum values were not obtained from the randomly
orientated grains. By comparing the R.I. values with the ones obtained fram
the extinction angles the groundmass crystals were found to be high tempera-
ture plagioclase as well.
Direct measurements of 2V of olivines and pyroxenes were made when-
ever possible. They were corrected for difference in R.I. between hemispheres
and the mineral according to the formula p sinV = n sinH. The measurements
were orthoscopic. The R.I. of the hemispheres used was n = 1.557 in all cases.
The R.I. of the central plate is n= 1.560. The majority of the measurements
was made With U.M. 2 objective and fully open diaphragm (Munro 1963).
The maximum possible error in actual reading of 2H is estimated ± 1°
(0.5° on each side), but the accuracy may be as small 'as ±3° in the case of
olivine corresponding to 6% Fa. The pyroxene measurements are more accurate
due their smaller optic axial angles, or ± 1.5°.
Measurements of R.I. of olivine and pyroxene were made on orientated
sections giving np. The accuracy in estimated ± 0.003. The values thus obtained
correspond reasonably well with those obtained from the 2Vs.
The carbonate present in the rock was determined by X-ray powder method
as aragonite. One olivine crystal was determined in this way as well, and was
found to be Fo 83%, Fa 17%. (Yoder 1957).
Modal analysis was made of representative sections. About 1000 points
were counted in each section. A fine interval eounter was used in most cases
(10 points per mm), but a coarser one (3 points per mm) was used to count
some of the coarser grained sections. The modes were determined for (a) com-
position of the groundmass, (b) phenocrysts, and (c) overall composition.
The density of numerous specimens wias measured by means of a Walker’s