Dr. Nagy I. Zoltán szerk.: Fragmenta Mineralogica Et Palaentologica 4. 1973. (Budapest, 1973)

!P- arc cos (v' . cos <& + v' . sin * ) Accuracy of measurement and compution and the applied designation The accuracy of the read in the U-table is about 0.5°. Computions were performed by applying five-figure table of circular function. The deviation of the optical symmetry axes from the perpendicular was within 0.5°. No corrections were performed since its error was within the limit of error of the measurement. The triangles of error of the twinning axes were rather small, the length of side has been mostly within 1°. The arithmetical mean of the components of the three vectors was used, this was converted into unit vector and this value was transformed. The Goldschmidt

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s <j> and p values given for albite and anortite were interpolated to the anortite percentage of the given plagioclase and these values were compared with the results of measurement and compution. It should be noted here that in case of determining the anortite content the fact may be attributed to the accuracy that in case of 75 to 90 per cent anortite content the optical vibration directions do not sensitively change to the chemical composition and within this range there are a few typical values, respectively. In the stereogram the curves of high and low temperatures levels almost coincide. The designation used in the computions is as follows: unit vector of the (010) face normal in the first complex unit vector of the (010) face normal in the second complex unit vector of the (001) face normal Mi MII.