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

Z, = unit vector of the [ooij zone axis in the first 0 complex ZJJ = unit vector of the [OOÍJ zone axis in the second 0 complex K = - L

°°J = unit vector of the Roc Tourné twinning a"° (010) xis 1 = unit vector of the (110) face normal —o T^ = unit vector of the (110) face normal f = unit vector of the (130) face normal z_

o = unit vector of the (130) face normal a_

o = unit vector of the highest optical elasticity direction or optical main vibration direction unit tion unit tion b = unit vector of the medium ODtical elasticity direc—o 1 £

o = unit vector of the lowest optical elasticity direcIn the right lower index of the letter denoting the vector the capital letter means the adequate twin member. The comma in the right upper part denotes the transformed value of the vector. In case of computing the bisectrix to denote the bisectrix vector the Goldschmidt

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s mark of that crystallographic direction was used which had been obtained in search of the twinning axis. Plagioclase twinning laws occurring in the andésite of the Visegrád Mountains First twin crystal Sign of rock sample: P.2.b. Place of sampling: uppermost region of the "Pálbükkös" quarry northwest of Pilisszentlászló. The