Szeberényi, H. Vincze

English

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TW IN LAW OF 'BÖRZSÖNY" W ITH M EASURABLE TWINNING -  AND COMPOSITION -  PLANE FROM HUNGARIAN ANDESITEH. V1NCZE SX E B E R Ű X Y IM inen.logical D ep artm en t, M useum  o f  X a tu ra i Ifislo ry , B u d ap est R eceived: 22. D ecem ber 197*iSUM M ARYf o r m a l  tw in-taw s acco rd ing  to  (1 lb) ant) ()30) respectively  acco rd ing  to  th e  le f t fo rm s o f  these could  be ostabhshcd  to r  th e  i .r s t  tu n e  for p tagioctase tw in -crvsta] g roups figuring  as and es .te  p o rp h y n c  ph en o cry sts (1!'74). A lthough  these  tw in-iaw s couid  be e s t ^ i s h e d  fm n . th e  m easured  op ttca) tla ta  b y  m eans of vectorgeom etrica l co m p u ta tio n s , no com position  -  respectively  tw n m .n g  -  p lanes co,.Id be  determ ined w ith th em . T he ptagioclase tw in  c ry s­ta l  dea lt w "h  m  w h a t follow s occurred  in o n e o f  the  an d esite  san .p lescoll.x-ted  in th e  Visegtdd tnoun tam s. W ith  th .s tw m c o m p lo x th ec o m p o s itio n  -  an d  tw in n in g  -  p lan o o f th e  B örzsöny, tw in-law , (110), betw een  tw o  tw inned  p a r ts  being  in im m edia te  c o n ta c t could  well be  detec- ted  and  m easured . V alues ob ta in ed  w ith th e  .ncasu .es a re  in good ag reem en t w ith  those o b ta .n ed  from  co m p u ta tio n s, p rov ing  th a t  n o rm al tw in  law s acco rd ing  to  (110) a n d  (130) respectively  accord ing  to th e ir  left fo rm sc an  be present w ith  p lngioclnsts found  in n a tu re .ihe  andesite sample eomes írom the Dümürkapu (juarry (Yisegrád mountains). The rock is a medium- or dark -grey, fresh pyroxene an­desite. Microscopically it contains microcavities the internal walls of which are lined a t places by green nontronite. The tex ture of the pyroxene andesite is microhcmicrystallic-porphyric. I ts  mineral components are: p/fopw Arse, or/Ao-awf/ cíóm-pyroaeMe a?td opcapte pttTcra/. The am ount and crystallization degree of the m atrix proves th a t it is an effusion rock and not a subvolcanic one. The porphyric components are: plagioclase pyro­xene; their dimension is: 150-1700 p. Between plagioclase and pvroxene many oriented intergrowths can be seen, and thev  appear sometimes as inclusions within each other.The rock is contain ing so c alled "cognate inclusions" also i e in ­clusions from  th e  dep th  (gabbro, diabase).On the evidence of the texture crystallization might have occurred in two magma chambers, bu t the solidification of the rock took place on the surface. Owing to  the idiomorphic development of the porphyric pheno­crysts it is probable th a t the formation had been stagnant in a ma"ma- chamber of high tem perature and the outflow took place afterwards. "In the crystallization process of plagioclases two generations can be observed. The bulk of plagioclase crystals is of zonal structure, bu t zoneless sections can also be encountered. The anorthito content of zonal plagioclases varies between 70 and 8 6 %.6 *Tiie plagiociases are of iiiaiied, taim iar habit aicording to tiie face (010). Twinned format ions are very frequent, twin piates are rather wide. Among tiie crystal faces types (110) ami (110) can frequentiy tie encoun­tered. Besides cieavages (0 0 1 ) and (0 1 0 ) aiso cleavages according to (1 1 0 ) and (1 1 0 ) are weli measurahie. Predom im ant composition faces are: the most frequent piane is (0 1 0 ) and some irreguiar composition pianes among the muiti-composed twin formations are aiso reiativeiv frequent.Many crystal groups consisting of a number of twin compiexes are present and their ratio as compared with tha t of "common" twins is high. Tiie compiicate compiexes consisting of several twin members show some­times a weii observabic matrix inclusion structure, which seems to  tie uniform as if it surrounded a common core.This rode was tiie source of tiie piagioclasc twin c-rystai on tiie thin sec­tion of which one can measure tiie ieft form of tiie "Bbrzsbny-twin-iaw". tiie (lib ) face as composition and twin piane. The method of vcctor-geomct- l'icat com putation for the interpretation of observed data  of piagiociase twin crystais was discussed in an article of mine puhiished in common with M r s () r k e n y i — B o n d o r L. (1971).Observed and computed da ta  of tiie piagiociase twin crysta) figuring in sampie Sz. 2 . are as lbiiows:Denotations used in tiie course of computation:.) /^  =  unit vector of tiie (bib) face norm al in tiie  firs t compiex..)/y^ =  unit vector of tiie (bl(') face normai in tiie second compiex.= unit vector of tiie [9b] [ zone axis in the first and second coni]) iex,A* =  — u n it vector o f tiie  "roc-tourne" tw in axis.(bib)=  unit vector of tiie (! ib) face norma), cq, -  unit vector of tiie direction of highest optica) eiasticity ,=  unit vector of tiie direction of medium optica) elasticity.Cg =  unit vector of tiie direction of iowest o]iticai eiasticity.The upper case te tter figuring as index in tiie right tower corner of tiie te tter denoting  th e  vector represents tiie corresponding tw in  member. Tiie apostrophe in tiie rigid upper corner denotes tiie transform ed vatuc of tiie vector. The m arking (witii ietters) o f tiie morphoiogica) directions agrees w ith the  (loicisclimicit m arking system .The measured d a ta  of tiie tw in crystais shown in Fig. 1. are as foiiows;I. =  first complex containing tiie m embers A, B, C. its  cieavage and  tw inning face is: d/y =  n =  il3..A\ li =  17.7°. i i =  second compiex containing tiie  m em bers D ami E.34 H . V IN C Z E  SZEBEBTENYl_________ T W IN  L A W  O F  „B Ö R Z S Ö N Y " W IT H  M E A S U R A B L E 85t'ç / . /. l 'lag ioclase  tw in c ry sta l in and esite  oi D nm örkapu .Its cleavage and twinning face according to (Old) is:th e  ( 1 1 0 ) twinning and composition plane appearing between the two com­plexes is as follows:The (1 1 0 ) direction appears in cither twin members in the form of growth lines, crystal faces and parting too.th e  (1 1 0 ) direction of the two complexes is alternatively parallel to  the direction of86 H. V IN C Z E  S Z E B E H E N Y IС/?<':д-?нсн0ег.'DfiPMí-MK?;Moer.'A und Б  : Я; of Carisbad; D and E : Aibite Б  and C : 7Í, o f B oc-Tonrnc.A and C : Aibite diy.A and  E  : tw in  T  o f Börzsöny.I I I  every twin-member, the orthogonality of the two optical sym m etrv- axes m ust be examined. I f  the angie subtended by them  is reaiiy 90°, then the product of their unit vectors is zero. This is, of course, impossible owing to the measurement errors. Up to a deviation of 0 .5 ° the numerical d a ta  should not- be changed, but in case of an error surpassing 0 .5 ° one lias to apply an adjusting procedure. Concerning the d a ta  published above no such deviations occurred, so th a t no use of adjusting was necessary.Computing of the twin axes is carried out on the basis of bisector vectors of the optical directions. The com putation of bisector vector tabes place by means ol addition, respectively subtraction of unit vectors of the corresponding directions. The obtained vector must be reduced to unit vector form.The bisectors of optical svnnnetrv-axes of A and H are:T W IN  L A W  O F  „B O R Z S O N Y " W IT H  M E A S U R A B L E  H7Owing to the closeness of the two [%]-s the mean value of the bisector of the directions [%,,] and is computed and used in w hat follows:The bisectors of optical directions of A and C twin-members are:! he un it vector computed on the basis of the measurements is:For the subsequent computations the measured Af, value will be used, because it is subtending an angle of 90° with the unit vector of Z,. From  these two directions the un it vector of the twin-axis of Roc-Tourné will be computed by means of vectorial multiplication:J lie twin axis of Roc-Tourné results from the optical d a ta  of B and C as folio ws:Because the value obtained by means of vectorial multiplication is the more accurate one, we will use it instead of the measured values, b u t we sec th a t the deviation is insignificant.SS H. V 1N CZE SZEB EK B N Y 1Ttie bisectors of the optical directions of twin-members D and E are:The average value is:The unit vector computed from the measured direction is:This iatter vatuc subtends the angie 89° 30 , with the twin axis of Larishad and so it is just to be accepted.The optica) bisectors between twin-members A and E are:The unit vector computed from the mean vahic is:The angie subtended by this with the twin-axis of Carstbad is 89° 56', i.e. it is more accurate, than  the 7',, vector computed from the measure­ment.The bisector of the (010) directions of the two coniptexes is:Let us transform  the vector 7',, into the coordinate system X l X (X-ax­is: twin-axis of Koc-Tourne, Y-axis: Atbite twin axis, X-axis: twin-axis off'artnhndl We obtain*The sign of the axes of the coordinate system XYX has been fixed basing on the Eu!cr angtes of first kind.The transform ed value of the 7* — twin-axis in system /. is:The Gotdschmidt vatues are:Thus, between the twin-members A and E twin-taw according to (1 10) can be observed, i.e. the )eft form of the "Bbrzsbny" twin-taw.Ttie observed da ta  are shown in the annexed stereogram (Fig. 2 .)T W IN  LA W  O F  ..B Ö R Z S Ö N Y " W IT H  M E A S U R A B L E S93. S tereogram  o f  a plagioclase t win c ry s ta l observed in  andesite  o f  D ö m örkapu .The interpolated values for the case of an anorthite content of 85% are for (1 1 0 ) as follows:1 he anortlute content oi the twin crystal corresponds on the migration curve to  83 — 84%, the observation points are lying along the HT curve within a distance of 1  mm.R E F E R E N C E SB a r t h .  T. W ., 1969: Feldspars, L ondon  (1 — 2til)H a r r i ,  C. 1950: A nw endung d e r V ekto rrechnung  a u fe in ig e  h äu fig  a u ftre te n d e  kristal- op tische Problem e. S. M. P . M. 30. (258 — 300).B u r r i, C-, 1950: C harak terisie rung  de r P lag iok lasop tik  durch  drei W inkel. . . S. M. P. M. 36. (5 3 9 -5 9 2 ) .B u r r i ,  C., op tischo O i'icn tii'n in j von H '] ')a l;r;n )< )i'itaus \iid e s itv f)tiK u < 'h iró(Iran ) . S. M. P. H .  48. (78)  -  8 0 ] ).B u r r  i, C.. 1072: Zur D efin ition  unt! B erechnung  d cro p tisch o n  O ricn tie rttn g v o n  I'lagiokbts- cn. S.A1. T . M. 52. (4 9 7 -5 1 4 ) .B u r r i ,  C-, 1974: Ghentisnnts ttnt) o[ttis;')te O rien tieru n g  zweier B y to w n its  au s  ishtndi- schem  O U vinttasaft. 8. iM. P. M. 54. (1 — !7).H t t r r i ,  C-, P a r k t : r ,  R. .  W c n k .  K<t .  t9 0 7 :) ) i t!o p tisc tte G rie n tie ru n g d e rP la g io k ta -  se. Bttsel. (1 -334).B t t r r i ,  C-. Ö r k é n y i - B t t t t d o r ,  L. ,  \  ) t t c x e - S x < t b e r é t t y i ,  JL , 197<):Hftctt- nerisctto A usw ertung  von [ [ 'is t 't io j 'c ra tm n o ttítu r rh c k 'tu e iU a ro  V ektortnethotlett. S. M. P . M. 50. ( t - 3 8 ) .H  a j ó s, Gy., 1 Int rochtct iot t  to  t h e gttotttetry. B u d ap est (in H un g arian  1 -  594).K o t  ta  S-, S x t r ó k t t v ,  ] \ .  ] ..  ]9 0 7 :M in cra ]o g ie (B u d a} < o s t.in H u n g arian , 1 — 930).K ö h l e r ,  A., 1950; B etncikttttgett über P t . tg io k ta s x a it ii t tg e .^ tin .l 'e tr .  H itt.  T sc h erttt .3 . Serit) 1. (347 — 352).K ö h l e r ,  A . R t t a x ,  P., 1915 —47: G e ttt tn k e ttü tte rd ie B itd ttn g  vet) l'elttspatxw illingen itt G esteinen. Verh. Geol. B und . W ien (103— t71)L e n g y e l .  K-, 1925: Petrogcnctischc  B eobactttungen  :ttt den A ndositen von der Utnge- b ttn g v o n P itisS 7 .o n tlá sz ló .l ''ö ld t. K ö x ! .5 5 ( tr ]S — )27, in f lt tn g a rii tn ) .Ö r k é n y i  — B o n  t i e r ,  L. .  V i n c x e  — S x  e b e  f é n y i ,  H ., 1974:(110), (110), (139) an d  (130) P lttg ite lase  Tw intting  itt A ndesite  front H u ngary , A eta Geol. Ac. Sc. H u n g . 18., ( 9 9 -  135).R  e i tt h  a  r d , H -, 1931: U niversale D rch tischn icthoden . Hasét.W e n k ,  E d , G l a t t s e r ,  A. Schwandet*, H ., l ' r o t n t n s d o r f ,  \ . ,  1972: T w in law s, o p tic o r io n ta tio n a n d  com position  o f p iag io rlascs front recks t2 <'5t, 14 053 and  14 310. Geoch. Costnoelt. A cta  1. (581 — 589).<J() H . V IN C Z E  S Z E B E K B N Y I

Twin law of "Börzsöny" with measurable twinning- and composition-plane from Hungarian andesite

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Twin law of "Börzsöny" with measurable twinning- and composition-plane from Hungarian andesite

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