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    STUDIA GEOLOGIA - Issue no. 3 / 2003  
         
  Article:   REACTION CORONAS AROUND QUARTZ XENOCRYSTS IN THE BASALTIC ANDESITE FROM DETUNATA (APUSENI MOUNTAINS), ROMANIA.

Authors:  NICOLAE HAR.
 
       
         
  Abstract:  Reaction coronas consisting of Al-poor pyroxene, tridymite and glass are developed around quartz xenocrysts of sedimentary origin hosted in olivine - bearing basaltic andesites from Detunata (Apuseni Mts. - Romania). The quartz xenocrysts reacted with the basaltic melt. The outer parts of the reaction coronas consist of a glassy matrix containing prismatic augite crystals developed perpendicular to the interface between the coronas and basaltic host rock. The inner parts of the coronas consist of isolated crystals of augite, fragments of quartz and tridymite crystals in a glassy groundmass. Tridymite is also present in fractures of the quartz xenocrysts. Microprobeanalyses and Raman spectroscopy were used to identify the composition of the coronas pyroxenes, glass, and silica polymorphs. The coronas pyroxenes have a homogeneous composition, typical for augite. The glass is highly siliceous (SiO2 = 72.0 – 76.8 wt.%) as the result of quartz dissolution, and high in alkalis (Na2O = 1.38 – 3.22 wt.%; K2O = 4.72 – 6.23 wt.%) and aluminum (Al2O3 = 9.31 – 12.18 wt. %). Tabular, twinned crystals of tridymite show a high content of alkalis (Na2O + K2O = 0.30 – 0.39 wt. %) and aluminum (Al2O3 = 0.54 – 0.86 wt. %). Sodium and potassium ions appear to fill structural channels and voids in the tridymite structure, charge balanced by the substitution of Al for Si. The Raman spectra of tridymite show the most representative peaks, of 403 and 422 cm-1. The diffusion processes and temperature were the controlling factors in the genesis of the newly-formed minerals in the reaction zones. Si4+ diffused from the quartz lattice, while Ca 2+, Mg 2+, Na + and K + migrated from the basaltic-andesitic melt towards the coronas. ...  
         
     
         
         
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