Authigenic mica in Early Miocene volcaniclastic rocks of the Macelj area, Hrvatsko Zagorje, Croatia
Tibljaš, Darko; Zagreb, Stjepan Šcavnicar; Padova, Gianmario Molin; Zagreb, Renata Slavkovic
published: Feb 5, 2004
In the Early Miocene zeolitized volcanicalstic rocks from NW part of Hrvatsko zagorje green authigenic mica occurs in thin veins and coatings of glass shard vesicles. X-ray powder pattern with widened but still relatively sharp reflections is characteristic for 1M micas, with Fe-rich octahedral sheet. The observed value d(060) is 1.508 Å. IR spectrum is characterized by sharp absorption bands in the OH stretching region, with two strongest bands at 3580 cm-1 and 3600 cm-1 ascribed to Al-Fe3+ and Al-Mg cationic environment of the OH groups. Microprobe analyses revealed that this is an interlayer-deficient dioctahedral mica, with Al as dominant cation in octahedral sheet, viM3+>1.2, and low tetrahedral substitution, with quite peculiar chemical composition that does not correspond ideally to any member of the mica group.