A Geoscience Digital Image Library

TitleThin section of trachyte porphyry
DescriptionCross polarized thin seciton of trachyte porphyry. The rock contains numerous large phenocrysts of feldspar, most of which are strongly kaolinized potash feldspar (probably sanidine originally); others consist of oligoclase, moderately altered to sericite and some epidote. Still others are agglomerated composites of oligclase and potash feldspar. The mafic phenocrysts (most of which probably were a dark slightly sodic amphibole initially) have been largely converted to a pseudomorphous aggregate of pale green pyroxene, epidote calcite, and chlorite. The fine grained matrix is an equigranular aggregate of sodic plagioclase, orthoclase, and minor quartz dotted with minute crystals of apatite, sphene, pyroxene pyrite and replaced by irregular aggregates of coarse calcite.
ChronostratigraphyTertiary (Post-Oligocene)
LocationUSA ▹ Colorado ▹ Teller. Near Cripple Creek.
PhotographerDarla Sondrol. 2002-04-03.
CollectionWard’s Collection of Classic North American Rocks #16.
Key wordstrachyte, porphyry, Cripple Creek, Colorado, thin section
Tech details1.3 MB. Thin section. Nikon FDX-35.
GeoDIL number2559