I recently returned from Norway with lots of rock samples which will be photographed in the near future. The photos will be added mostly to the old posts where appropriate but this way my regular readers will not see them. And I also miss an opportunity to learn something new from comments. So I thought that perhaps I should start writing shorter posts in addition to longer overviews of rock types.
Here is the first one of the new series which will contain mostly rock samples but also other interesting geological photos taken during my geotrips.
Graphic granite from Evje, Norway. Width of sample 9 cm.
Width of view 14 cm.
Graphic granite is a granitic pegmatite where large alkali feldspar crystals contain angular, linear, or wedge-shaped intergrowths of quartz. These intergrowths may resemble cuneiform writings, hence the name.
Quartz crystals are uniformly scattered in the feldspar crystals and not randomly oriented. These crystals grew simultaneously with alkali feldspar host and they are optically continuous (one crystal).
The term “pegmatite” was originally used to describe graphic granites by Haüy in 1822. Graphic granites may indeed be described as pegmatites even according to current usage but the term “pegmatite” nowadays has much wider meaning, encompassing also rocks which have no graphic texture nor do they have to be granitic in composition1.
Another sample of graphic granite from the same locality. Evje, Norway.
References
1. Jackson, J. A. (1997). Glossary of Geology