It's not just granites or other igneous rocks. A major rock-forming mineral is Potassium Feldspar (K-spar to geologists). The weathering products of K-spar inc

It's not just granites or other igneous rocks. A major rock-forming mineral is Potassium Feldspar (K-spar to geologists). The weathering products of K-spar include many varieties of clay. Those, in turn, wind up incorporated in sedimentary rocks When logging a well for permeability -- an activity that occurs for both hydrocarbon and geothermal projects -- detection of radio-emissions due to K is an indicator of sedimentary strata that are unlikely to have much permeability because the clays clog up the fluid-flow pathways. This has been today's lesson in geoscience. :D (Edit: typo.)