Abstract:
Rhyolites of Gohati area are the part of Peshawar Plain Alkaline Igneous
Province (PPAIP), exposed near Gohati village, district Swabi. The main exposure of
Gohati Rhyolite is present along the roadside appearing white to light grey in color and
highly fractured; however, massive portions are also present at certain places. These
rocks are studied in terms of their petrographic, physical and mechanical properties.
Petrographic studies show that the Gohati Rhyolite rocks (GR1, GR2 and GR3) are
extremely fine grained and display porphyritic texture. Quartz is the most abundant
mineral, distributed in the form of phenocrysts and appears to be floating in fine-grained
matrix. Abundant quartz is also present in fine grained ground mass along with the other
major minerals like plagioclase and alkali feldspar. Minor amount of rutile along-with
the trace amount of tourmaline, epidote, garnet and monazite is also present. The
mechanical and physical properties including Compressive Strength, Water
Absorption, Specific Gravity, Porosity, Los Angeles Abrasion, Durability and
Soundness Test of collected bulk samples (GR1, GR2 and GR3) of rhyolites were
determined. The values of all the mentioned properties fall in the range of
recommended values for their usage as an engineering material. Besides, the excess of
fine grained as well as amorphous silica may also lead to alkali silica reactivity which
also demerit these rhyolites to be used as an aggregate. Rhyolites exhibit greater
strength as compared to granites of the adjoining area, most probably because of their
fine grained nature, abundance of quartz, irregularity in shape of its constituent minerals
and variation in the grain size distribution.
