RELIANCE OF GEOMECHANICAL PROPERTIES ON PETROGRAPHIC CHARACTERISTICS OF GOHATI RHYOLITE, DISTRICT SWABI, PAKISTAN

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.