Assessment of mineralogical and petrographic factors affecting petro-physical properties, strength and cracking processes of volcanic rocks

Abstract

Microtextural features influence the petrophysical and mechanical properties of rocks, including the elastic properties and cracking processes under axial loads. Volcanic rocks with varying microtextural characteristics and compositions were evaluated to assess the effect of the main constituents' mineralogical, petrographic and micro-structural features on the petrophysical properties, elastic properties and strength. Furthermore, detailed analyses were also used to understand the cracking processes under axial loads. The analyses of the cracking processes were completed on thin sections obtained from the mechanically tested specimens.The results from the quantitative mineralogical and petrographic studies and observations from thin sections revealed that the mineral mass fractions have an effect on the specific gravity and loss-on-ignition (LOI) values. On the other hand, unconfined compressive strength (UCS) and elastic properties are mostly affected by petrographic variables (e.g., mineral content). The UCS values tend to decrease with a relative increase in the groundmass with respect to the phenocryst content and vice versa. Biotite is the only mineral that influences the UCS individually. Geometric features (e.g., Feret's diameter and perimeter) of opaque minerals and biotite are determined to be important constituents influencing the Young's modulus.Under axial loads, crack propagation is strongly dependent on the proportional distribution of the phenocryst and groundmass. It was observed that increasing groundmass content leads to predominantly axial cracks. The cracks tend to bend or propagate as a boundary crack when they reach the boundary of an unaltered phenocryst. Thus, axial and shear cracks co-exist with increasing amounts of phenocrysts, which hinders crack propagation. Occasionally, cracks can penetrate the altered or opaque phenocrysts, depending on the degrees of orientation (with respect to applied load) of the same minerals. Furthermore, the synthesis of the measurements obtained from UCS tests and thin section observations reveals that an increase in the amount of phenocrysts, which promote the formation of more shear cracks in addition to axial cracks, causes an increase in the radial strain and Poisson's ratio. The other factors investigated had minor effects on physical and mechanical properties of studied rocks.