Three-dimensional microstructures of slates from the Sierra Nevada Range, California, USA

Abstract

Slate is known to have complex microstructure, heterogeneous composition, and rich in phyllosilicates or sheet-like minerals. Due to its high durability, water resistance, and beautiful appearance, slate can be used as a building and roofing materials. Microstructures and mineralogy of slate are the critical factors used to evaluate the properties of roofing slate. This study aims to investigate mineralogy, 3D microstructures, and pore system of slates from Plumas county (sample A), Placer county (sample B), and Tuolumne county (sample C) of the Sierra Nevada Range in California, USA. The Range is in the ancient subduction and metamorphism zone but relatively little is known about the microstructures and properties of slates in this area. Results from X-Ray Diffraction (XRD) experiment suggests that quartz (50-77 vol%), muscovite (16-26 vol%), and biotite (8-25 vol%) are major minerals in all samples. Under polarizing microscope, quartz mostly show elongated and porphyroblastic texture in Sample A and C, which is a common texture of quartz-rich roofing slate. Conversely, minerals in sample B align the grains randomly and show low degrees of shape preferred orientation. Synchrotron X-ray Micro-Computed Tomography (Syn-MCT) is further used to illustrate 3D orientation and distribution of internal components such as minerals, organic materials, and pores. Muscovite and biotite grains in sample A and C align more parallel to the cleavage plane (315° and 0° respectively), indicating strong shape preferred orientation, which is a suitable fabric for roofing slate tile. The porosity of sample A (19.1 vol%) is relatively higher than sample C (10.5 vol%) due to lower metamorphic grade. Pores and organic materials are mostly isolated, showing no connectivity and suggesting low permeability. The abundance of quartz plays an important role in governing the high hardness of slates. Based on the calculation of quartz and phyllosilicates ratio, sample A and C can be categorized into a very hard slate type while sample B is classified into a hard regular. The sizes of pore, organic material, and phyllosilicates in both samples show similar trends where small grain/pore sizes (<50 μm3) dominate the majority of the pore/grain volume. Due to their high hardness, low porosity and permeability, and mineral preferred orientation make the sample A and C suitable for making building material, particularly as thick slabs for flooring and panelling.