Quartzite

Quartzite represents one of the most common and stable metamorphic products of the Earth's crust. It forms when quartz-rich sandstones (siliceous sediments deposited in marine or fluvial environments) undergo regional or contact metamorphism, typically at temperatures between 200 and 800 °C and variable pressures. During this process, original quartz grains are welded together through recrystallization, eliminating interstitial pores and creating an incredibly compact and monolithic rock. Unlike sandstone, which disintegrates easily under pressure, quartzite is practically insoluble to ordinary chemical agents and resists mechanical erosion magnificently.

The crystal structure remains trigonal (as in pure quartz), but individual crystals are so intimately connected that the rock behaves as a homogeneous body. Coloration varies from pure white to gray, pink, red (from iron oxides), yellow, or even blue, depending on impurities present during metamorphosis. Quartzite deposits are found in ancient mountain ranges and continental shields, often in association with schists, marbles, and gneisses. Quartzite has historically been used for paving, monumental sculptures, and construction material of the highest quality, owing to its nearly legendary durability.

Quartzite is a medium- to high-grade metamorphic rock characterized by an equigranular granoblastic texture with quartz grains typically ranging from 0.1 to 2 mm. Mineralogical composition is dominated by SiO₂ (>90%), with possible accessory minerals including feldspar, mica, magnetite, ilmenite, and rutile. The crystal system is trigonal (space group P3₁21 or P3₂21 for α-quartz). Hardness is 7 on the Mohs scale; density ranges from 2.65 to 2.75 g/cm³ depending on the degree of compaction and the presence of accessory minerals. The refractive index is approximately 1.544 (ω) and 1.553 (ε), with positive birefringence of ~0.009. Quartzite is practically opaque in thick samples and may show translucency in thin sections. Fracture is conchoidal or irregular; the rock exhibits no well-defined cleavage due to strong intergranular cementation. Under an optical microscope in transmitted light, grain boundaries are sharp and often characterized by sutures (sinuous contact lines) resulting from metamorphic recrystallization. Raman spectroscopy of quartz shows the characteristic band at ~465 cm⁻¹. The α-β quartz transition temperature (573 °C) provides information on metamorphic grade. Geothermometric analysis based on fluid inclusions and mineral associations allows estimation of the P-T conditions of formation.