Cuarzo

Quartz is a tectosilicate of silicon dioxide (SiO₂) that crystallises in the trigonal system and is among the most abundant minerals in the Earth’s crust.

In geodes it lines cavities with well-formed crystals. In Jalgaon it commonly coats basaltic cavities as microcrystalline chalcedony, associated with fluorapophyllite, heulandite and other zeolites. At Panasqueira it occurs as prismatic crystals with dolomite (CaMg(CO₃)₂), siderite (FeCO₃) and pyrite (FeS₂). Hyaline specimens from Arkansas show high clarity, very regular horizontal striations and terminations with combinations of rhombohedra. Low-temperature hydrothermal secondary chlorite coatings can impart colour and a matte texture.

Smoky quartz owes its colour to natural radiation. Amethyst is the violet variety, coloured by iron; at Iraí it develops the flower habit and began to be recognised and traded systematically from the mid-2000s. Chlorite inclusions can turn quartz green and produce matte surfaces and open growth; in the Himalaya such three-dimensional intergrowths are frequent. On Sérifos, microscopic actinolite inclusions produce green quartz with parallel-aligned crystals, a habit characteristic of the island. The blue quartz from Juanona has been attributed to extremely fine aerinite needles, a currently preferred but not yet settled hypothesis.

Chalcedony is a micro- to cryptocrystalline variety of quartz formed by fibrous aggregates; it precipitates in layers in volcanic cavities and generates botryoidal or stalactitic coatings. The so-called “grape agate” from Mamuju is not agate, but quartz/amethyst with radial growth. Hydrocarbon inclusions occur in three-phase cavities with liquid, gas and solid, and produce fluorescence. Amethyst does not come from quartz veins: it forms in vesicular andesite with gas-created cavities. In Spain, “jacinto de Compostela” refers to red authigenic floating quartzes, their colour due to clay inclusions, which occur as doubly terminated crystals or spheroidal “hedgehog” aggregates and are typically found in marls or gypsum.

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