Connoisseur’s Choice: Ferruginous Quartz, Tinejdad, Errachidia Province, Drâa-Tafilalet Region, Morocco

Abstract

Q like many minerals is allochromatic, meaning that it occurs in a wide variety of colors. In contrast, idiochromatic minerals such as azurite, realgar, and sulfur possess a characteristic color. The most common color varieties of quartz—amethyst and smoky quartz—are caused by the chromophores iron and aluminum that substitute for silicon atoms in the quartz structure. Other color varieties—prominently rose quartz (Clifford 2012)—are colored by mineral inclusions. The color, size, and boldness of the ferruginous quartz specimen featured here as figure 1 prompted preparation of this column. Quartz, more precisely known as alpha-quartz or low quartz, is the common, low-temperature, and low-pressure polymorph of silica, SiO2. It typically occurs as colorless to smoky formless grains in many igneous, sedimentary, and metamorphic rock types. Quartz has a Mohs hardness of 7 and is very resistant to chemical attack, so it survives weathering and is the principal component of sand and other sediments. Quartz can be pure silica; as such, it ideally is colorless and transparent with a vitreous luster. Amethyst, chrysoprase, citrine, rose quartz, sard, and smoky quartz are varietal names based on color. Its high hardness, attractive colors, and abundance make quartz an important gem mineral. Quartz crystallizes in the trigonal trapezohedral class (32) of the trigonal crystal system. This crystal class lacks a center of symmetry, so the inherent piezoelectric and pyroelectric properties of quartz make it useful in electronics. This class is also enantiomorphous, i.e., quartz occurs as either right-handed or left-handed crystals (which are mirror images of each other) due to a spiral in its atomic arrangement. Crystals are common and may reach more than 10 meters in length (Rickwood 1981; Rykart 1995).