Coquimbite

Chemical formula: AlFe<sup>3+</sup><sub>3</sub>(S<sup>6+</sup>O<sub>4</sub>)<sub>6</sub>(H<sub>2</sub>O)<sub>12</sub>·6H<sub>2</sub>O

Coquimbite is a rare, secondary iron and aluminum sulfate, forming characteristic purple or greenish crystals and granular aggregates.

## Characteristics Coquimbite is a hydrated iron and aluminum sulfate, found in the oxidation zones of ore deposits, especially in arid and desert climates. It forms well-developed, hexagonal crystals with a tabular or short-prismatic habit. More often, however, it occurs as granular, massive, crust-like aggregates or as efflorescences on other minerals. Its crystals rarely exceed a few millimeters, although specimens several centimeters in size are known. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 2.5. It is brittle and has a density similar to quartz, around 2.1 g/cm³. It exhibits a vitreous luster, and on cleavage surfaces, it may show a pearly luster. It is transparent to translucent. Coquimbite is water-soluble, which limits its occurrence to dry environments. ## Colors and Varieties The most characteristic and prized color of coquimbite is light purple or amethystine. It can also be pale pink, colorless, white, greenish, yellow, or brown. The coloration depends on impurities and formation conditions. No named commercial varieties are distinguished. ## History and Name The mineral's name comes from the Coquimbo region in Chile, where it was first found and described. The scientific description was made in 1841 by the German mineralogist August Breithaupt. The name directly refers to the type locality. ## Uses Coquimbite has no significant industrial application. It is of interest only to mineral collectors and scientific institutions as a secondary mineral of weathering zones.

Properties

Mohs hardness
2.5
Luster
Vitreous
Streak
White
Density
2.1
Cleavage
Imperfect on {1011}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Trigonal

Diagnostic features

## Identification Key diagnostic features of coquimbite include its characteristic purple or greenish color, trigonal (hexagonal) crystals, vitreous luster, and low hardness. The most important test is its water solubility, which distinguishes it from many other sulfate minerals. It often has a slightly astringent, metallic taste (taste testing is not recommended for safety reasons). ## Distinguishing from Similar Minerals Coquimbite is sometimes confused with purple fluorite or amethyst, but it is much softer than these. From other sulfates, such as halotrichite or alunite, it is distinguished by its crystal form and color intensity. Unlike many similar secondary minerals, it forms distinct, well-developed crystals. ## Crystal Forms Coquimbite crystals belong to the trigonal system and most often take the form of short, hexagonal prisms or thick tablets. They frequently occur as radial or granular aggregates, as well as crusts and efflorescences.

Geological environment

## Genesis Coquimbite is a secondary mineral, formed by the oxidation of sulfide iron minerals (mainly pyrite and marcasite) under low humidity conditions. It forms as a product of the evaporation of acidic mine waters in arid and desert climates. It is also found as a product of volcanic exhalations (fumaroles). ## Mineral Associations It most often co-occurs with other sulfates formed under similar conditions, such as copiapite, voltaite, halotrichite, melanterite, alunogen, gypsum, and native sulfur. These minerals together form characteristic, multicolored efflorescences and crusts. ## Localities The most important and historical localities of coquimbite are in Chile, in the Atacama (Tierra Amarilla) and Coquimbo regions. Beautiful specimens come from the Alcaparrosa mine in San Juan province, Argentina. It also occurs in the USA (California, Nevada), in Italy near Vesuvius and on Vulcano island, in Spain (Rio Tinto mine), in Germany (Harz Mountains), and in the Czech Republic.

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized coquimbite specimens are those with well-formed, sharp, transparent crystals of intense purple color. Druses and crystal sprays on a contrasting matrix are particularly sought after. Crystal size is very important – specimens with crystals exceeding a centimeter are rare and valuable. Association with other rare sulfates is also significant. ## Popular Localities Specimens from the Alcaparrosa mine in Argentina, which yield large, purple crystals, are considered the best in the world. Classic, though usually smaller, specimens come from the Atacama Desert in Chile. Specimens from volcanic fumaroles (e.g., from Italy) are valued for their specific genesis.

Care and storage

## Cleaning Coquimbite is water-soluble, so it must never be cleaned wet. To remove dust, use only a soft brush or compressed air from a safe distance. ## What to Avoid Avoid all contact with water, water vapor, and high humidity, which can cause it to dissolve or disintegrate. The mineral is sensitive to temperature changes and direct sunlight, which can cause it to fade. Protect it from chemicals and acids. ## Storage Coquimbite specimens must be stored in dry conditions. It is best to place them in a sealed, airtight box (such as a "membrane box" or with a moisture absorber, e.g., silica gel). Display should be in a closed display case, away from sources of moisture and direct light.

Sources

Read more