Crandallite

Chemical formula: CaAl₃(PO₄)(PO₃OH)(OH)₆

Crandallite is a calcium-aluminum phosphate mineral of the plumbogummite group, most commonly forming radial aggregates and crusts.

## Characteristics Crandallite is a hydrated calcium and aluminum phosphate belonging to the plumbogummite group. It rarely forms distinct, tabular or rhombohedral crystals. It usually occurs as radial or spherulitic aggregates, spherical concretions, crusts, as well as earthy and chalk-like masses. ## Physical properties This mineral is characterized by a hardness of 5 on the Mohs scale. Its density is approximately 2.78 g/cm³. The luster is variable, from vitreous on crystal faces to dull or chalky in the case of earthy aggregates. It is translucent to opaque. ## Colors and varieties Crandallite most often takes on a yellow, white, or gray color. In transmitted light, it is colorless. No named varieties of this mineral are distinguished. ## History and name The mineral is named in honor of Milan L. Crandall Jr. (1883-1936), a mining engineer from Knight-Crandall, who provided the first samples for study. The mineral was described in 1917 by G.F. Loughlin and W.T. Schaller based on specimens from the Brooklyn mine in the Tintic district, Utah, USA. ## Uses Crandallite has no significant industrial application. However, it is an interesting mineral sought after by collectors specializing in rare minerals and phosphates.

Properties

Mohs hardness: 5
Luster: Vitreous, Dull, Chalky
Streak: White
Density: 2.78
Cleavage: On {0001}, perfect.
Fracture: Irregular/Uneven
Transparency: Translucent,Opaque
Crystal system: Trigonal

Diagnostic features

## Identification Crandallite is most easily recognized by its characteristic forms of occurrence – radial, spherical aggregates and crusts. Its color (usually yellowish or white), dull or vitreous luster, and hardness of 5 on the Mohs scale are also helpful. It reacts with acids. ## Distinguishing from similar minerals It can be confused with other minerals from the plumbogummite group (e.g., plumbogummite, goyazite), from which differentiation requires advanced chemical analysis (EDS/XRD). It is distinguished from calcite by its higher hardness and lack of vigorous effervescence in cold, dilute hydrochloric acid. It is distinguished from variscite by its higher hardness and density. ## Crystal forms Crystals are rare, usually occurring as thin, hexagonal tablets or rhombohedra. Most commonly found are radial and spherulitic aggregates, as well as massive, earthy or chalk-like concretions and crusts.

Geological environment

## Genesis Crandallite is a secondary mineral. It forms in the oxidation zones of ore deposits, in phosphate-rich sedimentary rocks (phosphorites), and as a result of low-temperature hydrothermal processes. It can also form from the interaction of phosphate-rich solutions with igneous rocks, such as carbonatites and pegmatites. ## Mineral associations It co-occurs with minerals such as variscite, wavellite, apatite, strengite, limonite, quartz, kaolinite, as well as other phosphates from the plumbogummite group. ## Locations The most important localities worldwide include the discovery site in Utah (USA), as well as Bolivia (Siglo XX mine), Brazil (Minas Gerais), Germany (Hesse), Australia (Queensland), Senegal, and the Democratic Republic of Congo.

Rarity

Not very common

For collectors

## Quality criteria The most valued by collectors are specimens with well-formed, even if small, crystals. Well-formed, spherical aggregates with intense color, contrasting with the rock matrix, are also attractive. Specimens from the classic Utah locality are historically important. ## Popular localities Among the most famous localities from which collector specimens originate are the mines in the Tintic district in Utah (USA), the Siglo XX mine in Llallagua (Bolivia), where crandallite forms attractive spherulites, and some localities in Brazil.

Care and storage

## Cleaning Crandallite specimens should be cleaned very carefully, preferably with a soft brush dry. If necessary, compressed air can be used to remove dust from crevices. Avoid washing with water, especially in the case of earthy and friable aggregates. ## What to avoid Avoid contact with acids and other chemicals that may damage the mineral. Crandallite is relatively soft and brittle, so it must be protected from impacts and scratches. It should not be heated or subjected to ultrasound. ## Storage Specimens should be stored in separate, padded boxes or display cases, away from harder minerals. Protect from dust and moisture. Due to its perfect cleavage, avoid pressure on crystal surfaces.