Meta-aluminite
Chemical formula: Al<sub>2</sub>S<sup>6+</sup>O<sub>4</sub>(OH)<sub>4</sub>·5H<sub>2</sub>O
Meta-aluminite is a rare, hydrated aluminum sulfate, forming white, earthy aggregates that are a dehydration product of aluminite.
Description
## Characteristics Meta-aluminite is a secondary mineral from the sulfate group, chemically a hydrated aluminum sulfate. It occurs as very fine-grained, compact masses resembling chalk or clay. Its aggregates are typically white, dull, and opaque. Due to its structure and low hardness, it is a very brittle and delicate mineral to the touch. It often forms efflorescences and crusts on rock surfaces. ## Physical Properties This mineral is extremely soft, and its hardness is too low to be precisely determined on the Mohs scale, though it is estimated to be around 1. It is very light, with a density of approximately 1.89 g/cm³. It is characterized by a dull or earthy luster and is opaque. It does not exhibit cleavage, and its fracture is earthy. ## Colors and Varieties Meta-aluminite occurs almost exclusively in white, less commonly with a grayish or yellowish tint, caused by impurities. It does not form color varieties or have commercial names. ## History and Name The mineral's name refers to its chemical composition (aluminum, Lat. *aluminium*) and its close relationship with aluminite. The prefix "meta-" indicates a lower degree of hydration compared to aluminite. The mineral was first described in 1968 by Clifford Frondel based on specimens found in Temple Mountain in Emery County, Utah (USA). ## Uses Meta-aluminite has no industrial application. Its significance is solely scientific and collectible, as a rare product of weathering processes.
Diagnostic features
## Identification Meta-aluminite most often forms white, dull, and earthy aggregates resembling chalk. It is very soft, light, and brittle. A characteristic feature, similar to aluminite, is its tendency to stick to the tongue due to its porous structure. It dissolves in acids. ## Distinguishing from Similar Minerals Macroscopic differentiation of meta-aluminite from aluminite is practically impossible and requires advanced analytical methods, such as X-ray diffraction (XRD), to determine the degree of hydration. It can also be confused with other white, earthy minerals, e.g., kaolinite, which, however, does not dissolve in acids. ## Crystal Forms Well-formed crystals are extremely rare and appear as microscopic fibers or lamellae. Typically, this mineral occurs in compact, earthy, or chalk-like masses.
Geological environment
## Genesis Meta-aluminite is a secondary mineral, formed by the dehydration of aluminite. It forms in weathering zones of aluminum-rich rocks, such as clays or shales, under the influence of sulfate-rich waters. These waters often originate from the oxidation of sulfide minerals, mainly pyrite. It is found as efflorescences and crusts in coal mines, on mine walls, and in fractures of sedimentary rocks. ## Mineral Associations It most often co-occurs with its precursor – aluminite. It is also accompanied by other sulfate minerals formed under similar conditions, such as gypsum, epsomite, alunogen, as well as pyrite and marcasite. ## Localities The most important and classic occurrence is its type locality – Temple Mountain in Utah, USA. It has also been reported in several locations in Europe, including brown coal mines near Tatabánya in Hungary and in Germany.
Rarity
Rare
Collector aspects
## Quality Criteria As a rare and visually unstriking mineral, meta-aluminite is valued primarily by systematic collectors. The value of a specimen depends primarily on the confirmation of its identity by analytical methods. The richness of the aggregate on the rock matrix and co-occurrence with other rare secondary sulfates enhance its attractiveness. ## Popular Localities The most sought-after specimens by collectors, serving as reference material for this species, come from the type locality in Temple Mountain, Utah, USA.
Care and storage
## Cleaning Meta-aluminite specimens are extremely sensitive to water and mechanical damage. Cleaning should be limited to very gently removing dust with a soft brush or blowing it off with an air blower. Absolutely no water or chemical agents should be used. ## What to Avoid Avoid contact with water and other liquids in which the mineral may dissolve. It is very soft and brittle, so it must be protected from impacts, scratching, and abrasion. It is also sensitive to changes in humidity – storage in too dry or too humid an environment can lead to further alterations. ## Storage It is recommended to store specimens in closed, padded containers or "micromount" boxes, which protect them from physical damage, dust, and fluctuations in ambient humidity.