Grandaite

Chemical formula: Sr²⁺₂Al(As⁵⁺O₄)₂(OH)

Grandaite is a rare strontium and aluminum arsenate, distinguished by its intense, orange color and small, spherical aggregates.

## Characteristics Grandaite is an arsenate belonging to the brackebuschite supergroup. It forms very small, spherical or hemispherical aggregates, whose diameter rarely exceeds 0.5 mm. The individual crystals comprising these aggregates are platy or tabular and have an elongated shape. Due to their small size, the features of individual crystals are difficult to observe without specialized equipment. ## Physical Properties This mineral is characterized by a hardness ranging from 6-6.5 on the Mohs scale, making it relatively hard. Its density, calculated based on the chemical formula and unit cell parameters, is 4.378 g/cm³. The luster of grandaite is described as resinous to vitreous. It is a translucent mineral. ## Colors and Varieties Grandaite occurs in characteristic, vivid colors, from bright orange, through salmon, to brown. The streak, which is the color of the powdered mineral, is reddish-brown. No named varieties of this mineral have been identified. ## History and Name The mineral's name comes from the Italian mineral collector, Federico Granda (born 1968), in recognition of his contribution to the study of the mineralogy of the Western Alps. Grandaite was approved as a new mineral species by the International Mineralogical Association (IMA) in 2012. The type locality, from which it originates and where it was first described, is the Valletta mine in Italy.

Properties

Mohs hardness
6-6.5
Luster
Resinous
Streak
Reddish brown
Density
4.378
Fracture
Irregular/Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Key diagnostic features of grandaite are its unique form of occurrence as small, spherical aggregates, its intense orange to brown color, and its reddish-brown streak. A hardness in the range of 6-6.5 is also an important clue. However, final identification requires advanced analytical methods, such as Raman spectroscopy or chemical analysis (EDS). ## Distinguishing from Similar Minerals Grandaite can be confused with other arsenates of similar color and form, such as mimetite or some varieties of wallkillite. Distinguishing it from these minerals based on visual characteristics is practically impossible and requires specialized studies. Geological context and associated minerals can provide additional clues. ## Crystal Forms Grandaite forms characteristic spherical or hemispherical aggregates composed of very small, radially arranged tabular or platy crystals.

Geological environment

## Genesis Grandaite is a secondary mineral, formed in the oxidation zones of ore deposits. In the type locality (Valletta mine), it formed as a result of hydrothermal processes that altered quartz and manganese-rich rocks. Its chemical composition (presence of strontium and arsenic) is directly related to the specific chemical composition of hydrothermal solutions in this location. ## Mineral Associations This mineral co-occurs with other rare minerals, such as arsenogoyazite, arsenoflorencite-(Ce), agardite-(Y), filipstadite, hematite, quartz, and unidentified minerals from the microlite group and apatite supergroup. ## Localities The only confirmed and described occurrence of grandaite in the world is its type locality - the Valletta mine (Vallone della Valletta), located in the municipality of Canosio, in the province of Cuneo, Piedmont, Italy.

Rarity

Very rare

For collectors

## Quality Criteria As a microscopic mineral, the main criterion for evaluating specimen quality is the richness and aesthetics of the aggregates on the rock matrix. The most prized specimens are those with numerous, well-formed, and intensely colored grandaite spherules, clearly contrasting with the background. The presence of rare associated minerals is also important. ## Popular Localities The only source of grandaite specimens is its type locality, the Valletta mine in Italy. All specimens available on the collector's market originate from this single location.

Care and storage

## Cleaning Due to the small size and rarity of specimens, mechanical cleaning is highly inadvisable. If absolutely necessary, compressed air can be used to remove loose dust. Contact with water and any chemicals should be avoided. ## What to Avoid As an arsenate, grandaite may be sensitive to acids. Ultrasonic cleaners, steam cleaners, and sudden temperature changes should be avoided. Specimens should be protected from moisture and direct, prolonged exposure to sunlight, which could theoretically affect its color. ## Storage Grandaite specimens, due to their microscopic nature, are best stored in specialized, sealed "micromount" boxes. This protects them from dust, mechanical damage, and contact with other minerals.

External references

Sources

Read more