Hidalgoite

Chemical formula: Pb²⁺Al₃(S⁶⁺O₄)(As⁵⁺O₄)(OH)₆

Hidalgoite is a rare lead aluminum arsenate-sulfate, forming small, vitreous crystals of a characteristic greenish color.

## Characteristics Hidalgoite is a mineral from the beudantite group, chemically classified as a hydrated lead aluminum arsenate-sulfate. It most often occurs as very small, rhombohedral or pseudocubic crystals, which rarely exceed 1 mm in size. It also forms aggregates in the form of crusts, coatings, and earthy masses. Its appearance is usually inconspicuous, and identification requires specialized research. ## Physical Properties This mineral is characterized by a Mohs hardness of 4.5. It has a vitreous to resinous luster, and on fracture surfaces, it can be dull. It is translucent. The density of hidalgoite is approximately 3.71 g/cm³. ## Colors and Varieties Typical colors of hidalgoite include shades from white, through pale green and pistachio green, to emerald green. No named varieties are distinguished. ## History and Name The mineral's name comes from the state of Hidalgo in Mexico, where its type locality is located - the San Pascual mine in the municipality of Zimapán. It was described and recognized as a new mineral in 1953. ## Uses Due to its rarity and occurrence in the form of small crystals, hidalgoite has no industrial application. It is of purely scientific and collectible significance.

Properties

Mohs hardness: 4.5
Luster: Vitreous, Resinous
Streak: White
Density: 3.71
Fracture: Irregular/Uneven,Conchoidal
Transparency: Translucent
Crystal system: Trigonal

Diagnostic features

## Identification Identifying hidalgoite under amateur conditions is very difficult due to the small size of the crystals and its resemblance to many other secondary minerals. Identification relies on chemical (EDS) and X-ray (XRD) analysis. It is distinguished by its greenish color, relatively high density, and co-occurrence with other arsenates in the oxidation zones of ore deposits. ## Distinguishing from Similar Minerals It can be confused with other minerals from the beudantite group, such as beudantite itself, segnitite, or corkite, as well as mimetite or pyromorphite. Distinguishing it from these is possible almost exclusively by advanced analytical methods that allow for the precise determination of its chemical composition (arsenic to sulfur ratio and the presence of lead and aluminum). ## Crystal Forms Hidalgoite crystals are usually very small, taking the form of rhombohedra that can resemble cubes (pseudocubic). However, it most often forms microcrystalline coatings, crusts, and earthy aggregates.

Geological environment

## Genesis Hidalgoite is a secondary mineral, formed in the oxidation (weathering) zones of polymetallic ore deposits rich in arsenic and lead. It forms as a result of the reaction of acidic, sulfate-rich waters with primary minerals containing arsenic, lead, and aluminum. ## Mineral Associations It often co-occurs with other secondary minerals of the oxidation zone, such as beudantite, anglesite, jarosite, plumbogummite, as well as quartz, barite, and limonite. ## Localities The most important and type locality is the San Pascual mine in Zimapán, Hidalgo state, Mexico. It is also known from several other localities worldwide, including the Tsumeb mine in Namibia, Gold Hill in Utah (USA), Broken Hill in New South Wales (Australia), and some sites in Germany and Greece.

Rarity

Rare

For collectors

## Quality Criteria For micromineral collectors, specimens with well-formed, sharp crystals of intense green color, set on a contrasting rock matrix, are most desirable. Due to the small size of the crystals, their quality under magnification is crucial. Specimens from the type locality (Zimapán, Mexico) are particularly prized. ## Popular Localities The most known and valued specimens come from the type locality in Mexico. Specimens from other global localities, such as Tsumeb or Gold Hill, are also sought after by specialists and collectors of rare minerals.

Care and storage

## Cleaning Hidalgoite specimens should be cleaned very carefully, using a soft brush to remove dust. Due to its chemical composition (lead and arsenic content) and possible reactivity, contact with water and any chemical agents should be avoided. After each contact with the mineral, thorough hand washing is recommended. ## What to Avoid Avoid ultrasonic cleaners, steam cleaning, and contact with acids and other chemicals that may damage or dissolve the mineral. Protect it from high temperatures and scratching by harder minerals. ## Storage Hidalgoite specimens are best stored in closed, stable containers or display cases to protect them from dust and mechanical damage. It is recommended to separate it from other minerals to avoid mutual scratching. Due to its toxic components, it should be stored out of reach of children and pets.