Guildite

Chemical formula: Cu²⁺Fe³⁺(S⁶⁺O₄)₂(OH)·4H₂O

Guildite is a rare, secondary copper and iron sulfate, forming characteristic, tabular crystals of an intense chestnut-brown color.

## Characteristics Guildite is a hydrated copper and iron sulfate, occurring in the oxidation zones of ore deposits. It forms very small, but well-formed crystals, most often tabular or platy, less frequently prismatic. They usually appear as single, dispersed crystals or small, loose aggregates on the surface of other minerals. Its most characteristic feature is an intense, chestnut-brown to reddish-brown color. ## Physical Properties Guildite crystals are brittle and exhibit a vitreous luster. This mineral is transparent to translucent. Its Mohs hardness is 2.5, making it relatively soft. The density ranges from 2.72-2.74 g/cm³. ## Colors and Varieties Guildite occurs in a characteristic palette of colors, from chestnut-brown, through reddish-brown, to yellowish-brown. No color or commercial varieties are distinguished. ## History and Name The mineral was named in 1928 in honor of Frank Nelson Guild (1870-1939), professor of mineralogy and petrography at the University of Arizona, who first described this mineral. It was discovered in the Jerome mine in Yavapai County, Arizona, USA. ## Uses Due to its rarity and small crystal size, guildite has no industrial application. It is solely an object of interest for collectors specializing in rare minerals or minerals from specific localities (so-called "micromounts").

Properties

Mohs hardness
2.5
Color
Brown, yellow
Luster
Vitreous
Streak
Pale yellow
Density
2.72
Cleavage
On {001} and {100}, perfect.
Fracture
Conchoidal
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Guildite is identified by its characteristic chestnut-brown color, tabular crystal habit, and specific occurrence – in the oxidation zones of sulfide deposits, often in a dry, desert climate. Its softness and reaction to water are also key diagnostic features. ## Distinguishing from Similar Minerals It can be confused with other secondary iron sulfates, such as jarosite; however, jarosite usually has a more yellow color and a different crystal habit (trigonal). Butlerite also has a similar composition and color but crystallizes in the monoclinic system and forms different aggregate habits. Accurate differentiation often requires advanced analytical methods. ## Crystal Forms Guildite crystals are usually very small, with a thickness of less than a millimeter. They take the form of thin, tabular or platy crystals with a rectangular or hexagonal outline. They occur as single crystals or small, radial aggregates.

Geological environment

## Genesis Guildite is a secondary mineral, formed by the oxidation of sulfide copper and iron deposits (mainly pyrite and chalcopyrite) under conditions of low humidity and a desert climate. It often forms as a product of mine fires, where high temperatures accelerate chemical reactions. ## Mineral Associations It most commonly co-occurs with other secondary sulfates, such as copiapite, butlerite, voltaite, chalcanthite, melanterite, as well as with gypsum, halite, and native sulfur. ## Localities The most important and classic locality is the United Verde mine in Jerome, Yavapai County, Arizona, USA, which is its type locality. It is also known from other mines in dry regions, e.g., from Chuquicamata in Chile.

Rarity

Very rare

For collectors

## Quality Criteria The most prized guildite specimens are those with sharply terminated, well-formed, and transparent crystals of an intense, chestnut-brown color. Rich aggregates of crystals on a contrasting matrix or in association with other rare sulfates are also highly valued. Due to the microscopic size of the crystals, specimens are usually evaluated under magnification. ## Popular Localities By far the most sought-after and classic specimens come from the United Verde mine in Jerome, Arizona (USA). Material from this historic locality is considered exemplary for this species.

Care and storage

## Cleaning Guildite specimens should be cleaned with utmost care, using only a soft brush to remove dust. Due to its solubility in water, all contact with liquids, including distilled water and alcohol, should be avoided. ## What to Avoid The mineral is sensitive to moisture and can be damaged or dissolved upon contact with water. Ultrasonic cleaners, all chemical agents, acids, and bases should be avoided. It should also be protected from high temperatures and direct sunlight, which can lead to its dehydration and color change. ## Storage Guildite requires storage in a dry place, preferably in a sealed, closable "membrane box" or in a display case away from moisture. Exposure should take place in a closed display cabinet away from sources of heat and light.

External references

Sources

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