Emmonsite

Chemical formula: Fe<sup>3+</sup><sub>2</sub>(Te<sup>4+</sup>O<sub>3</sub>)<sub>3</sub>·2H<sub>2</sub>O

Emmonsite is a rare iron tellurite, forming characteristic, minute crystals and coatings of an intense yellowish-green color.

## Characteristics Emmonsite is a hydrated iron tellurite, occurring in the oxidation zones of ore deposits rich in tellurium. It forms aggregates of minute, acicular or bladed crystals, which often arrange themselves into radial, rosette-like, or spherulitic forms. It is also found as earthy or powdery coatings and crusts on the host rock. Its most characteristic feature is its vivid, bright, yellowish-green to greenish-yellow color. ## Physical Properties Emmonsite crystals are very small and brittle. The mineral exhibits a vitreous luster, and on fracture surfaces, it can be dull or earthy. It is a relatively soft and light mineral, with a density of approximately 4.5 g/cm³. It is transparent to translucent, although in earthy aggregates it appears opaque. ## Colors and Varieties The dominant and virtually sole color of emmonsite is an intense yellowish-green, in various shades. No color or commercial varieties are distinguished. ## History and Name The mineral was first described in 1885 by Whitfield Hillebrand. The name "emmonsite" was given in honor of Samuel Franklin Emmons (1841-1911), an American economic geologist working for the United States Geological Survey, who studied the deposits from which the first mineral samples originated (Tombstone, Arizona, USA). ## Uses Emmonsite has no industrial application. Its significance is purely scientific and as a collector's item, as a rare and aesthetic secondary mineral.

Properties

Mohs hardness
5
Luster
Vitreous
Streak
Pale greenish yellow
Density
4.5
Cleavage
Perfect on {010}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The key diagnostic feature of emmonsite is its characteristic, bright, yellowish-green color and its mode of occurrence – most often as minute, acicular crystals forming radial aggregates, rosettes, or crusts. Its occurrence in the oxidation zones of tellurium deposits is also an important clue. ## Distinguishing from Similar Minerals Emmonsite is sometimes confused with other secondary tellurium minerals of similar color, such as mackayite (usually greener, different crystals) or poughite (more yellow, different habit). Distinguishing it from these minerals without advanced analysis (e.g., XRD) can be very difficult or impossible, especially for microscopic specimens. ## Crystal Forms Emmonsite crystallizes in the monoclinic system. It forms elongated, acicular or bladed crystals, often with a rhombic cross-section. These crystals rarely occur individually, usually combining into radial, stellar, or spherulitic aggregates. It is also often found in the form of thin crusts and coatings.

Geological environment

## Genesis Emmonsite is a secondary mineral, forming exclusively in the oxidation (weathering) zones of hydrothermal ore deposits rich in tellurides. It forms under low temperature and pressure conditions as a result of the weathering of primary tellurium minerals (e.g., tellurobismuthite, altaite, hessite) in the presence of iron minerals (e.g., pyrite). ## Mineral Associations This mineral often co-occurs with other oxidation products of tellurides. Typical associated minerals include: native tellurium, mackayite, poughite, rodalquilarite, as well as quartz, barite, hematite, goethite, and various iron sulfates. ## Localities The most important and classic localities for emmonsite are the mines in the Tombstone area, Cochise County, Arizona, USA (type locality). It is also known from the El Plomo mine in Honduras; Moctezuma in Mexico; the Delamar mine in Lincoln County, Nevada, USA; as well as the Kankberg mine in Sweden and the Kochbulak deposit in Uzbekistan.

Rarity

Rare

For collectors

## Quality Criteria Specimens with well-formed, distinct crystals forming aesthetic, radial aggregates are most valued by collectors. The intensity and purity of the yellowish-green color significantly increase the value of a specimen. Contrast with the rock matrix and the presence of rare associated minerals are also important. Due to the microscopic nature of most crystals, specimens with crystals visible to the naked eye are particularly sought after. ## Popular Localities The best specimens in the world are considered to be those from the historical localities in Tombstone, Arizona (USA) and from the mines in Moctezuma (Sonora, Mexico). Specimens from these places, especially those acquired in the 19th and early 20th centuries, fetch high prices and are prized additions to advanced systematic mineral collections.

Care and storage

## Cleaning Emmonsite specimens are extremely delicate and brittle. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove loose dust. Wet cleaning, and especially the use of ultrasonic cleaners, is strongly discouraged. ## What to Avoid Avoid contact with water and any chemicals that could damage or dissolve the minute crystals. The mineral is sensitive to shocks, impacts, and abrasion. It should be protected from high temperatures and sudden temperature changes. ## Storage Specimens should be stored under stable conditions, in a sealed "micromount" box, which protects them from dust, mechanical damage, and humidity. It should not be exposed to direct sunlight, although its particular sensitivity to light is not known.

External references

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