Epidote-(Sr)

Chemical formula: CaSrFe<sup>3+</sup>Al<sub>2</sub>(Si<sub>2</sub>O<sub>7</sub>)(SiO<sub>4</sub>)O(OH)

A rare mineral from the epidote group, in which strontium (Sr) dominates over calcium (Ca) in one of the structural sites.

## Characteristics Epidote-(Sr) is a mineral belonging to the epidote group, characterized by the dominance of strontium in one of the cationic positions. It forms elongated, prismatic crystals with a rhombic cross-section, often gathered in radial aggregates. Crystals are usually small, reaching up to 2 mm in length. Visually, it is very similar to other minerals in this group, especially epidote and clinozoisite. ## Physical Properties Epidote-(Sr) crystals exhibit a luster ranging from vitreous to resinous. The mineral is translucent. Its Mohs hardness is approximately 6-7, and its density is higher than that of typical epidote due to the presence of heavier strontium. ## Colors and Varieties Epidote-(Sr) has a characteristic dark green to blackish-green color. The streak is greenish-gray. No varieties have been distinguished. ## History and Name The mineral's name refers to its chemical composition – it is an epidote in which strontium (Sr) is the dominant element in a specific structural position. It was officially described in 1990 by P. Bonazzi, G. Menchetti, and V. V. Zaykov. The type locality (locus typicus) is the Saureiskoe deposit in the Southern Urals, Russia.

Properties

Mohs hardness
6-7
Luster
Vitreous to Resinous
Streak
Greenish-gray
Density
3.6-3.7
Cleavage
Perfect on {001}
Fracture
Uneven
Transparency
Translucent
Crystal system
Monoclinic

Diagnostic features

## Identification Identifying epidote-(Sr) based on visual characteristics is practically impossible and requires advanced analytical methods such as X-ray microanalysis (EDS/WDS) or X-ray diffraction (XRD). The dark green color and prismatic crystal habit are typical for the entire epidote group. ## Distinguishing from Similar Minerals It can be confused with epidote, piemontite-(Sr), and other minerals from the epidote group. Distinguishing it from epidote requires confirmation that strontium dominates over calcium in the A2 site. It differs from piemontite-(Sr) by the dominance of iron over manganese. ## Crystal Forms It forms prismatic, elongated crystals, often acicular or rod-like. They usually occur as radial or tangled aggregates.

Geological environment

## Genesis Epidote-(Sr) forms as a result of low-grade metamorphic processes and hydrothermal processes. In its type locality (Urals), it was found in sulfide veins cutting greenschists that underwent greenschist facies metamorphism. ## Mineral Associations This mineral coexists with quartz, calcite, epidote, piemontite-(Sr), thuringite (an iron-rich variety of chlorite), hematite, pyrite, and chalcopyrite. ## Localities Confirmed occurrences of this mineral are extremely rare. The most important ones include: - Saureiskoe deposit, Baimak District, Bashkiria, Southern Urals, Russia - type locality. - Nežichov mine, Rompach, Sobotín, Czech Republic. - Kombat mine, Grootfontein, Namibia.

Rarity

Very rare

For collectors

## Quality Criteria The quality of a collector's specimen primarily depends on the abundance of crystals on the rock matrix and their development. Specimens with distinct, well-formed, radial aggregates of dark green crystals, contrasting with the lighter host rock, are most valued. Due to its rarity, every documented specimen has scientific and collector's value.

Care and storage

## Cleaning It is recommended to clean only with a soft, dry brush or compressed air to remove dust. Due to its rarity and potential fragility of aggregates, avoid washing in water and using ultrasonics. ## What to Avoid Avoid contact with acids and other chemicals that may react with the mineral. Avoid sudden temperature changes and mechanical vibrations that could damage delicate crystals. ## Storage Specimens should be stored in stable conditions, in a closed display box, to protect them from dust, mechanical damage, and direct sunlight, which, however, does not cause fading.

External references

Sources

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