Ecdemite

Chemical formula: Pb₂+₆As₃+₂O₇Cl₄

Ecdemite is a rare lead, arsenic, and chlorine mineral, occurring as soft, yellowish masses or small crystals.

## Characteristics Ecdemite is a rare, secondary halide mineral containing lead, arsenic, and chlorine. It typically forms granular, massive aggregates or crusts. Less commonly, it is found as small, imperfect tabular crystals, often grouped into irregular aggregates. Its appearance is usually inconspicuous, and specimens rarely reach large sizes. ## Physical Properties This mineral is very soft and heavy due to its high lead content. It is characterized by a greasy or resinous luster. It is opaque or at most translucent in thin fragments. Its brittleness and low hardness make it very delicate. ## Colors and Varieties Ecdemite most often ranges in color from light yellow to greenish-yellow, sometimes transitioning to grayish or greenish. No named varieties are distinguished. ## History and Name The mineral's name comes from the Greek word *ἔκδημος* (ekdēmos), meaning "unusual" or "rare," referring to its atypical chemical composition and rarity. It was first described in 1877 by the Swedish mineralogist Lars Johan Igelström based on samples from the Långban mine in Sweden. In the same year, independently, it was described by the German mineralogist Gerhard vom Rath as "heliophyllite" based on material from the Harstig mine in Sweden. The name ecdemite took precedence. ## Uses Ecdemite has no industrial applications. It is of purely scientific interest and is valued by specialized collectors of rare minerals.

Properties

Mohs hardness: 2.5-3
Luster: Greasy
Streak: Pale yellow
Density: 7.2-7.6
Cleavage: Good on {001}
Fracture: Uneven
Transparency: Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Ecdemite is identified by its characteristic yellow or greenish-yellow color, greasy luster, very high density, and low hardness. It occurs in a specific geological environment, in association with other lead and arsenic minerals. ## Distinguishing from Similar Minerals It can be confused with other secondary lead minerals of similar color, such as mimetite or vanadinite, but it is significantly softer than them. Its greasy luster also distinguishes it, whereas the aforementioned minerals have a more vitreous or resinous luster. Final identification often requires chemical analysis. ## Crystal Forms It forms thin, tabular crystals, often with hexagonal or octagonal outlines, as well as irregular lamellae. Crystals are usually small and grouped into rosette-like or haphazard aggregates. It more commonly occurs as granular and massive aggregates.

Geological environment

## Genesis Ecdemite is a secondary mineral formed in the oxidation zones of ore deposits, in metamorphic rocks rich in manganese, lead, and arsenic. It forms as a result of the alteration of primary minerals containing these elements. ## Mineral Associations It most commonly co-occurs with minerals such as: barite, calcite, hausmannite, magnetite, mimetite, hedyphane, franklinite, willemite, and other rare minerals from Långban and Franklin. ## Localities The most important and classic localities for this mineral are the mines in the Långban region (e.g., Långban, Harstig) in Värmland, Sweden. It is also known from historical sites in the USA - Franklin and Sterling Hill in New Jersey. Apart from these two famous localities, its occurrence is extremely limited.

Rarity

Rare

For collectors

## Quality Criteria The most prized specimens by collectors are those with well-formed, even if small, crystals of an intense, pure yellow color. Association with other rare minerals, especially from classic localities like Långban or Franklin, is also important. Specimens with visible, sharply defined crystals on a contrasting matrix are much more highly valued than massive aggregates. ## Popular Localities By far the most sought-after and valuable specimens come from the historical, classic localities: Långban in Sweden and Franklin in New Jersey, USA. Material from these places is considered the standard for this species.

Care and storage

## Cleaning Ecdemite is extremely delicate and chemically sensitive. Cleaning should be kept to an absolute minimum. If necessary, a very soft brush can be used to remove dust. Avoid water and any chemical agents. ## What to Avoid Absolutely avoid contact with water, acids, detergents, and other chemicals. The mineral is very soft, susceptible to scratches and mechanical damage. Due to its lead and arsenic content, avoid inhaling dust and wash hands after any contact with the specimen. ## Storage Ecdemite specimens should be stored in a dry, stable environment, preferably in a closed display box, away from other minerals that could scratch it. It should be protected from shocks and vibrations.

Sources

IMA Mineral List (CNMNC)