Lehmannite
Chemical formula: Na₁₈Cu²⁺₁₂Ti⁴⁺O₈(As⁵⁺O₄)₈FCl₅
Lehmannite is a very rare sodium, copper, and titanium arsenate, forming dark green, tabular crystals in volcanic exhalation products.
Properties
- Mohs hardness
- 4.5-0
- Streak
- Greyish green with an olive hue.
- Density
- 3.676
- Fracture
- Irregular/Uneven
- Transparency
- Translucent,Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Field identification of lehmannite is impossible due to the microscopic size of its crystals. Identification requires advanced laboratory methods, such as chemical composition analysis (EDS) and X-ray diffraction (XRD). In a collection, it can be preliminarily identified based on its characteristic appearance (dark green, tabular crystals in aggregates) and confirmed locality (Arsenatnaya fumarole). ## Distinguishing from Similar Minerals Lehmannite co-occurs with many other rare arsenates from the Tolbachik fumaroles, such as arsmirandite, erykłaksmanite (erythrosiderite), or kozyrevskite. Distinguishing it from these minerals is only possible through chemical and crystallographic analysis. Visual similarity is too great to serve as a basis for certain identification. ## Crystal Forms Lehmannite forms thin, tabular or bladed crystals with an elongated habit. These crystals combine into radial, stellate, or chaotic aggregates, and also form thin crusts on the rock surface.
Geological environment
## Genesis Lehmannite is a mineral of volcanic origin, formed by sublimation from volcanic gases in a fumarolic environment. It crystallizes at high temperatures (approximately 450-500°C) in exhalation zones rich in arsenic, copper, and chlorine. ## Mineral Associations This mineral occurs in association with other rare arsenates and oxides formed under similar conditions. It most commonly co-occurs with hematite, tenorite, sanidine, as well as arsmirandite, erykłaksmanite (erythrosiderite), lammerite, urusovite, and johillerite. ## Localities The only confirmed occurrence of lehmannite in the world is its type locality - Arsenatnaya fumarole, located on the second cinder cone of Tolbachik volcano on the Kamchatka Peninsula in Russia.
Rarity
Very rare
For collectors
## Quality Criteria The quality of a lehmannite specimen is primarily assessed based on the richness and development of crystals within the aggregate. The most prized specimens are those with clearly visible, well-formed, radial crystal clusters on a contrasting rock matrix. The size of the aggregates and the intensity of the color also influence the specimen's attractiveness. ## Popular Localities All lehmannite specimens available on the collector's market come from a single location in the world – the fumarole of Tolbachik volcano in Kamchatka. It is a mineral typical of this locality and has not yet been found anywhere else.
Care and storage
## Cleaning Lehmannite specimens are extremely delicate and small, therefore they should not be cleaned mechanically or chemically. Any attempts at cleaning risk destroying or losing the crystals. They should be left undisturbed on their original rock matrix. ## What to Avoid Contact with water, chemicals, ultrasound, and any vibrations or impacts should be avoided. The mineral is sensitive to changes in humidity and temperature, which can occur in fumarolic environments, so it should be stored under stable conditions. ## Storage Micromount lehmannite specimens should be stored exclusively in specialized "micromount" boxes, which protect them from dust, moisture, and mechanical damage. Display should only take place under a microscope.