Phoenicochroite
Chemical formula: Pb<sup>2+</sup><sub>2</sub>O(Cr<sup>6+</sup>O<sub>4</sub>)
Phoenicochroite is a rare chromate mineral with a striking, bright red color, most often forming aggregates of small, bladed crystals.
Properties
- Mohs hardness
- 3-3.5
- Luster
- Resinous to adamantine
- Streak
- Brick red
- Density
- 7.01
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification The most important diagnostic feature of phoenicochroite is its intense, bright red color and brick-red streak. Small, bladed crystals forming fan-shaped aggregates are also characteristic. High density, noticeable even in small specimens, is also helpful in identification. ## Distinguishing from Similar Minerals Phoenicochroite is sometimes confused with crocoite (PbCrO₄), with which it often co-occurs. Crocoite, however, has a more orange hue and forms better-developed, prismatic crystals with a different habit. The streak of crocoite is orange-yellow, while that of phoenicochroite is brick-red. It is distinguished from realgar by its higher density and crystal form. ## Crystal Forms Phoenicochroite crystals are usually very small, thin, and tabular or bladed, often with pseudohexagonal outlines. They form fan-shaped, radial, or stellate aggregates, as well as thin crusts and coatings on the host rock.
Geological environment
## Genesis Phoenicochroite is a secondary mineral, forming in the oxidation zones (so-called iron caps) of hydrothermal lead-rich deposits (mainly galena), where chromium compounds are also present. It forms under conditions of low carbon dioxide concentration. ## Mineral Associations This mineral most often co-occurs with other secondary lead and chromium minerals. Its typical associates include crocoite, wauquelinite, pyromorphite, cerussite, galena, as well as iron and manganese oxides (limonite, goethite). ## Localities The most important and historical locality for phoenicochroite is Berezovskiy Zavod (Serebryanaya mine) in the Urals, Russia. Other known localities include the Callenberg mine in Saxony (Germany), the Adelaide mine in Dundas, Tasmania (Australia), and several places in Arizona (USA), e.g., the Potter-Cramer mine.
Rarity
Rare
For collectors
## Quality Criteria The most highly prized phoenicochroite specimens are those with well-formed, even if small, crystals of intense, pure red color, forming aesthetic, fan-shaped aggregates. Contrast with light host rock or co-occurrence with other rare minerals, such as crocoite, significantly increases the value of the specimen. Crystal size is crucial – specimens with crystals exceeding a few millimeters are exceptionally rare and sought after. ## Popular Localities Classic and most desired specimens come from Berezovsk, in the Urals, Russia. These are the historical standard for this mineral. In recent years, interesting specimens from Dundas, Tasmania, have also appeared on the market.
Care and storage
## Cleaning Phoenicochroite specimens should be cleaned with the utmost care, using only a soft brush to remove dust. Due to its brittleness and solubility in acids, contact with water and any chemical agents should be avoided. Ultrasonic cleaning is absolutely forbidden. ## What to Avoid The mineral is sensitive to acids, in which it dissolves. Sudden temperature changes and exposure to moisture should be avoided. As a mineral containing lead and chromium (VI), it is toxic – avoid inhaling dust and wash hands after any contact with the specimen. ## Storage Collector's specimens of phoenicochroite should be stored under stable conditions, in a dry place, away from direct sunlight. It is best to place them in a sealed display box to protect them from dust and mechanical damage.