Delvauxite
Chemical formula: CaFe<sup>3+</sup><sub>4</sub>(PO<sub>4</sub>)<sub>2</sub>(OH)<sub>8</sub>·4-5H<sub>2</sub>O
Delvauxite is a rare, amorphous iron phosphate, occurring as resinous, yellow-brown to reddish-brown masses and coatings.
Properties
- Mohs hardness
- 2.5
- Luster
- Resinous to vitreous
- Streak
- Yellowish brown
- Density
- 1.85 - 2.0
- Cleavage
- None
- Fracture
- Conchoidal to uneven
- Transparency
- Translucent to opaque
- Crystal system
- Amorphous
Diagnostic features
## Identification Delvauxite is recognized by its characteristic mode of occurrence (spherical, reniform aggregates), resinous luster, and relatively low hardness. Its color, from yellow-brown to reddish-brown, is also helpful in identification. As an amorphous mineral, it exhibits no crystal faces or cleavage. ## Distinguishing from similar minerals - **Limonite/Goethite**: These iron oxyhydroxides can form aggregates similar in shape and color. However, delvauxite usually has a more resinous luster and is significantly lighter (lower density). Goethite often also has a fibrous internal structure, which delvauxite lacks. - **Beraunite/Rockbridgeite**: Other iron phosphates that can co-occur with delvauxite. However, they usually form, even if small, visible crystals of a different habit and have higher hardness. ## Crystal forms As an amorphous substance, delvauxite does not form crystals. It occurs exclusively as compact, reniform, spherical (spherulitic), botryoidal aggregates, and as coatings and crusts.
Geological environment
## Genesis Delvauxite is a secondary mineral, formed in the oxidation zones of deposits containing iron sulfides (e.g., pyrite, marcasite) and phosphate minerals (e.g., apatite). It forms as a result of chemical weathering in an environment rich in iron and phosphorus, often in sedimentary rocks such as clays, sandstones, or lignite deposits. ## Mineral associations It most commonly co-occurs with minerals from which it forms or which crystallize under similar conditions. These include gypsum, melanterite, vivianite, limonite, goethite, as well as other, rarer iron phosphates. ## Localities The most important and historical localities are in Belgium, in the vicinity of Liège (Berneau, Visé). This mineral has also been found in the Czech Republic (Stříbro, Nýřany), Germany (Hesse, North Rhine-Westphalia), Hungary, and in small quantities in other localities in Europe and the USA.
Rarity
Rare
For collectors
## Quality criteria The collector appeal of delvauxite primarily depends on the locality and the form of the aggregates. Specimens from historical, classic localities in Belgium and the Czech Republic are most valued. Well-formed, spherical or reniform aggregates with intense, resinous color and luster are more desirable than shapeless masses. Specimen size is also important, although large, aesthetic aggregates are exceptionally rare. ## Popular localities Classic specimens, often sought after by collectors, come from Berneau and Visé in Belgium. Significant finds also come from the coal mine in Nýřany in the Czech Republic, where delvauxite formed spectacular, spherical aggregates.
Care and storage
## Cleaning Delvauxite is a mineral sensitive to water and chemicals. Cleaning should be kept to an absolute minimum. If necessary, a very soft, dry brush can be used to remove dust. Avoid contact with water, which can cause partial dissolution or structural changes. ## What to avoid - Water and other liquids, including ultrasonic cleaners. - Sudden changes in temperature and humidity. - Chemicals, acids, and detergents. - Prolonged exposure to direct sunlight, which can cause fading. ## Storage Delvauxite specimens are best stored in stable conditions, in closed boxes or display cases, to protect them from dust and humidity changes. Due to its brittleness and low hardness, it should be kept away from harder minerals that could scratch it.