Neltnerite
Chemical formula: CaMn<sup>3+</sup><sub>6</sub>O<sub>8</sub>(SiO<sub>4</sub>)
Neltnerite is a rare mineral from the braunite group, a calcium and manganese oxysilicate, forming black, metallic grains and massive aggregates.
Properties
- Mohs hardness
- 6
- Luster
- Metallic
- Streak
- Black
- Density
- 4.63
- Cleavage
- None
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Tetragonal
Diagnostic features
## Identification Neltnerite can be preliminarily identified by its black color, metallic luster, and occurrence as massive aggregates or embedded grains. Its high density is also characteristic. It often co-occurs with other manganese minerals, which is an important field indicator. However, definitive identification requires advanced methods such as chemical analysis (EDS) and X-ray diffraction (XRD). ## Differentiation from Similar Minerals Neltnerite is visually very similar to braunite, with which it forms intergrowths and is genetically related. Distinguishing these two minerals without specialized equipment is practically impossible. It can also be confused with other black, metallic manganese oxide minerals, such as hausmannite or hollandite. It differs from them in chemical composition and crystal structure. ## Crystal Forms Well-formed neltnerite crystals are extremely rare. It typically forms massive, granular aggregates or irregular grains embedded in the rock. Observed crystal forms have a pseudo-octahedral habit.
Geological environment
## Genesis Neltnerite is a hydrothermal mineral associated with contact metamorphism processes. It forms in manganese deposits that have been subjected to hydrothermal solutions at relatively low temperatures. In its type locality (Tachgagalt), it occurs in veins cutting Precambrian rhyolitic volcanic tuffs. ## Mineral Associations This mineral often co-occurs with other manganese minerals. Its most common associates are braunite, marokite, hausmannite, gaudefroyite, quartz, calcite, barite, and various manganese oxides. ## Localities The most important and well-documented occurrence of neltnerite worldwide is its type locality – the Tachgagalt mine, located near Ouarzazate in the Anti-Atlas Mountains of Morocco. This is the primary source of specimens of this mineral. Outside of Morocco, its presence has also been reported in the Wessels mine in Hotazel, in the Kalahari Manganese Field in the Republic of South Africa.
Rarity
Rare
For collectors
## Quality Criteria The collector appeal of neltnerite is primarily related to its rarity. The most prized specimens are those in which neltnerite forms rich, massive aggregates or clearly visible grains contrasting with the lighter host rock. Due to the lack of well-formed crystals, form is not the main evaluation criterion. Specimens with documented association with other rare minerals from this locality, such as gaudefroyite or marokite, are also highly valued. ## Popular Localities The only commercially significant source of neltnerite specimens is its type locality: the Tachgagalt mine in Morocco. Specimens from South Africa are rare on the collector's market.
Care and storage
## Cleaning Neltnerite specimens can be cleaned with a soft, dry brush to remove dust. For heavier soiling, distilled water may be used, but prolonged soaking should be avoided. After wet cleaning, the specimen should be thoroughly dried. ## What to Avoid Contact with acids and strong chemical agents, which can damage its surface, should be avoided. Despite its relatively high hardness, it should be protected from impacts and contact with harder minerals (e.g., quartz, corundum) to prevent scratches. ## Storage Neltnerite specimens are best stored in separate, padded boxes or display cases to prevent abrasion and mechanical damage. It is not sensitive to sunlight or humidity changes, so it can be displayed under standard conditions.