Merelaniite
Chemical formula: Pb<sup>2+</sup><sub>4</sub>Mo<sup>4+</sup><sub>4</sub>V<sup>3+</sup>Sb<sup>3+</sup>S<sup>2-</sup><sub>15</sub>
Merelaniite is a rare, lead-black mineral with a metallic luster, known for its flexible, coiled crystals resembling scrolls.
Properties
- Mohs hardness
- 2
- Luster
- Metallic
- Streak
- Black
- Density
- 6.16
- Cleavage
- Perfect on {0001}
- Fracture
- Micaceous
- Transparency
- Opaque
- Crystal system
- Trigonal
Diagnostic features
## Identification Identification of merelaniite is almost exclusively possible based on its unique morphology. The characteristic, scroll-like crystals with a metallic luster, resembling papyrus scrolls, are its main diagnostic feature. It occurs in the form of cylindrical or tubular aggregates. ## Distinguishing from Similar Minerals Cylindrite is the only mineral with a similar, coiled morphology, but it differs in chemical composition and typically forms larger, more massive aggregates. Final differentiation requires advanced analytical methods, such as electron microprobe (EDS/WDS), due to merelaniite's complex and specific chemical composition. ## Crystal Forms Merelaniite forms unique, cylindrical crystals in the shape of scrolls or tubes. These scrolls are the result of stresses within the crystal structure, causing very thin layers of the mineral to coil. Individual crystals rarely exceed 1 mm in length and several tens of micrometers in diameter.
Geological environment
## Genesis Merelaniite forms under conditions of high-grade metamorphism. Its typical environment is graphite- and diopside-rich lenses in gneisses that have been subjected to high temperatures and pressures. The mineral crystallizes from hydrothermal fluids in the late stages of metamorphic processes. ## Mineral Associations This mineral occurs in close association with graphite, in which it is often embedded. It is also accompanied by diopside, alabandite, tanzanite (zoisite), quartz, wurtzite, and other rare sulfides such as patronite. ## Localities The only confirmed locality for merelaniite in the world is the Merelani Hills in the Arusha region of Tanzania. This is the type locality for this mineral.
Rarity
Extremely rare
For collectors
## Quality Criteria The quality of a merelaniite specimen is primarily assessed based on the perfection and size of its unique, coiled crystals. The most desirable are well-formed, sharp, metallic-lustered scrolls that clearly contrast with the surrounding graphite or other host mineral. The size of the scroll itself is also important – specimens exceeding 1 mm are considered exceptional. The aesthetic arrangement of several crystals on a matrix significantly increases its collector's value. ## Popular Localities The only source of merelaniite specimens is its type locality: the Merelani Hills in Tanzania. All specimens available on the collector's market come from this single location, making them exceptionally sought after by collectors specializing in rare minerals and minerals from specific localities.
Care and storage
## Cleaning Merelaniite specimens are extremely delicate and typically microscopic, occurring as inclusions in other minerals, such as graphite. They do not require and should not be cleaned in a traditional manner. Any attempt at mechanical cleaning will almost certainly destroy the fragile and flexible crystal. ## What to Avoid Avoid all chemicals, ultrasonics, and sudden temperature changes. The crystals are very soft and susceptible to mechanical damage, including scratches and crushing. They should be protected from dust, which can act as an abrasive material. ## Storage The best way to store merelaniite is to leave it in its original form, i.e., as an inclusion in the host mineral. Specimens should be stored exclusively in specialized "micromount" boxes or perky boxes to protect them from physical damage, dust, and vibrations.