Derbylite
Chemical formula: Fe<sup>3+</sup><sub>4</sub>Ti<sup>4+</sup><sub>3</sub>Sb<sup>3+</sup>O<sub>13</sub>(OH)
Derbylite is a rare iron, titanium, and antimony oxide, forming small, black crystals with a metallic luster.
Properties
- Mohs hardness
- 6.5-7
- Luster
- Metallic to submetallic
- Streak
- Dark brown
- Density
- 4.95
- Cleavage
- Imperfect on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Derbylite is identified based on its characteristic crystal form (sharp, pseudo-octahedral crystals), black color, metallic luster, and high hardness. Its specific occurrence environment is also key, especially in association with other titanium and antimony minerals. Due to its small size, positive identification almost always requires laboratory analysis (e.g., EDS). ## Distinguishing from Similar Minerals It can be confused with other black, metallic minerals such as magnetite, ilmenite, or hematite. Derbylite differs from them by its characteristic, sharp crystal form and lack of magnetism (unlike magnetite). Final differentiation from rarer oxides requires specialized studies. ## Crystal Forms Crystals are usually very small, less than a millimeter in size. They take the form of sharp, steep pyramids, often resembling distorted octahedra (pseudo-octahedra). They occur as single, overgrown crystals or in small groups on the surface of the matrix rock.
Geological environment
## Genesis Derbylite is a mineral of hydrothermal or metamorphic origin. It forms in veins cutting through iron and manganese ore deposits that have undergone metamorphic processes. Its crystallization is associated with the presence of solutions rich in titanium and antimony. ## Mineral Associations This mineral often co-occurs with hematite, rutile, anatase, brookite, quartz, and other rare antimony minerals. In its type locality (Brazil), it was found in association with tripuhyite. ## Localities The most important and classic derbylite locality is in the iron ore mines near Ouro Preto in Minas Gerais state, Brazil. This is its type locality. Beyond Brazil, its occurrence has also been confirmed in Buca della Vena, Italy, and in the Vysoká-Lhota mine in the Czech Republic.
Rarity
Very rare
For collectors
## Quality Criteria The most prized derbylite specimens are those with sharp, well-formed, and lustrous crystals that stand out clearly from the matrix rock. Due to the small size of the crystals, their perfect form and lack of damage are crucial. Contrast with a light matrix, e.g., quartz, enhances the visual appeal of the specimen. The size of individual crystals, even if exceeding 1 mm, significantly impacts value. ## Popular Localities Specimens from the historical type locality in Minas Gerais, Brazil, are by far the most sought after by collectors. Specimens from other, few localities, such as those from Italy, are also valued due to their rarity.
Care and storage
## Cleaning Derbylite specimens, due to their small and brittle crystals, should be cleaned very carefully. The safest method is to use compressed air to remove dust. If wet cleaning is necessary, a soft brush moistened with distilled water can be used, followed by thorough drying of the specimen. ## What to Avoid Contact with acids and other aggressive chemicals should be avoided. Ultrasonic cleaning is absolutely not recommended, as it can cause small crystals to detach from the matrix rock. The mineral is stable, but extreme temperatures should be avoided. ## Storage Derbylite specimens are best stored in sealed "micromount" boxes, which protect them from dust and mechanical damage. A label with precise locality information is crucial for the specimen's value.