Brownmillerite
Chemical formula: Ca<sub>2</sub>Fe<sup>3+</sup>AlO<sub>5</sub>
A rare iron calcium aluminate, occurring mainly in artificial materials such as Portland cement.
Properties
- Mohs hardness
- 5.5
- Luster
- Metallic
- Streak
- Brown
- Density
- 3.74-3.77
- Cleavage
- Good on {010}
- Fracture
- Uneven
- Transparency
- Opaque
- Crystal system
- Orthorhombic
Diagnostic features
## Identification Field identification of brownmillerite is impossible. It requires specialized laboratory techniques, such as reflected light microscopy, X-ray microanalysis (EDS/WDS), or X-ray diffraction (XRD). In polished samples under a microscope, in reflected light, it can be recognized by its characteristic anisotropy and color. ## Distinguishing from Similar Minerals It can be confused with other opaque, dark minerals in the rock, such as magnetite, ilmenite, or other oxides. Certain distinction is only possible through chemical and structural analysis. ## Crystal Forms It forms microscopic, tabular or lath-like crystals, often arranged in irregular clusters or aggregates. It occurs as inclusions in other minerals, e.g., in gehlenite or larnite.
Geological environment
## Genesis Brownmillerite is a mineral of high-temperature contact metamorphism. It forms as a result of thermal alteration of impure marls and limestones under sanidinite facies conditions, in direct contact with a magmatic intrusion or in xenoliths. It requires temperatures exceeding 800°C. ## Mineral Associations It most often co-occurs with minerals typical of the sanidinite facies, such as larnite, spurrite, gehlenite, mayenite, wollastonite, calcite, and perovskite. ## Localities The most important and classic natural brownmillerite localities include the Mayen region in the Eifel mountains (Germany), where it was discovered. It has also been reported in the Hatrurim Formation in Israel and the Palestinian Territories, in Jordan, as well as in volcanic xenoliths on Mount Vesuvius (Italy) and in the Fuji region (Japan).
Rarity
Extremely rare
For collectors
## Quality Criteria From a collector's perspective, the value of a brownmillerite specimen depends almost exclusively on its scientific and documentary value. The most desirable samples are those from confirmed, classic localities where brownmillerite forms richer concentrations or is clearly visible in association with other rare minerals. The quality of the mineral itself (color, form) is of secondary importance due to its microscopic nature. ## Popular Localities The most prized specimens by specialized collectors come from the type locality in Mayen (Germany) and from the Hatrurim complex (Israel/Palestine), known for the occurrence of many rare high-temperature minerals.
Care and storage
## Cleaning Due to its extreme rarity and most often microscopic nature of occurrence in the host rock, specimens do not require and should not be subjected to cleaning. Any attempt at mechanical or chemical intervention risks destroying the mineral. ## What to Avoid Avoid all acids, which can react with the mineral or surrounding rock. Also avoid ultrasound and sudden temperature changes. ## Storage Specimens should be stored under stable conditions, in a collector's box, protected from dust and mechanical damage. These are most often already polished rock samples (so-called 'polerki') or thin sections intended for microscopic examination.