Brownmillerite is a rare oxide mineral with chemical formula Ca2(Al, Fe)2O5. It is named for Lorrin Thomas Brownmiller (1902–1990), chief chemist of the Alpha Portland Cement Company, Easton, Pennsylvania.
The chemical compound was first recognized in 1932 and named for the chemist who identified it. The naturally occurring mineral form of the compound was first recognized in 1964 for occurrences in the Bellerberg volcano, Ettringen, Mayen-Koblenz, Germany.
- Category: Oxide mineral
- Formula: Ca2(Al, Fe)2O5
- Crystal system: Orthorhombic
- Crystal class: Dipyramidal (mmm).
- Color: Reddish brown
- Crystal habit: As minute square platelets; massive
- Diaphaneity: Semitransparent
- Specific gravity: 3.76
Discovery and occurrence
At the type locality, the mineral occurs within limestone blocks that are contained in a volcanic flow. The limestone blocks had undergone thermal metamorphism. The mineral also occurs in the thermally altered strata of the Hatrurim Formation of Israel. Minerals associated with brownmillerite in the Mayen locality include calcite, ettringite, wollastonite, larnite, mayenite, gehlenite, diopside, pyrrhotite, grossular, spinel, afwillite, jennite, portlandite, and jasmundite. In an Austrian occurrence near Kloch, melilite, mayenite, wollastonite, kalsilite, and corundum are found. Within the Hatrurim area spurrite, larnite and mayenite are associated.
The mineral is similar to the calcium aluminoferrite phases which are commonly found as components of Portland cement.