Brownmillerite
Red/red-brown crystals of brownmillerite from Caspar quarry, Bellerberg Volcano, Ettringen, Mayen, Eifel Mts, Rhineland-Palatinate, Germany
General
Category	Oxide mineral
Formula (repeating unit)	Ca2(Al,Fe)2O5
IMA symbol	Bmlr[1]
Strunz classification	4.AC.10
Dana classification	7.11.2.1
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Ibm2
Unit cell	a = 5.57 Å, b = 14.52 Å, c = 5.34 Å; Z = 4
Identification
Color	Reddish brown
Crystal habit	As minute square platelets; massive
Diaphaneity	Semitransparent
Specific gravity	3.76
Optical properties	Biaxial (−)
Refractive index	nα = 1.960 nβ = 2.010 nγ = 2.040
Birefringence	δ = 0.080
Pleochroism	Distinct; X = Y = yellow-brown; Z = dark brown
2V angle	75° (measured)
References	[2][3][4][5]

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.

Discovery and occurrence

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.^[4]

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.^[3]

The mineral is similar to the calcium aluminoferrite phases which are commonly found as components of Portland cement.^{[citation needed]}

Use as a catalyst

Brownmillerite has been found to be a highly active oxygen evolution reaction catalyst in neutral pH.^[6]