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Red/red-brown crystals of brownmillerite from Caspar quarry, Bellerberg volcano, Ettringen, Mayen, Eifel Mts, Rhineland-Palatinate, Germany
Category Oxide mineral
(repeating unit)
Strunz classification 04.AC.10
Dana classification
Crystal symmetry Orthorhombic dipyramidal
H-M symbol: (2/m 2/m 2/m)
Space group: Ibmm
Unit cell a = 5.57 Å, b = 14.52 Å, c = 5.34 Å; Z=2
Color Reddish brown
Crystal habit As minute square platelets; massive
Crystal system Orthorhombic
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 [1][2][3]

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[edit]

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.[2]

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.[1]

The mineral is similar to the calcium aluminoferrite phases which are commonly found as components of Portland cement.