Bredigite

Overview

Bredigite is a rare calcium magnesium nesosilicate that forms at high temperatures where limestone is baked by intruding magma. It is one of a small family of "high-temperature" calcium silicates, closely allied to larnite, and is of interest chiefly to mineralogists and cement chemists rather than to the wider gem and crystal trade. Specimens are almost always small, pale grains embedded in metamorphic rock, so bredigite is a name encountered far more often in scientific papers than on collector shelves.

Composition & structure

Bredigite has the formula Ca₇Mg(SiO₄)₄, placing it among the nesosilicates, in which isolated SiO₄ tetrahedra are linked through calcium and magnesium cations rather than to one another. Compositionally it is close to dicalcium silicate (Ca₂SiO₄), and it was historically discussed alongside the polymorphs of that compound; the modern accepted formula recognises essential magnesium in its structure. It crystallises in the orthorhombic system, and its grains are typically colourless to grey with a vitreous lustre.

Formation & occurrence

Bredigite is a product of high-temperature contact metamorphism. It forms where basaltic or dolerite magma intrudes and bakes impure limestones and dolostones, driving off carbon dioxide and recrystallising the rock into exotic calcium-silicate assemblages. At its type locality, Scawt Hill in County Antrim, it occurs with larnite, spurrite, gehlenite, melilite, perovskite and magnetite. Comparable occurrences are known from pyrometamorphic and combustion-metamorphic settings, including the Hatrurim Complex of the Levant, and the mineral has also been reported from some industrial slags, reflecting its kinship with the calcium silicates of Portland cement clinker.

Identification & similar species

Because bredigite occurs as small, pale grains intergrown with other calcium silicates, it is rarely identified by eye. It closely resembles larnite, with which it commonly coexists, and reliable identification depends on optical microscopy, electron microprobe analysis or X-ray diffraction. Its association with spurrite, gehlenite and melilite in a baked-limestone setting is a useful contextual clue, but the species itself is a laboratory determination rather than a hand-specimen one.

Notable localities & collecting

The classic localities lie in the contact-metamorphosed limestones of County Antrim, Northern Ireland, with Scawt Hill being the original type locality and nearby Ballycraigy and Carneal also yielding the mineral. Related occurrences are recorded in Scotland and in the wider Hatrurim "Mottled Zone" of Israel and neighbouring areas. Bredigite is not a display mineral and is essentially unavailable in commerce; interest in it is scientific, centred on understanding cement chemistry and high-temperature metamorphism.