Majorite

Majorite is an unusual garnet in which silicon (Si4+) enters the octahedral site that aluminum, iron, or chromium occupy in ordinary garnets. This 'silicate garnet' arrangement only becomes stable at the extreme pressures of Earth's mantle transition zone.

It is one of the dominant minerals in the deep mantle, where pyroxene dissolves into a majoritic garnet structure with increasing depth. Synthetic and natural majorite is isometric to slightly tetragonal and very hard.

Natural majorite was first identified in shocked meteorites, where impact pressures briefly reach mantle-like conditions, and the mineral is named after geophysicist Alan Major.

Majorite forms only at very high pressures, roughly those of the mantle transition zone (about 250-660 km depth). There, ordinary pyroxene progressively dissolves into garnet, producing majoritic compositions that make up much of the deep mantle mineralogy.

In the rock record, natural majorite is preserved in two settings: as microscopic grains in the shock-melted veins of meteorites, where impacts generate transient ultra-high pressures, and as rare inclusions in diamonds that crystallized at great depth.

It was first described from the Coorara meteorite, and majoritic garnet inclusions in 'superdeep' diamonds provide direct samples of the transition zone.

Majorite is essentially never identified by eye in the field; it occurs as tiny grains in shock veins of meteorites or as microscopic inclusions in diamond. Confirmation requires electron microprobe and structural analysis showing excess silicon in the octahedral site.

In meteorites it appears as purple to violet or colorless garnet associated with other high-pressure phases like ringwoodite and wadsleyite. The presence of these companions and the shock context distinguish majorite from ordinary garnet, which it can resemble chemically before analysis.

Majorite has no commercial use; it is far too rare and microscopic. Its value is entirely scientific.

As a major mineral of the mantle transition zone, majorite is central to understanding deep-Earth seismology, mantle convection, and the composition of the planet's interior. Majoritic inclusions in superdeep diamonds and shock-vein grains in meteorites are prized research material that records pressures and depths unreachable by drilling.