bredigite rhonite

Rhönite

Formula: Ca₄[Mg₈Fe³⁺₂Ti₂]O₄[Si₆Al₆O₃₆]
Inosilicate (chain silicate), rhönite group
Crystal System: Triclinic
Specific gravity: 3.4 to 3.76 measured
Hardness: 5 to 6
Colour: Brown-red, black to reddish black
Environments

Igneous environments
Extraterrestrial environments

Rhönite occurs in silica-undersaturated, alkalic, mafic to intermediate igneous rocks, as a primary mineral, or as an alteration of amphiboles. It is also developed at the contact of alkalic basalt and limestone. Associated minerals include titanium-bearing augite, kaersutite, alkalic feldspar, diopside, forsterite, spinel, perovskite, titanium-bearing magnetite and magnesioferrite (HOM).

Localities

At the Hut Point Peninsula, Ross Island, Ross Archipelago, Victoria Land, Eastern Antarctica, the McMurdo Volcanic Group consists of undersaturated alkaline volcanics which range in composition from alkali olivine basalt and basanite to trachyte and phonolite.
A detailed examination revealed no primary rhönite in the surface flows, but rhönite grains were found in three samples of porphyritic olivine basanite as microphenocrysts which rarely reach 0.2 mm in length. The rhönite is usually intergrown with or surrounded by pyroxene (AM 60.722-725).

At Puy de Barbeire, Massif Central, France, rhönite occurs in nepheline dolerites (Dana).

at Puy Foretier, Haute-Loire, Auvergne-Rhône-Alpes, France, rhönite occurs in an inclusion of pyroxenite in basalt (Dana).

At the type locality, the Rhön Mountains, Germany, rhönite occurs in nepheline basanite (Dana).

At the Allende meteorite, Pueblito de Allende, Chihuahua, Mexico, rhönite has been found which contains both Ti³⁺ ande Ti⁴⁺ (Dana).

At Pulling Point, Dunedin City, Otago Region, South Island, New Zealand, rhönite occurs within alteration rims on kaersutite crystals, together with titanium-bearing augite, olivine, plagioclase, apatite and titanium-bearing magnetite. These minerals make up coarse-grained xenoliths to 1 cm in a nepheline hawaiite. The host rock is composed of a groundmass of plagioclase, pale green clinopyroxene, interstitial nepheline and titanium-bearing magnetite, in which are set phenocrysts of titanium-bearing augite, olivine, and kaersutite (AM 60.722-725).

In the Koloa Volcanic Series, Kauai Island, Kauai County, Hawaii, USA, rhönite occurs only in the alkali gabbro as tiny phenocrysts or as epitaxial overgrowths on crystals of Fe-bearing diopside (AM 69.57-68).

At Location 159, Big Bend National Park, Brewster county, Texas, USA, the intrusion in which rhönite is found is a sill composed of two rock types, a variety of basanite and a dark syenite overlying it. The rhönite is found only in the syenite, where it is present in significant amounts of 5% to 15%. The syenite is usually porphyritic with phenocrysts of titanium-bearing augite, rhönite, olivine, nepheline and plagioclase, set in a groundmass of K-felspar, microlites and zeolites. Phenocrysts of rhönite up to 4 mm in length have been observed. Rhönite, titanium-bearing augite, and plagioclase crystallised early, preceding nepheline and K-felspar. The rhönite is commonly intergrown with titanium-bearing augite, and rarely with olivine (AM 55.864-874).

On the Moon, grains of rhönite have been discovered in magmatic inclusions in augite grains of the lunar regolith from Mare Crisium, returned to Earth by the Russian Luna 24 spacecraft. These rhönite grains are up to 8 μm long (AM 93.488-491).

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