Fayalite is listed from 330 localities on Mindat. It is the Fe²⁺ end member in the olivine group and forms a solid solution series towards the Mg-end member forsterite, and intermediate members are often called olivine. Fayalite is much rarer than forsterite, and most of the of the olivines are forsterites. Unlike forsterite, fayalite also forms a series towards the Mn-end member tephroite, and it is sometimes oxidized to form the rare ferric olivine laihunite.

Fayalite occurs, as shown in this article, as small perfect crystals in vugs in rhyolites, often associated with tridymite or other silica-rich phases. It is also found in metamorphic iron deposits, often associated with tephroite in Fe-Mn ores, and in alkaline and acidic rocks such as syenites and granites. The largest crystals are found in pegmatites, and large crystals up to 30 cm are found in Quirra, Sardinia. Similarly sized crystals are known from granite and syenite pegmatites in the Czech Republic, Poland and the Pikes Peak area in Colorado, USA and other places. These crystals are normally anhedral and only available as dark brown to black, more or less massive aggregates. Fayalite is also present in some meteorites.

Any large, well formed crystal of fayalite will therefore be very rare, except possibly as a byproduct of copper production. It is estimated that for every ton of copper produced, 2.2 tons of slag are generated, with a large portion of this being fayalite. Examples of crystallized fayalite slags can be seen on the Mindat page of the Baia Sprie mine, Romania.

Australia

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Eastern Hill, Mount Anakie, Anakie, City of Greater Geelong, Victoria, Australia

Mount Anakie consists of three relatively low scoria cones and a small, enclosed depression - probably a maar. The scoria cones are aligned southeast/northeast over a distance of 5 km, and they belong to the quarternary Western District Volcanic Plains, which contain approximately 400 eruptions. Volcanoes in this district began erupting lava flows about 4.5 million years ago, while Mt. Anakie itself last erupted 1.5 million years ago.

The source of mineral specimens from Mount Anakie is a large quarry operated in the easternmost of the cones. This cone is partly stabilized by layers and dykes of lava in the upper slopes. The scoria of all cones contain abundant rounded bombs with cores of basalt, olivine, granite or clay. The Anakies are an important source of iherzolite xenoliths and rare composite xenoliths. Granitic rocks underlie the volcanoes and form tors and slabs at “Fairy Park”, adjacent to the scoria cones. Well formed minerals, like fayalite, form microcrystals in voids in the scoria or in xenoliths.

References:

Victorian Resources Online, State of Victoria

Germany

Caspar quarry, Ettringen, Vordereifel, Mayen-Koblenz, Rhineland-Palatinate, Germany

Volcanism in Eifel is a part of a series of intra-plate volcanic fields in Central Europe that have been active throughout the Tertiary and Quaternary and is largely related to rifting of the Rhine Graben and broad uplift of pre-Tertiary basement in the Rhenish Shield. Xenoliths and ejecta of various rocks are common, and host a wide range of different minerals.

Fayalite is much rarer in the Eifel area than the Mg-olivine forsterite. The dark brown to black, transparent to opaque crystals up to 2 mm large found in sanidinite xenoliths in Emmelberg are generally considered the best from the area. Associated minerals include magnetite, apatite, biotite and fine nepheline crystals. The fayalite from Bellerberg is occasionally partly oxidised to the rare ferric olivine group mineral Laihunit.

The fayalite is distinguished from the more common forsterite by chemical analyses, but the color and more pronounced pinacoid faces of the crystals may give indications that the olivine mineral is fayalite.

References:

Blaß, G., Emmerich F.J., and Graf. H.W. (2006): Die Minerale der Vulkaneifel, DVD.

Ettringer Bellerberg, Ettringen, Vordereifel, Mayen-Koblenz, Rhineland-Palatinate, Germany

Italy

Mount Somma, Somma-Vesuvius Complex, Naples, Campania, Italy

Pollena quarries, Pollena Trocchia, Mount Somma, Somma-Vesuvius Complex, Naples, Campania, Italy

A large number of mineral species have been found in the Somma-Vesuvius Complex. Multiple cycles of magma, with variable composition originating from magma chambers at different depths, have created a range of mineral-forming environments. Many of the eruptions have brought to the surface xenoliths or loose ejecta of rocks formed in the contact between different magmas and the rocks surrounding the magma chambers.

Fayalite is a relatively rare mineral in the Somma-Vesuvius complex, found only in lavas from the 1631 and 1906 eruptions. In the 1631 lavas, fayalite occurs as a primary mineral, and may be found as tabular crystals up to 5 mm with sodalite and apatite. It has also been found associated with magnetite, sanidine, augite and sodalite in some leucotephrite blocks ejected in 1906.

References:

Burri, Conrad and Di Girolamo, Pio (1974): Contributo alla conoscenza delle lave della grande eruzione del Vesuvio del 1631. Societa Italiano Mineralogia e Petrologia, Vol 30, pp 705-739.

Pelloux, Albert (1927): The minerals of Vesuvius. American mineralogist Vol. 12, pp 14-21.

Montenero Quarry, Onano, Viterbo Province, Lazio, Italy

The Montenero quarry is operated to extract material for road gravel and mass for the agricultural industry from pyroclastic flows in the Vulsini Volcanic District.

The Vulsini mainly consists of two large, partly overlapping calderas, Bolsena and Latera, developed between 0.6-0.1Ma. Their activity was dominated by explosive Plinian and pyroclastic flow-forming eruptions and subordinate lava flows characterized by a wide range of compositions belonging to the leucitite, basanite and shoshonite magmatic suites.

The Montenero quarry lies in the overlapping section between these calderas, and a large number of minerals have formed beautiful micro-crystals in cavities in pyroclastic blocks and in lavas. Fayalite is one of these minerals, and it forms small tabular crystals of an orange-brown color.

References:

Nappi, G., L. Valentini and M. Mattioli (2004): Field Trip Guide Book - P09: Ignimbritic Deposits in Central Italy: Pyroclastic Products of the Quaternary Age and Etruscan Footpaths (pdf). Italian Agency for the Environmental Protection and Technical Services (APAT).

Signoretti, E. and Pucci, R., (2007): Gita sociale e ricerca mineralogica sul terreno nella cava di Montenero, Onano (VT), Il Cercapietre, 1-2/2007, 61-67.

Simei S., Acocella V., Palladino D.M. and Trigila R. (2006): Evolution and structure of Vulsini calderas (Italy). Geophysical Research Abstracts, Vol. 8, 09302.

New Zealand

Hendersons Quarry, Mount Ngongotaha, Rotorua District, Bay of Plenty Region, New Zealand

The Henderson quarry is worked in the rhyolites of the Ngongotaha Dome,which lies in the northeast part of the Ngongotaha Volcanic Complex in the Taupo vulcanic zone. The Ngongotaha Dome is thought to be the oldest dome of the complex. The quarry provides an almost complete cross section of the dome and exposes an area of approximately 10,000 m² of fresh rhyolite. The Henderson Quarry consequently gives both petrologists and mineral collectors a unique opportunity to study the interior of a volcanic dome.

The dome interior comprises crystalline, generally flow-banded rhyolites which commonly grade into devitrified (containing crystallized lithophysae) obsidian and is cut by obsidian dikes. In voids in these lithophysae (see mindat glossary), a large number of minerals have crystallized from gaseous phases.

Fayalite (Fa88, with minor Mg and Mn) is the most common ferro-magnesian mineral present, and it grows on tridymite. Crystals are up to 4 mm in size and commonly equant. In general, fayalite is altered (oxidised) to a large degree, being opaque rather than clear brown, and it often exhibits a considerable iridescence. In some instances, numerous parallel grooves of uniform thickness are present on crystal surfaces
perpendicular to the c-axis.

References:

Beresford, Stephen Willis (1997): Volcanology and Geochemistry of the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand. PhD thesis, University of Canterbury.

Richnow, Jens (1999): Eruptional and post-eruptional processes in rhyolite domes. PhD thesis, University of Canterbury.

Tapuaeharuru Bay, Lake Taupo, Taupo District, Waikato Region, New Zealand

The Taupo Volcanic Zone is a 300 km-long rift zone, resulting from westward subduction of the Pacific lithosphere beneath the Australian platform. It is filled by voluminous Quaternary volcanism: 80% rhyolite, 20% andesite and dacite, and < 1% high-alumina basalt. It is the most productive and active rhyolitic volcanic system on the earth, and there have been at least 34 major rhyolitic ignimbrite eruptions from an overlapping succession of at least eight calderas. The eruptions are divided into "old" from ca .2.0 Ma to 0.36 Ma, and "young" from 0.36 Ma onwards.

Fayalite is found in small mm-sized crystals in vugs in the rhyolite lavas, associated with tridymite. The fayalite has formed in the late stage gaseous phase and may show signs of weathering. At Tapuaeharuru Bay, fayalite can be found in flow-banded rhyolite showing obvious gas bubbles on the western side of the bay.

References:

Mindat locality page.

Beresford, Stephen Willis (1997): Volcanology and Geochemistry of the Kaingaroa Ignimbrite, Taupo Volcanic Zone, New Zealand. PhD thesis, University of Canterbury.

Richnow, Jens (1999): Eruptional and post-eruptional processes in rhyolite domes. PhD thesis, University of Canterbury.

Wilson, Colin,J. N., Blake S., Charlier, B.L.A. and Sutton, A.N. (2006): The 26.5 ka Oruanui Eruption, Taupo Volcano, New Zealand: Development, Characteristics and Evacuation of a Large Rhyolitic Magma Body. Journal of Petrology, Vol 47., pp 35-69.

Western Shore, Lake Taupo, Taupo District, Waikato Region, New Zealand

Fayalite is found in vugs in massive banded rhyolite, near the tops of individual flows. See also text above.

References:

Mindat locality page.

USA

Coso Hot Springs, Inyo County, California, USA

Little Lake Perlite deposits (Desert Materials Corp. deposit), Sugarloaf Mountain, Inyo County, California, USA

The Coso volcanic field is located at the western margin of the Basin and Range Province, about 200 km north of Los Angeles. The area contains 39 high-silica rhyolite domes and lava flows that are mostly about 300,000 years old. The area is still active, as demonstrated by an active geothermal field. Mindat distinguishes between minerals found in the geothermal fields (Coso Hot Springs) and in the rhyolite domes and lavas (various claims and mines), but most authors and collectors do not make this distinction.

Faylite is found within cavities in Iithophysae, in an obsidian flow a few miles west of Coso Hot Springs. The Iithophysae are very numerous in some portions of the obsidian, reaching up to one and a half inches in diameter. Most of the lithophysae are quite regular, nearly spherical in outline. In those that are not massive, there are minute spherulites of cristobalite with platy crystals of tridymite and (some times) platy, brown crystals of fayalite up to 3mm. Chemical analyses confirm the material to be fayalite, with Fa>85 and Speziale et al. (2004) published an electron microprobe analysis corresponding to (Fe0.94Mn0.06)₂SiO₄

The obsidian is associated with rhyolite in the majority of the volcanic domes.

References:

Manley, Curti. R. and Bacon, Charles R. (2000): Rhyolite Thermobarometry and the Shallowing of the Magma Reservoir, Coso Volcanic Field, California. Journal of Petrology, Vol. 41, pp 149–174.

Murdoch, Joseph and Web b, Robert W. (1940): Notes on some minerals from southern California, II, American Mineralogist, Vol. 25, pp 549-555.

Speziale, Sergio, Duffy, Thomas S., and Angel, Ross J. (2004): Single-crystal elasticity of fayalite to 12 GPa. Journal of Geophysical Research: Solid Earth (1978-2012), Vol. 109, Issue B12,15 pages.

RevisionHistory

Revision no date description editor 1.0 Aug 2014 First Draft Olav Revheim

Reviewed by Becky Coulson Aug 2014