"Rock of the Month # 92, posted February 2009" ---

Orbicular textures occur but rarely in igneous rocks. When they do, they may be spectacular, and are exhibited in museums as examples of decorative stones. They are found most frequently in granitic rocks, such as granite and granodiorite, tonalite and diorite. They are rarely reported in gabbros and ultramafic rocks, carbonatites, chromite ores, syenites, and hornfels. They have also been described rarely in volcanic rocks which tend, like the intrusive examples, to be of felsic composition.

This orbicular granite (or maybe diorite?) is the principal feature of a small conservation park ("Santuario de la Naturaleza - granito orbicular"), roughly equidistant between Copiapo and Chanaral, 80 km from each town in the narrow coastal strip of northern Chile. The Museo Mineralogico in Copiapo, at the time of my visit, was the home of a fine public mineral collection. The entrance courtyard contained a fine m-scale piece of a local orbicular granite. The featured example displays many orbicules, typically about 7 cm in diameter, with rare rounded enclaves of diorite some 30 cm wide. The mineralogy includes quartz-rich cores in darker rims and pink matrix composed of biotite, hornblende, magnetite, orthoclase K-feldspar and other minerals. The rock is cut by a thin, vertical diabase dyke. The contact with a finer-grained granitic phase was visible at the tideline when these photos were taken, on 22 March 1996.

Historically, much work on orbicular rocks hails from Scandinavia, but an informal survey of 50 or so publications shows occurrences across India, Canada and the U.S.A., plus a smaller number of examples in Papua New Guinea, Africa (Zimbabwe) Latin America (Chile and Peru) and elsewhere. American examples are reported from Vermont to New Mexico, California and Idaho, while in Canada they occur from Labrador to Ontario, Manitoba and the Yukon.

Orbicular rocks are the subject of a beautifully-illustrated review of examples from Finland (Lahti, 2005). Some two-thirds of the examples in the book are boulder occurrences, which seems appropriate as the Finns are probably the world's most diligent students and pursuers of glacially-derived boulder trains.

Factors invoked to explain the growth of orbicules, not always mutually exclusive, include:

1) Assimilation of hornfels xenoliths and progressive destruction of xenolithic cores, with comb layering (Thompson and Giles, 1974), 2) Liquid immiscibility (Roedder, 1979), 3) Undercooling in magma, forcing crystallization to occur only on solid objects, forming orbicules and comb layers (Vernon, 1985), 4) Comb textures and unidirectional growth (Durant and Fowler, 2002), and 5) Heterogeneous nucleation in supercooled magma (Ort, 1992).

Selected References, in chronological order

Dale,TN (1909) The granites of Vermont. USGS Bull. 404, 138pp.

Thompson,TB and Giles,DL (1974) Orbicular rocks of the Sandia Mountains, New Mexico. BGSA 85, 911-916.

Roedder,E (1979) Silicate liquid immiscibility in magmas. In `The Evolution of the Igneous Rocks: Fiftieth Anniversary Perspectives' (Yoder,HS editor). Princeton University Press, 588pp., 15-57.

Vernon,RH (1985) Possible role of superheated magma in the formation of orbicular granitoids. Geology 13, 843-845.

Ort,MH (1992) Orbicular volcanic rocks of Cerro Panizos: their origin and implications for orb formation. BGSA 104, 1048-1058.

Samson,IM and Sinclair,WD (1992) Magmatic hydrothermal fluids and the origin of quartz-tourmaline orbicules in the Seagull batholith, Yukon Territory. In `Granitic Pegmatites' (Martin,RF and Cerny,P editors), Can.Mineral. 30 part 3, 937-954.

Durant,DG and Fowler,AD (2002) Origin of reverse zoning in branching orthopyroxene and acicular plagioclase in orbicular diorite, Fisher Lake, California. Mineral.Mag. 66, 1003-1019.

Lahti,SI (editor) (2005) Orbicular Rocks in Finland. Geol.Surv.Finland, 177pp.

Singh,LG and Vallinayagam,G (2008) Orbicular rhyolite of Dhiran area, Malani igneous suite, Barmer district, western Rajasthan. J.Geol.Soc.India 71, 67-72.