Orbicular rocks

Orbicular rocks are a type of intrusive igneous rock deposit, occurring primarily at the outer margin of plutonic dikes, which are cooled from magma. Orbicular rocks contain inclusions called orbicules that are composed of concentrated minerals layered in concentric spheres around a central core. Most orbicular deposits have been uncovered from geologic strata in Finland, though they have been found on every continent.

Occurrence of Orbicular Rocks

Orbicular rocks are igneous rocks that contain orb-like formations as inclusions within the rock body. Igneous rocks form when magma generated within the mantle of the earth rises toward the surface and cools to form solid structures. Orbicular rocks form within intrusive or plutonic igneous rock bodies, which are deposits that form when magma solidifies beneath the earth’s surface rather than breaking through the crust to become lava.

Orbicular rocks can form within a variety of different types of intrusive rock deposits and most commonly form from magma that is high in silica, a molecule composed of silicon and oxygen that makes up the vast majority of most igneous rock. Orbicular rocks have been found in dikes, which are intrusive rock formations that form when magma rises toward the surface perpendicular to the existing rock, forming a horizontal or vertical wedge within the parent rock.

The vast majority of orbicular rocks come from small dikes, and orbicular sections of the deposit are generally found to occur at the margin of the igneous deposit. Orbicular rocks generally occur only within portions of an igneous deposit that are 3 to 5 meters (10 to 16.5 feet) in width and less than 5 m in depth. Many orbicular rocks have been discovered in isolated boulders that are the remains of ancient igneous dikes that have eroded and split into individual rocks.

In rare occasions, orbicular rocks have been found in sills, which are horizontal deposits of igneous rock that form within layers of the parent rock and are generally oriented parallel to the earth’s crust. Alternatively, orbicular rocks may be part of larger intrusive deposits called batholiths, which result from the combination of subterranean magma pockets and that may extend for many kilometers beneath the earth’s surface.

Orbicular rocks have been found in more than one hundred locations worldwide, but they are a relatively rare type of igneous formation; the exact geologic setting required for orbicular rock formation is still poorly understood. The vast majority of orbicular rocks have been uncovered in igneous deposits from Europe, specifically in Finland. This is not believed to represent a unique feature of Finland’s geologic environment but rather is the result of dedicated efforts by Finnish geologists to complete detailed surveys of that country’s mineralogical terrain. Significant quantities of orbicular rocks also have been uncovered from igneous deposits in the United States and in Indonesia.

Chemical Composition of Orbicular Rocks

In general, the chemical petrology of orbicular rocks is similar to that of the igneous formations in which the rocks have been discovered. Morphological variations in the orbicular deposits are therefore related to the concentration and depositional orientation of the molecules within the formation, rather than to the presence of unique molecules not found in the surrounding parent rock.

Igneous rocks are classified according to both the chemical structure of the molecules found within the rock and the overall size of grains within the rock. Granitic rock is one of the most common types of igneous rock that contains orbicular inclusions. Granitic rock has visible grains and is composed largely of minerals derived from the molecule silica. Granitic rocks are classified as felsic, which are silicate rocks containing large quantities of feldspar, a common type of silicate mineral. Felsic rocks also contain other types of silica-based minerals, such as quartz and micas. Granitic orbicular rocks are the most common type of orbicular inclusion.

Orbicular deposits also can be found in mafic rocks, which are igneous rocks that contain a higher proportion of heavier elements, such as magnesium and iron. Unlike felsic rocks, mafic rocks contain lower levels of potassium but have higher levels of calcium and sodium along with iron and other metal deposits. Some igneous rocks are ultramafic, meaning they contain less than 45 percent silica, with the rest of the rock consisting largely of heavier elements like iron. In one rare occurrence, orbicular inclusions formed within a sample of carbonatite found in Finland. Carbonatite is a type of igneous rock containing high proportions of carbonate minerals, a molecular salt formed from carbonic acid.

Most orbicular rock deposits that are found are made of feldspar, one of the most common types of rock-forming minerals, accounting for more than 60 percent of the earth’s crust. Feldspar is composed largely of silica molecules, with such elements as potassium, calcium, and sodium mixed into their matrix. Feldspar samples tend to be either rich in potassium, called K-feldspars, or rich in sodium or calcium.

Plagioclase is a type of feldspar rich in either calcium or sodium, with low potassium content. Plagioclase is one of the most common types of rock and is found in a variety of igneous deposits. Orbicular rocks also may contain quartz, which is a crystalline variety of silica that generally forms in superheated aqueous environments or within magma pockets.

Orbicular rocks contain inclusions of dark minerals within their matrices, which may represent different mineral types. Among the most common mineral inclusions in orbicular rocks is horneblende, which is a dark form of amphibole, a type of mineral consisting of silica molecules joined to either iron or magnesium atoms. Horneblende occurs in two basic varieties—one that is rich in iron, called ferrohornblende, and one that is rich in magnesium, called magnesiohorneblende.

Alternatively, orbicular rocks may contain biotite, which is a type of silicate rock from the group of minerals known as micas. The dark color of biotite is derived from iron atoms bonded to the silicate molecules within the rock. Biotite also may contain sheets of magnesium and aluminum atoms within the rock, further contributing to the mineral’s dark color.

Structure of Orbicular Rocks

Orbicular plutonic rocks contain orb-like nodules of mineral called orbicules, which contain layers of minerals in a radial array surrounding an internal core, sometimes called the nucleus. The outer portion of an orbicule consists of concentric circles of minerals called shells.

The number of shells within an orbicule varies, and some orbicules may have only a single shell while others may have hundreds of shells surrounding the core. Between the shells are layers of minerals called subshells, which contain molecules similar to those of the surrounding rock but that may be organized differently.

The central core of an orbicule may consist of a cluster of small crystals, usually composed of quartz or a similar silica-rich mineral. Alternatively, some orbicules have cores consisting of one or a few large crystals, again generally derived from the quartz family of minerals. Some orbicular cores consist of fragments of parent rock, called xenoliths, which may differ from the type of rock contained within the remaining part of the orbicule, including the minerals of the shells and subshells.

Outer shells are usually composed of feldspar or plagioclase and therefore appear as lighter segments of rock. The portion between shells consists of plagioclase or granite with some darker minerals mixed into the matrix, making these areas somewhat darker than the outer shells. Subshells generally consist of plagioclase bonded to horneblende or biotite, making them darker than the outer shells or the area between shells. The typical orbicule therefore consists of alternating bands of light- and dark-colored rock surrounding the inner core, which tends to be light in color and to consist of quartz crystals or other silicate minerals. Many orbicule cores contain small xenoliths that generally appear as dark flecks or nodules within the crystals of the core.

The size of orbicules is related to the type of igneous rocks in the surrounding deposit. Large orbicules tend to develop within felsic rocks containing high proportions of quartz and feldspar. Orbicules in this type of sediment can range from 10 to 40 centimeters (4 to 16 inches) in diameter. Mafic igneous deposits generally contain smaller orbicules; similarly, ultramafic rocks contain the smallest and least developed orbicules.

Formation of Orbicules

Several hypotheses explain the origin of orbicules within igneous formations. Initially, the bands of various minerals were thought to be related to properties of liquid diffusion that affect the formation of mineral layers during the solidification process.

An alternative theory, developed in the 1970s, sees orbicules as the result of xenoliths of other rock types or crystals becoming part of (included in) a flow of magma. When this occurs, the magma surrounding the included segments cools such that “comb layering” occurs, in which crystals form like the teeth of a comb perpendicular to the surface of the inner core. Alternating layers within the orbicules are then seen as the result of successive periods of cooling, leading to comb layering of minerals around the forming nodule.

While comb layering appears to play a role in the formation of the shells and subshells contained within orbicules, chemical investigation has found that not all orbicules contain xenoliths within their core. Theories of orbicule genesis focus on the phenomenon of undercooling, which occurs when a sample of magma rapidly cools to temperatures significantly below those needed for solidification. In undercooling, crystals may form within the body of the magma and serve as attractors for layers of cooling material that form comb layers around the crystal structure. As this process continues, chemical properties within the mineral components cause them to layer into discrete segments of different types of minerals.

The orientation of orbicules within the matrix of an igneous deposit indicates that orbicules form while the surrounding rock is still cooling and that orbicules can shift within the rock matrix after formation. In some cases orbicules have been found where the outer shell has broken apart, perhaps after encountering disruptive contact with other orbicules as the inclusions shifted within the pliable rock matrix.

In general, the processes underlying the formation of orbicules is still an area of concerted research. The precise physical and chemical conditions required for the formation of orbicules appear to be relatively narrow in scope, as orbicular rock formations are rare among igneous deposits. There appears to be a narrow range of temperatures and pressures during which orbicule formation is possible, which explains why orbicules are relatively rare and why they tend to form only in certain portions of a developing dike, where temperatures fall within this range.

Uses of Orbicular Rocks

Orbicular rocks contain attractive patterns of light and dark concentric circles, so they have been prized by rock collectors and are often used as decorative stones. Some orbicular rocks have been used as an alternative to marble for making counters and tabletops. A variety of collectible minerals can be made from polished and cut orbicular rocks.

Because of their rarity, orbicular rocks cannot be collected by amateurs in most locations in which they are found. In Finland, for instance, where the largest variety of orbicular rocks have been found, provisions prohibit removing orbicular specimens from rock quarries, thereby preserving the remaining orbicular deposits for geologic research.

Principal Terms

biotite: type of layered, silicate mineral composed of silica molecules bonded with magnesium and iron and appearing in dark brown and black varieties

carbonatite: silicate mineral consisting of reduced levels of silica and rich in carbonate molecules formed from carbonic acid

dike: type of plutonic rock formation consisting of cooled magma oriented perpendicular to the earth’s surface and generally proceeding at a diagonal through the layers of surrounding parent rock

felsic: mineral group characterized by high proportions of silicate minerals, including quartz, feldspar, plagioclase, and micas

horneblende: type of silicate rock characterized by its dark color, which is the result of inclusions of iron and magnesium within the rock matrix

mafic: igneous deposits of rocks consisting of silica enriched with high levels of iron, magnesium, sodium, and calcium and with low levels of potassium-rich feldspar

orbicule: formation within orbicular igneous rocks consisting of similar materials to the parent rock; organized in concentric layers of crystals surrounding a core comprising smaller crystal clusters, larger individual crystals, or fragments of other rock types

petrology: branch of geology that studies the formation and classification of rock types

plagioclase: silicate mineral formed from molecules of silica bonded with sodium, aluminum, and calcium

plutonic: rock formed from magma that cools in pockets beneath the earth’s surface

Bibliography

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