Spherulite
Encyclopedia
In petrology
, spherulites icon are small, rounded bodies that commonly occur in vitreous
igneous rock
s. They are often visible in specimens of obsidian
, pitchstone
and rhyolite
as globules about the size of millet seed or rice grain, with a duller luster
than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.
the spherulites are of circular outline and are composed of thin divergent fibers that are crystalline as verified with polarized light. Between crossed Nicol
s, a black cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crosshairs; as the microscope stage is rotated the cross remains steady; between the black arms there are four bright sectors. This shows that the spherulite consists of radiate, doubly refracting fibers that have a straight extinction; the arms of the black cross correspond to those fibers that are extinguished. The aggregate
is too fine-grained to directly determine what minerals it is composed of using visible light microscopy.
or silicified. In the center of a spherulite there may be a crystal
(e.g. quartz
or feldspar
) or sometimes a cavity. Occasionally spherulites have zones of different colors, and while most frequently spherical, they may also be polygon
al or irregular in outline. In some New Zealand
rhyolites the spherulites send branching cervicorn processes (like stags horns) outwards through the surrounding glass of the rock. The name axiolites is given to long, elliptical or band-like spherulites.
, megaspherulites, which range in diameter from 0.30 to 4.3 meters occur within a thick layer of rhyolitic vitrophyre
. Megaspherulites as large as 0.91 meter occur within rhyolite
exposures on Steens Mountain, Oregon
and ones as large as 1.83 meters in diameter occur within welded tuffs exposed near Klondyke, Arizona
. The best known occurrence of megaspherulites are stone balls, which range in diameter from 0.61 to 3.35 meters, found around Cerro Piedras Bola in the Sierra de Ameca between Ahualulco de Mercado
and Ameca, Jalisco
. As often happens with considerably smaller spherulites, these megaspherulites have been released by weathering from an ash flow tuff
, in which they originally formed, to create natural stone balls.
and other places. The characteristic radiate fibrous structure is usually conspicuous, but the fibers are interrupted by cavities that are often arranged as to give the spherulite a resemblance to a rosebud with folded petals separated by arching interspaces. Some of these lithophysae are several centimeters or more in diameter. Tridymite
, fayalite
and other minerals in the lithophysae may be precipitates from the vapor phase that occupied the cavities. The fibers of these coarse spherulites are alkali
feldspar
(sanidine
or anorthoclase
) and tridymite.
s and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide.
, crystals of quartz and feldspar can crystallize in intricate intergrowths with geometric forms, sufficiently coarse to be easily distinguishable by means of the microscope. Often the quartz or the feldspar of the intergrowth extinguishes simultaneously with a crystal of either of these minerals lying in the center of the aggregate. These intergrowths crystallize from a melt, not in the solid state.
s there are straight or feathery feldspar crystals (usually oligoclase
) forming pale colored spherulites a quarter to half an inch in diameter. The same rocks often contain similar aggregates of plumose skeleton crystals of augite
. Many volcanic rocks have small lath-shaped crystals of feldspar or augite diverging from a common center.
Petrology
Petrology is the branch of geology that studies rocks, and the conditions in which rocks form....
, spherulites icon are small, rounded bodies that commonly occur in vitreous
Glass
Glass is an amorphous solid material. Glasses are typically brittle and optically transparent.The most familiar type of glass, used for centuries in windows and drinking vessels, is soda-lime glass, composed of about 75% silica plus Na2O, CaO, and several minor additives...
igneous rock
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
s. They are often visible in specimens of obsidian
Obsidian
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth...
, pitchstone
Pitchstone
Pitchstone is a dull black glassy volcanic rock formed when viscous lava or magma cools swiftly. It is similar to but coarser than obsidian. It is a volcanic glass with a conchoidal fracture , a resinous lustre, and a variable composition. Its colour may be mottled, streaked, or uniform brown,...
and rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
as globules about the size of millet seed or rice grain, with a duller luster
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
than the surrounding glassy base of the rock, and when they are examined with a lens they prove to have a radiate fibrous structure.
Structure
Under the microscopeMicroscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
the spherulites are of circular outline and are composed of thin divergent fibers that are crystalline as verified with polarized light. Between crossed Nicol
Nicol prism
A Nicol prism is a type of polarizer, an optical device used to produce a polarized beam of light from an unpolarized beam. See polarized light. It was the first type of polarizing prism to be invented, in 1828 by William Nicol of Edinburgh...
s, a black cross appears in the spherulite; its axes are usually perpendicular to one another and parallel to the crosshairs; as the microscope stage is rotated the cross remains steady; between the black arms there are four bright sectors. This shows that the spherulite consists of radiate, doubly refracting fibers that have a straight extinction; the arms of the black cross correspond to those fibers that are extinguished. The aggregate
Aggregate (composite)
Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes...
is too fine-grained to directly determine what minerals it is composed of using visible light microscopy.
Occurrence
Spherulites are commonest in silica-rich glassy rocks. Sometimes they compose the whole mass; more usually they are surrounded by a glassy or felsitic base. When obsidians are devitrified, the spherulites are often traceable, though they may be more or less completely recrystallizedRecrystallization (geology)
In geology, solid-state recrystallization is a metamorphic process that occurs under situations of intense temperature and pressure where grains, atoms or molecules of a rock or mineral are packed closer together, creating a new crystal structure. The basic composition remains the same...
or silicified. In the center of a spherulite there may be a crystal
Crystal
A crystal or crystalline solid is a solid material whose constituent atoms, molecules, or ions are arranged in an orderly repeating pattern extending in all three spatial dimensions. The scientific study of crystals and crystal formation is known as crystallography...
(e.g. quartz
Quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...
or feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
) or sometimes a cavity. Occasionally spherulites have zones of different colors, and while most frequently spherical, they may also be polygon
Polygon
In geometry a polygon is a flat shape consisting of straight lines that are joined to form a closed chain orcircuit.A polygon is traditionally a plane figure that is bounded by a closed path, composed of a finite sequence of straight line segments...
al or irregular in outline. In some New Zealand
New Zealand
New Zealand is an island country in the south-western Pacific Ocean comprising two main landmasses and numerous smaller islands. The country is situated some east of Australia across the Tasman Sea, and roughly south of the Pacific island nations of New Caledonia, Fiji, and Tonga...
rhyolites the spherulites send branching cervicorn processes (like stags horns) outwards through the surrounding glass of the rock. The name axiolites is given to long, elliptical or band-like spherulites.
Megaspherulites
Occasionally spherulites are found that are many centimeters and, even more rarely, up to two or three meters in diameter. Those spherulites, which are more than 20 centimeters in diameter, are called megaspherulites. Near Silver Cliff, ColoradoSilver Cliff, Colorado
Silver Cliff is a Statutory Town that is the most populous town in Custer County, Colorado, United States. The population was 512 at the 2000 census.-History:...
, megaspherulites, which range in diameter from 0.30 to 4.3 meters occur within a thick layer of rhyolitic vitrophyre
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
. Megaspherulites as large as 0.91 meter occur within rhyolite
Rhyolite
This page is about a volcanic rock. For the ghost town see Rhyolite, Nevada, and for the satellite system, see Rhyolite/Aquacade.Rhyolite is an igneous, volcanic rock, of felsic composition . It may have any texture from glassy to aphanitic to porphyritic...
exposures on Steens Mountain, Oregon
Steens Mountain
Steens Mountain is a large fault-block mountain in the southeastern part of the U.S. state of Oregon. Located in Harney County, it stretches some and rises from an elevation of about above the Alvord Desert to its peak at...
and ones as large as 1.83 meters in diameter occur within welded tuffs exposed near Klondyke, Arizona
Arizona
Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...
. The best known occurrence of megaspherulites are stone balls, which range in diameter from 0.61 to 3.35 meters, found around Cerro Piedras Bola in the Sierra de Ameca between Ahualulco de Mercado
Ahualulco de Mercado
Ahualulco de Mercado is a town and municipality, in Jalisco in central-western Mexico. The municipality covers an area of 134.22 km².As of 2005, the municipality had a total population of 21,465....
and Ameca, Jalisco
Ameca, Jalisco
Ameca is a city and municipality, in Jalisco in central-western Mexico. The municipality covers an area of 685.73 km². The city is bisected by the Ameca River which drains to the Pacific Ocean near Puerto Vallarta....
. As often happens with considerably smaller spherulites, these megaspherulites have been released by weathering from an ash flow tuff
Tuff
Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...
, in which they originally formed, to create natural stone balls.
Lithophysae
Very large and cavernous spherulites are called lithophysae; they are found in obsidians at Lipari, in Yellowstone ParkYellowstone National Park
Yellowstone National Park, established by the U.S. Congress and signed into law by President Ulysses S. Grant on March 1, 1872, is a national park located primarily in the U.S. state of Wyoming, although it also extends into Montana and Idaho...
and other places. The characteristic radiate fibrous structure is usually conspicuous, but the fibers are interrupted by cavities that are often arranged as to give the spherulite a resemblance to a rosebud with folded petals separated by arching interspaces. Some of these lithophysae are several centimeters or more in diameter. Tridymite
Tridymite
Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo,...
, fayalite
Fayalite
Fayalite is the iron-rich end-member of the olivine solid-solution series. In common with all minerals in the olivine group, fayalite crystallizes in the orthorhombic system with cell parameters a 4.82 Å, b 10.48 Å and c Å 6.09.Iron rich olivine is a relatively common constituent of acidic and...
and other minerals in the lithophysae may be precipitates from the vapor phase that occupied the cavities. The fibers of these coarse spherulites are alkali
Alkali
In chemistry, an alkali is a basic, ionic salt of an alkali metal or alkaline earth metal element. Some authors also define an alkali as a base that dissolves in water. A solution of a soluble base has a pH greater than 7. The adjective alkaline is commonly used in English as a synonym for base,...
feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
(sanidine
Sanidine
Sanidine is the high temperature form of potassium feldspar 4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures...
or anorthoclase
Anorthoclase
The mineral anorthoclase is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8....
) and tridymite.
Artificial glass
Artificial glass sometimes crystallizes and contains spherulites that may be as large as a marble. As the glass has little similarity in chemical composition to volcanic obsidians, these spherulites when analyzed throw little light on the mineral nature of spherulites in rocks. They show, however, that in viscous semi-solid glasses near their fusion point crystallization tends to nucleate at certain centers and to spread outwards, producing spherulitic structures. Many saltSalt
In chemistry, salts are ionic compounds that result from the neutralization reaction of an acid and a base. They are composed of cations and anions so that the product is electrically neutral...
s and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide.
Geometric forms
As illustrated in the entry on granophyreGranophyre
Granophyre is a subvolcanic rock that contains quartz and alkali feldspar in characteristic angular intergrowths such as those in the accompanying image....
, crystals of quartz and feldspar can crystallize in intricate intergrowths with geometric forms, sufficiently coarse to be easily distinguishable by means of the microscope. Often the quartz or the feldspar of the intergrowth extinguishes simultaneously with a crystal of either of these minerals lying in the center of the aggregate. These intergrowths crystallize from a melt, not in the solid state.
Variolites
Another group of radiate fibrous growths resembling spherulites in many respects consists of minute feathery crystals spreading outwards through a fine grained or glassy rock. In the varioliteVariolite
Variolites are a group of dark green basic igneous rocks that, especially on weathered surfaces, exhibit pale colored spots, or spherules, that give them a pockmarked appearance...
s there are straight or feathery feldspar crystals (usually oligoclase
Oligoclase
Oligoclase is a rock-forming mineral belonging to the plagioclase feldspars. In chemical composition and in its crystallographic and physical characters it is intermediate between albite and anorthite . The albite:anorthite molar ratio ranges from 90:10 to 70:30.Oligoclase is a high sodium...
) forming pale colored spherulites a quarter to half an inch in diameter. The same rocks often contain similar aggregates of plumose skeleton crystals of augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
. Many volcanic rocks have small lath-shaped crystals of feldspar or augite diverging from a common center.