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In
petrology
Petrology () is the branch of geology that studies rocks and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous and metamorphic petrology are commonly taught together ...
, spherulites () are small, rounded bodies that commonly occur in
vitreous
Vitreous may refer to:
Materials
* Glass, an amorphous solid material
** Vitreous enamel, a material made by fusing powdered glass to a substrate by firing
* Vitreous lustre, a glassy luster or sheen on a mineral surface
Biology
* Vitreous body, ...
igneous rock
Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...
s. They are often visible in specimens of
obsidian
Obsidian () is a naturally occurring volcanic glass formed when lava extrusive rock, extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Obsidian is produced from felsic lava, rich in the lighter elements s ...
,
pitchstone
Pitchstone is a dark coloured, glassy volcanic rock formed when felsic lava or magma cools quickly. Since it is a volcanic glass, pitchstone may have a conchoidal fracture. Pitchstones may also contain phenocrysts, in which case it is a form of v ...
, and
rhyolite
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
as globules about the size of
millet
Millets () are a highly varied group of small-seeded grasses, widely grown around the world as cereal crops or grains for fodder and human food. Most species generally referred to as millets belong to the tribe Paniceae, but some millets al ...
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.
Structure
Under the
microscope
A microscope () is a laboratory instrument used to examine objects that are too small to be seen by the naked eye. Microscopy is the science of investigating small objects and structures using a microscope. Microscopic means being invisibl ...
the spherulites are of circular outline and are composed of thin divergent fibers that are
crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...
as verified with
polarized light. Between crossed
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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 or aggregates may refer to:
Computing and mathematics
* collection of objects that are bound together by a root entity, otherwise known as an aggregate root. The aggregate root guarantees the consistency of changes being made within the ...
is too fine-grained to directly determine what minerals it is composed of using visible light microscopy.
Occurrence
Spherulites are most common 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
recrystallized or silicified. In the center of a spherulite there may be a
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
(e.g.
quartz
Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical form ...
or
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
) or sometimes a cavity. Occasionally spherulites have zones of different colors, and while most frequently spherical, they may also be
polygon
In geometry, a polygon () is a plane figure that is described by a finite number of straight line segments connected to form a closed ''polygonal chain'' (or ''polygonal circuit''). The bounded plane region, the bounding circuit, or the two toge ...
al or irregular in outline. In some
New Zealand
New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island count ...
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, Colorado
The Town of Silver Cliff is the Statutory Town that is the most populous municipality in Custer County, Colorado, United States. The population was 587 at the 2010 census, up from 512 in 2000.
History
Silver Cliff was formed in the late 187 ...
, 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
Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained groundmass. The mineral ...
exposures on
Steens Mountain, Oregon and ones as large as 1.83 meters in diameter occur within welded tuffs exposed near Klondyke,
Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
. 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
Sierra (Spanish for "mountain range" and "saw", from Latin '' serra'') may refer to the following:
Places Mountains and mountain ranges
* Sierra de Juárez, a mountain range in Baja California, Mexico
* Sierra de las Nieves, a mountain range i ...
between
Ahualulco de Mercado
Ahualulco de Mercado is a City 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.
Name origin
Ahualulco comes from the Nahuat ...
and
Ameca, Jalisco
Ameca ( nah, Amecatl "string of water") 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 ...
. As often happens with considerably smaller spherulites, these megaspherulites have been released by weathering from an ash flow
tuff
Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock cont ...
, in which they originally formed, to create natural stone balls.
Lithophysae
Very large and cavernous spherulites are called
lithophysae
A lithophysa (plural lithophysae, from Greek ''lithos'' "stone" + ''physan'' "to blow") is a felsic volcanic Rock (geology), rock with a spherulitic structure and interior cavity with concentric chambers. Its outer shape is spherical or lenticula ...
; they are found in obsidians at Lipari, in
Yellowstone Park
Yellowstone National Park is an American national park located in the western United States, largely in the northwest corner of Wyoming and extending into Montana and Idaho. It was established by the 42nd U.S. Congress with the Yellowstone ...
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 is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is Si O2. Tridymite was first described ...
,
fayalite
Fayalite (, commonly abbreviated to Fa) 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 (space group ''Pbnm'') with cell para ...
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
In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...
feldspar
Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) feldsp ...
(
sanidine
Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system ...
or
anorthoclase
The mineral anorthoclase ((Na,K)AlSi3O8) 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 ...
) 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
salt
Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quantitie ...
s and organic substances exhibit the same tendency, yielding beautiful spherulite crystallizations when melted and cooled rapidly on a microscopic slide.
Variolites
Variolites
Variolites are mafic, igneous, and typically volcanic rocks, e.g. tholeiite, basalt or komatiite, that contain centimeter-scale spherical or globular structures, called ''varioles'', in a fine-grained matrix. These structures are lighter colore ...
are a type of radiate fibrous growth, resembling spherulites in many respects, consisting of minute feathery crystals spreading outwards through a fine grained or glassy rock. In variolites there are straight or feathery feldspar crystals (usually
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 ( Na Al Si3 O8) and anorthite ( CaAl2Si2O8). The albite:an ...
) 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 is a common rock-forming pyroxene mineral with formula . 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.
References
*{{1911, wstitle=Spherulites
*Baird, Bill, 1990
''Stone Spheres'' The Edinburgh Geologist, no 24 (Spring)
*Heinrich, P.V., 2007
Megaspherulites.'' PDF version, 1.4 MBBackBender's Gazette. vol. 38, no. 7, pp. 8–12.
*Smith, R.K., R.L. Tremallo, and G.E. Lofgren, 2000, ''Megaspherulite Growth: Far From Equilibrium Crystallization'', GeoCanada 2000 - The Millennium Geoscience Summit, Canadian Society of Exploration Geophysicists Annual Meeting.
*Smith, R.K., R.L. Tremallo, and G.E. Lofgren, 2001
''Growth of megaspherulites in a rhyolitic vitrophyre'' American Mineralogist, v. 86, n. 5–6, p. 589–600 (May 2001)
*Rodríguez, E.A., 2002
''The fantastic balls in el Cerro Piedras Bola (Jalisco)'' México desconocido # 305, July 2002. Last visited February 11, 2008.
Chiricahua Mountains
Igneous rocks
Petrology