Post by 1dave on Nov 22, 2020 13:36:50 GMT -7
THE BACKBENDER'S GAZETTE JULY 2007 - Megaspherulites
by Paul V. HeinrichMember of the Houston Gem & Mineral Society
Rockhounds, volcanologists, and other people who either collect or study vol-canic rocks and minerals are quite familiar with more-or-less spherical bodies which are commonly found in glassy, typically rhyolitic lavas and felsic weldedash flow tuffs (ignimbrites). These spherical bodies, called “spherulites,” consist of radiating masses of either acicular crystals (also known as spherulites) of feldspar,different polymorphs of quartz, or combination of both arranged around a nucleuswithin its center. In some cases, these spherical bodies do not exhibit a distinct radialcrystalline texture. However, they are still considered spherulites because they share acommon origin with and often occur together with spherulites exhibiting radial tex-tures.Spherulites are a very common feature found in obsidian, pitchstone, vitrophyre, andignimbrites. The “snowflakes” of snowflake obsidian are a typical example of spheru-lites found in a rhyolitic volcanic rock. Typically, these spherulites range in size froma few millimeters to just less than one centimeter. However in very rare cases, spheru-lites greater than 20 cm in diameter (called “megaspherulites”) form decimeter- and meter-scale size natural stone balls as discussed by Smith et al. (2001a) and Tremallo(1998).
Silver Cliff, Colorado
The best documented example of megaspherulites, as described in detail by Smith etal. (2001a, 2001b), Tremallo (1998) Tremallo et al. (1998), are found in the Black Obsidian Quarry just north of Silver Cliff, Custer County, Colorado. These light gray to light brownish gray megaspherulites, which range in diameter from 0.21 to 4.3meters, occur within a black to greenish black vitrophyre with microscopic albite and biotite phenocrysts. This vitrophyre is the middle unit of a 76- to 106-meter thick Middle Tertiary rhyolitic lava flow. The megaspherulites consist of fine to very fine grained radiating masses of acicular sanidine with interstital quartz; 3 to 4 millimeterveins of feldspar; and secondary purple fluorite and manganese oxide dendrites. The acicular sanidine occurs as compound, fan-shaped masses which form radiating co-lumnar-like jointing and cone structures (Smith et al., 2001a, 2001b).
Cerro Piedra Bola, Jalisco State, Mexico
The most spectacular known example of megaspherulites are stone balls, which range in diameter from 1.4 to 2 meters, and are found on and around Cerro Piedra Bola(Stirling 1969a, 1969b). It lies within the Sierra de Ameca about 6.2 miles southwest of Ahualulco de Mercado, Jalisco State, Mexico in the area of 20º 39’ 13.7” N, 104º03’ 27”W. Contrary to some descriptions, these stone balls occur not only in sphericalto semispherical shapes but as pear-shapes, cojoined twins, and dumbbells. In addition they are composed of devitrified volcanic material. According to Stirling (1969b),individual stone balls were encased in ash-flow deposits. Regionally, these tuffs havebeen dated to be 20 to 32 million years old (Frey 2007). According to Stirling (1969a,9 THE BACKBENDER'S GAZETTE JULY 2007 1969b), Dr. Robert L. Smith of the United States Geological Survey conducted a detailed petrographic analysis of samples from these stone balls. Unfortunately, the re-port that discussed these analyses was never published. It and the other field notes,pictures, samples, and petrographic thin sections of Dr. Smith and Dr. Stirling pertaining to their research appeared to have been lost. Currently, efforts are being made to protect these natural stone balls and to make them more accessible to tourists.
Klondyke, Arizona
Simon (1962) has briefly described megaspherulites, which are similar in nature tothe stone balls of Cerro Piedra Bola, Jalisco State, Mexico, from the Santa Teresa - Turnbull Mountains near Klondyke, Graham County, Arizona. They occur in the east side of a gully on the west flank of these mountains in the center of Section 18, T. 6S.,R. 20 E. about 610 meters east of the road to Imperial Mountain and about 8 kilometers north of Klondyke. These megaspherulites have an average of about 1 meter with some individual examples that are more than 2 meters in diameter. They consist of spheruloitic, radiolitic, and axiolitic aggregates of cristobilite, mica, orthoclase, plagioclase, and quartz. They occur in a black vitropyhre, which is about 12 meters thick and contains phenocrysts of feldspar and sparse biotite and quartz. This vitrophyre is the middle layer of a partially devitrified welded ash flow tuff, which is over 20 to 27 meters thick (Simon 1962).
Other Reported Megaspherulites
Megaspherulites have been reported from other locations in the world. Fuller (1931)reported the presence of megaspherulites, as large as 3 feet (0.9 meters) in diameter,within Tertiary “laminated rhyolites” exposed within Steens Mountain, Harney County,Oregon. In addition, Walker and Scheller (2004) reported the presence of megaspherulites within outcrops of the basal part of the Precambrian Tile Red rhyo-lite in the St. Francois Mountains of Missouri. In both cases details about the physical characteristics of these megaspherulites have not been published. Stirling (1969a,1969b) also reported that megaspherulites with maximum diameters of only 0.6 metershave been found at six sites in outcrops of ash flow tuffs within an unspecified 1,300square kilometer area around Los Alamos, New Mexico.
Rockhound State Park, New Mexico
One place where rock hounds can observe and collect specimens of spherulites isRockhound State Park near Deming, New Mexico (McLemore and Dunbar, 2000,Dunbar and McLemore, 2001, 2002). These spherulites range in size from 1 mm toabout 30 cm in diameter. The spherulites, which are larger than 20 cm in diameter, aretechnically small megaspherulites. They occur in rhyolitic lavas. Many consist of con-centrically zone dark grey to pinkish material surrounding a reddish core. Other spheru-lites of similar material are partly hollow. In a third group of these spherulites, thisvoid space has been filled with agate, chalcedony, and quartz crystals. The concentri-cally banded portions of these spherulites have been shown by microprobe analysis toconsist of intergrown quartz, alkali feldspar, plagioclase feldspar, and magnetite(Dunbar and McLemore 2002).
by Paul V. HeinrichMember of the Houston Gem & Mineral Society
Rockhounds, volcanologists, and other people who either collect or study vol-canic rocks and minerals are quite familiar with more-or-less spherical bodies which are commonly found in glassy, typically rhyolitic lavas and felsic weldedash flow tuffs (ignimbrites). These spherical bodies, called “spherulites,” consist of radiating masses of either acicular crystals (also known as spherulites) of feldspar,different polymorphs of quartz, or combination of both arranged around a nucleuswithin its center. In some cases, these spherical bodies do not exhibit a distinct radialcrystalline texture. However, they are still considered spherulites because they share acommon origin with and often occur together with spherulites exhibiting radial tex-tures.Spherulites are a very common feature found in obsidian, pitchstone, vitrophyre, andignimbrites. The “snowflakes” of snowflake obsidian are a typical example of spheru-lites found in a rhyolitic volcanic rock. Typically, these spherulites range in size froma few millimeters to just less than one centimeter. However in very rare cases, spheru-lites greater than 20 cm in diameter (called “megaspherulites”) form decimeter- and meter-scale size natural stone balls as discussed by Smith et al. (2001a) and Tremallo(1998).
Silver Cliff, Colorado
The best documented example of megaspherulites, as described in detail by Smith etal. (2001a, 2001b), Tremallo (1998) Tremallo et al. (1998), are found in the Black Obsidian Quarry just north of Silver Cliff, Custer County, Colorado. These light gray to light brownish gray megaspherulites, which range in diameter from 0.21 to 4.3meters, occur within a black to greenish black vitrophyre with microscopic albite and biotite phenocrysts. This vitrophyre is the middle unit of a 76- to 106-meter thick Middle Tertiary rhyolitic lava flow. The megaspherulites consist of fine to very fine grained radiating masses of acicular sanidine with interstital quartz; 3 to 4 millimeterveins of feldspar; and secondary purple fluorite and manganese oxide dendrites. The acicular sanidine occurs as compound, fan-shaped masses which form radiating co-lumnar-like jointing and cone structures (Smith et al., 2001a, 2001b).
Cerro Piedra Bola, Jalisco State, Mexico
The most spectacular known example of megaspherulites are stone balls, which range in diameter from 1.4 to 2 meters, and are found on and around Cerro Piedra Bola(Stirling 1969a, 1969b). It lies within the Sierra de Ameca about 6.2 miles southwest of Ahualulco de Mercado, Jalisco State, Mexico in the area of 20º 39’ 13.7” N, 104º03’ 27”W. Contrary to some descriptions, these stone balls occur not only in sphericalto semispherical shapes but as pear-shapes, cojoined twins, and dumbbells. In addition they are composed of devitrified volcanic material. According to Stirling (1969b),individual stone balls were encased in ash-flow deposits. Regionally, these tuffs havebeen dated to be 20 to 32 million years old (Frey 2007). According to Stirling (1969a,9 THE BACKBENDER'S GAZETTE JULY 2007 1969b), Dr. Robert L. Smith of the United States Geological Survey conducted a detailed petrographic analysis of samples from these stone balls. Unfortunately, the re-port that discussed these analyses was never published. It and the other field notes,pictures, samples, and petrographic thin sections of Dr. Smith and Dr. Stirling pertaining to their research appeared to have been lost. Currently, efforts are being made to protect these natural stone balls and to make them more accessible to tourists.
Klondyke, Arizona
Simon (1962) has briefly described megaspherulites, which are similar in nature tothe stone balls of Cerro Piedra Bola, Jalisco State, Mexico, from the Santa Teresa - Turnbull Mountains near Klondyke, Graham County, Arizona. They occur in the east side of a gully on the west flank of these mountains in the center of Section 18, T. 6S.,R. 20 E. about 610 meters east of the road to Imperial Mountain and about 8 kilometers north of Klondyke. These megaspherulites have an average of about 1 meter with some individual examples that are more than 2 meters in diameter. They consist of spheruloitic, radiolitic, and axiolitic aggregates of cristobilite, mica, orthoclase, plagioclase, and quartz. They occur in a black vitropyhre, which is about 12 meters thick and contains phenocrysts of feldspar and sparse biotite and quartz. This vitrophyre is the middle layer of a partially devitrified welded ash flow tuff, which is over 20 to 27 meters thick (Simon 1962).
Other Reported Megaspherulites
Megaspherulites have been reported from other locations in the world. Fuller (1931)reported the presence of megaspherulites, as large as 3 feet (0.9 meters) in diameter,within Tertiary “laminated rhyolites” exposed within Steens Mountain, Harney County,Oregon. In addition, Walker and Scheller (2004) reported the presence of megaspherulites within outcrops of the basal part of the Precambrian Tile Red rhyo-lite in the St. Francois Mountains of Missouri. In both cases details about the physical characteristics of these megaspherulites have not been published. Stirling (1969a,1969b) also reported that megaspherulites with maximum diameters of only 0.6 metershave been found at six sites in outcrops of ash flow tuffs within an unspecified 1,300square kilometer area around Los Alamos, New Mexico.
Rockhound State Park, New Mexico
One place where rock hounds can observe and collect specimens of spherulites isRockhound State Park near Deming, New Mexico (McLemore and Dunbar, 2000,Dunbar and McLemore, 2001, 2002). These spherulites range in size from 1 mm toabout 30 cm in diameter. The spherulites, which are larger than 20 cm in diameter, aretechnically small megaspherulites. They occur in rhyolitic lavas. Many consist of con-centrically zone dark grey to pinkish material surrounding a reddish core. Other spheru-lites of similar material are partly hollow. In a third group of these spherulites, thisvoid space has been filled with agate, chalcedony, and quartz crystals. The concentri-cally banded portions of these spherulites have been shown by microprobe analysis toconsist of intergrown quartz, alkali feldspar, plagioclase feldspar, and magnetite(Dunbar and McLemore 2002).
