Greeen Glass from Thomas Range in Utah
Mar 22, 2022 18:11:15 GMT -5
RWA3006, herchenx, and 1 more like this
Post by 1dave on Mar 22, 2022 18:11:15 GMT -5
www.mindat.org/mesg-103952.html
Have any of you collected this?
GREEN GLASS
pubs.usgs.gov/pp/0415/report.pdf
Occurrence and relations.-Throughout the main volcanic mass in the Thomas and Dugway Ranges there are widely scattered outcrops of what may briefly be called "green glass," for although it varies considerably in color, most of it is green or greenish gray. In its two largest areas, one of which is in the north-central part of the Thomas Range and the other about a mile east of Colored Pass, the green glass is in the upper part of the rhyolite of the third subgroup and in the overlying vitric tuff. Smaller exposures of this rock, ranging down to areas too small to show on plate 1, occur in parts of other subgroups or cut across them. The green glass is exposed in various places· from the base of the mountains to an elevation of about 6,200 feet. Except in a narrow band on the crest of-Antelope Ridge, it does not cap any of the ridges.
The green glass forms extremely irregular bodies, the thicknesses of which vary from a few feet to more than a thousand feet within short horizontal distances, as, for instance, on the south side of Pyramid Peak, Dugway Range (pl. 1).
The green glass is less resistant than most of the lavas, and many of the areas underlain by it are grassy swales. Its exposures are generally discontinuous and tend to form low rounded knobs.
Contacts of the green glass with other rocks, especially with other members of the younger volcanic group, are exposed in a number of places. On the west side of the mouth of Topaz Valley, green glass cuts the rhyolite and tuff of the fifth subgroup, which is the youngest in the area. In the central part of the Thomas Range the third and fourth subgroups are cut or replaced by this rock. In the Dugway Range, rhyolite of the first subgroup, rhyolite and tuff of the second, and breccia of the third are cut by green glass.
The green glass is not necessarily all of the same age; where it cuts across a rock, as it does on the top of the Antelope Ridge and on the south side of Pyramid Peak, it is evidently younger than that rock, but the age relation is less clear where the green glass is conformable with the other members of a subgroup. Where it is conformable, the green glass forms a layer below the obsidian layer and occupies the position where tuff or breccia is commonly found. The lower boundary of the green glass may conform to what would be the···lower boundary
100 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND DUGWAY RANGES, UTAH
of the pyroclastic beds if they were present, or it may be found much lower in the underlying rhyolite.
In places the green glass grades along strike into tuff Its contacts generally cut the overlying and underlying volcanic rocks at a small angle. In some places the boundaries of the green glass are so sharp that they can be located within a few inches. In many places, on the other hand, green glass and some other rock are separated by a gradational zone several hundred feet wide. Gradation of white vitric tuff into green glass passes through the following stages :
first, parts of the tuff .take on a slightly greenish cast; then the greenish cast deepens and spreads all through the rock, whose texture becomes more uniform and less distinctly pyroclastic; and finally the rock is a pale-green glass with a spheroidal texture. The alteration of tuff appears to be partly a result of impregnation with silica. Similar changes take place in volcanic breccia. In rhyolitic lava, however, the changes are different. Gray aphanitic rhyolite changes first either to a gray porphyritic rhyolite or almost abruptly to a brown porphyritic rock with a groundmass containing varying amounts of dull-brown glass, which changes in turn to a porphyritic spherulitic rock and then to a green spherulitic glass. The transitions from one of these rocks to the next are generally abrupt and in some places there is much intelayering of the various rocks.
Because green glass cuts most of the rocks with which it is in contact, it might be regarded as the youngest rock in the younger volcanic group. Its relations to the other rocks appears, however, to be somewhat anomalous, inasmuch as it cuts sharply across other kinds of volcanic rock in some places and grades into them in others. Some of its characteristics appear to indicate that it was formed by replacement. The writers believe that the green glass may have formed, as suggested by T. A. Steven of the U.S. Geological Survey, in and near the vents and that it consists not only of the vent fillings but also of partially remelted adjacent wall rocks, with the possible addition of some. new material. The generally steep dips of flow layers in the green glass, suggesting that its parent magma moved steeply upward, would be explained by this theory of origin. Some of the larger green glass bodies, moreover, such as those on Pyramid Peak and in the north-central part of the Thomas Range, occur in areas that probably contained vents.
The green glass is rarely seen in contact with the volcanic rocks of the older group or with the underlying Paleozoic sedimentary rocks, but at several places in the southern part of the Dugway Range, along the east flank of the volcanic rocks, it overlies upturned Cambrian rocks.
Lithology.-The material mapped as green glass includes not only the typical green spheroidal glass but also such gradational rocks as the brown porphyritic rock and the brown, gray, and greenish-gray spheroidal rocks, but no rocks in which a pyroclastic texture is visible. In short, it includes not only typical spheroidal green glass but all transitional material whose derivation is uncertain, and this transitional material covers a larger · total area than the glass.
The material here called green glass is mostly dark greenish gray, but in some places it is grayish green, light gray, brownish gray, brown, or reddish brown.
The greenish-gray rock appears, megascopically, to consist almost wholly of clear glass broken by numerous perlitic fractures (fig. 3<8). It is these fractures, rather than the presence of spherulites, that gives the rock its spheroidal texture. In some places, however, the rock contains many spherulites. Spherulites are in fact about as numerous, volume for volume, in the green glass unit as a whole as they are in any of the other volcanic rocks. The spherulites range in size from tiny pellets visible only under the microscope to large nodules 6 inches in diameter. In hand specimens they commonly appear to consist of light-brown or purplish-gray partly devitrified glass, and are darker colored than the glass that surrounds them. · In some thin sections the glass in the spherulites is light brown and is surrounded by clear glass (fig. 38), in others it is medium brown and is surrounded by a light-brown glass. Under the microscope the spherulites are seen to consist of radiating fibers that mostly have a lower refractive index than balsam; they are presumed to be mainly potassium feldspar, although some quartz is present. Spherulites or parts of spherulites commonly form on or around earlier formed phenocrysts. The larger ones (over half an inch in diameter) commonly have hollow centers. These cavities may be lined with quartz crystals, minute calcite crystals, or botryoidal masses of opal and chalcedony (fig. 39). Agate, the banded form of chalcedony, is rare, although it is found in a few localities. It is most common on the flats about 1 mile north of the northern end of the Thomas Range. Collectors have dug a number of pits in this area (see prospect symbols on plat e 1),· and a brief article on collecting has been written by I ves (1946b, p. 411-415).
In some parts of the glass, conspicuous flow structures are brought out by the alinement of microlites or crystallites or by color-layering in the glass (fig.-1:0). The color layers range from microscopic size to several feet in width. Some bands may be entirely spherulitic material, and in other s spherulites are absent.
Small open pockets or cavities, as much as 4 inches across, are plentiful and commonly contain rectangular crystals of tridymite or, in a few places, wedge-shaped twins. Cristobalite is also found in some of the cavities.
Few phenocrysts are visible in hand specimens, but in thin sections (fig. 40) it can be seen that small phenocrysts of quartz and sanidine commonly make up from 10 to 25 percent of the rock. Some of this rock contains larger and more conspicuous phenocrysts than the typical green glass, though the phenocrysts are of the same kinds and about equal in average abundance in the two kinds of rock. Quartz in subhedral to euhedral crystals, from 0.08 to 1.5 mm across, is very scarce in some specimens but makes up as much as 20 percent of others. Sanidine in subhedral to euhedral crystals, from 0.2 to 2.25 mm long, has about the same range in abundance. The embayment of both quartz and sanidine crystals in places by glass shows that they were formed before the glass solidified. Plagioclase phenocrysts form at most about 3 percent of the rock. They are generally smaller than those of quartz or sanidine, and some are enclosed in sanidine. Their composition ranges from An 20 to An 33 and averages about An 26 • Small more or less ragged crystals of biotite occur in almost every specimen, but nowhere make up more than 1 percent of the rock. Magnetite, in small rounded anhedral, generally occurs only in minute quantity. Some thin sections contain a very few small euhedral crystals of sphene and zircon.
Chemical composition.-A chemical analysis of one specimen of typical dark greenish-gray spheroidal
102 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND. DUGWAY RANGES, UTAH
glass collected about 1,000 feet northwest of Colored Pass (table 15, no. 14) shows that this rock has a rhyolitic composition and that it falls into the same category in Rittmann's chemical classification (1952, p. 95) as most of the rhyolites of the younger volcanic group. The analysis reveals, however, that the green glass is lower in Si02 , Al2 0 3 , and Na2 0 than any rhyolite of the younger group. Nevertheless, the norm of the green glass is higher in quartz than that of about half of the rhyolites of the younger group and higher in albite than the norms of most of the rhyolitic rocks in the older group. The biggest difference in composition between this rock and the other rocks of rhyolitic composition is in the water content.
In chemical analyses the water content. is divided into H 2 0- and H 2 0+ with H 2 0- .representing the water driven off by heating the rock to about 105°. H2O- would ordinarily represent water held in pore spaces, but some of the H 2 0- in this glass, which has the second highest H 2 0- content of all the analyzed rocks in this district and little pore space, may represent water dissolved in the glass itself. The H2O+ content of the green glass (4.05 percent) is the highest found in any rock of this district and is at least six times as great as that found in any of the rhyolites except the obsidian (2.34 percent) (table 15, no. 8).
The specific gravity and index of refraction of two selected specimens of clean dark greenish-gray spheroidal glass of similar appearance, in which some crystallites were visible, were determined, with the results shown in table 10.
TABLE 10.-Specific gravity and index of refraction of two specimens of green glass from the Thomas Range, Utah
[Specific gravity determinations were made on a Berman balance and are the average
of three fragments from the same specimen. Refractive index determinations were made in white light and have an accuracy of ±0.003]
Specimen Location Description Specific gravity Reflective Index
---
MH8-8-54 _____ 1,000 feet north of Dark greenish-gray 2. 30 1. 485 Colored Pass. glass containing some crystallites.
WJC-12-54 ____ Top of south end of Dark greenish-gray 2. 36 1.50 Antelope Ridge. glass containing abundant crystallites.
The specimen from north of Colored Pass ( MHS-8-54) is from the sample on which the chemical analysis (table 15, no. 14) was made. Inasmuch as the refractive indices of these two specimens of green glass are similar, the difference in specific gravity is probably due to the presence of compounds other than H 2 0. The difference in specific gravity is considerably greater than that found between any two specimens of obsidian (table 9). The second specimen of green glass (WJC-12-54) is similar in specific gravity and refractive index to some of the obsidian specimens, and therefore presumably resembles them In chemical composition. If it does, both specimens of green glass are rhyolitic but differ considerably In composition. This difference may be due either to differences in their parent magmas or to the incorporation and remelting of chemically different wall rocks.
Have any of you collected this?
GREEN GLASS
pubs.usgs.gov/pp/0415/report.pdf
Occurrence and relations.-Throughout the main volcanic mass in the Thomas and Dugway Ranges there are widely scattered outcrops of what may briefly be called "green glass," for although it varies considerably in color, most of it is green or greenish gray. In its two largest areas, one of which is in the north-central part of the Thomas Range and the other about a mile east of Colored Pass, the green glass is in the upper part of the rhyolite of the third subgroup and in the overlying vitric tuff. Smaller exposures of this rock, ranging down to areas too small to show on plate 1, occur in parts of other subgroups or cut across them. The green glass is exposed in various places· from the base of the mountains to an elevation of about 6,200 feet. Except in a narrow band on the crest of-Antelope Ridge, it does not cap any of the ridges.
The green glass forms extremely irregular bodies, the thicknesses of which vary from a few feet to more than a thousand feet within short horizontal distances, as, for instance, on the south side of Pyramid Peak, Dugway Range (pl. 1).
The green glass is less resistant than most of the lavas, and many of the areas underlain by it are grassy swales. Its exposures are generally discontinuous and tend to form low rounded knobs.
Contacts of the green glass with other rocks, especially with other members of the younger volcanic group, are exposed in a number of places. On the west side of the mouth of Topaz Valley, green glass cuts the rhyolite and tuff of the fifth subgroup, which is the youngest in the area. In the central part of the Thomas Range the third and fourth subgroups are cut or replaced by this rock. In the Dugway Range, rhyolite of the first subgroup, rhyolite and tuff of the second, and breccia of the third are cut by green glass.
The green glass is not necessarily all of the same age; where it cuts across a rock, as it does on the top of the Antelope Ridge and on the south side of Pyramid Peak, it is evidently younger than that rock, but the age relation is less clear where the green glass is conformable with the other members of a subgroup. Where it is conformable, the green glass forms a layer below the obsidian layer and occupies the position where tuff or breccia is commonly found. The lower boundary of the green glass may conform to what would be the···lower boundary
100 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND DUGWAY RANGES, UTAH
of the pyroclastic beds if they were present, or it may be found much lower in the underlying rhyolite.
In places the green glass grades along strike into tuff Its contacts generally cut the overlying and underlying volcanic rocks at a small angle. In some places the boundaries of the green glass are so sharp that they can be located within a few inches. In many places, on the other hand, green glass and some other rock are separated by a gradational zone several hundred feet wide. Gradation of white vitric tuff into green glass passes through the following stages :
first, parts of the tuff .take on a slightly greenish cast; then the greenish cast deepens and spreads all through the rock, whose texture becomes more uniform and less distinctly pyroclastic; and finally the rock is a pale-green glass with a spheroidal texture. The alteration of tuff appears to be partly a result of impregnation with silica. Similar changes take place in volcanic breccia. In rhyolitic lava, however, the changes are different. Gray aphanitic rhyolite changes first either to a gray porphyritic rhyolite or almost abruptly to a brown porphyritic rock with a groundmass containing varying amounts of dull-brown glass, which changes in turn to a porphyritic spherulitic rock and then to a green spherulitic glass. The transitions from one of these rocks to the next are generally abrupt and in some places there is much intelayering of the various rocks.
Because green glass cuts most of the rocks with which it is in contact, it might be regarded as the youngest rock in the younger volcanic group. Its relations to the other rocks appears, however, to be somewhat anomalous, inasmuch as it cuts sharply across other kinds of volcanic rock in some places and grades into them in others. Some of its characteristics appear to indicate that it was formed by replacement. The writers believe that the green glass may have formed, as suggested by T. A. Steven of the U.S. Geological Survey, in and near the vents and that it consists not only of the vent fillings but also of partially remelted adjacent wall rocks, with the possible addition of some. new material. The generally steep dips of flow layers in the green glass, suggesting that its parent magma moved steeply upward, would be explained by this theory of origin. Some of the larger green glass bodies, moreover, such as those on Pyramid Peak and in the north-central part of the Thomas Range, occur in areas that probably contained vents.
The green glass is rarely seen in contact with the volcanic rocks of the older group or with the underlying Paleozoic sedimentary rocks, but at several places in the southern part of the Dugway Range, along the east flank of the volcanic rocks, it overlies upturned Cambrian rocks.
Lithology.-The material mapped as green glass includes not only the typical green spheroidal glass but also such gradational rocks as the brown porphyritic rock and the brown, gray, and greenish-gray spheroidal rocks, but no rocks in which a pyroclastic texture is visible. In short, it includes not only typical spheroidal green glass but all transitional material whose derivation is uncertain, and this transitional material covers a larger · total area than the glass.
The material here called green glass is mostly dark greenish gray, but in some places it is grayish green, light gray, brownish gray, brown, or reddish brown.
The greenish-gray rock appears, megascopically, to consist almost wholly of clear glass broken by numerous perlitic fractures (fig. 3<8). It is these fractures, rather than the presence of spherulites, that gives the rock its spheroidal texture. In some places, however, the rock contains many spherulites. Spherulites are in fact about as numerous, volume for volume, in the green glass unit as a whole as they are in any of the other volcanic rocks. The spherulites range in size from tiny pellets visible only under the microscope to large nodules 6 inches in diameter. In hand specimens they commonly appear to consist of light-brown or purplish-gray partly devitrified glass, and are darker colored than the glass that surrounds them. · In some thin sections the glass in the spherulites is light brown and is surrounded by clear glass (fig. 38), in others it is medium brown and is surrounded by a light-brown glass. Under the microscope the spherulites are seen to consist of radiating fibers that mostly have a lower refractive index than balsam; they are presumed to be mainly potassium feldspar, although some quartz is present. Spherulites or parts of spherulites commonly form on or around earlier formed phenocrysts. The larger ones (over half an inch in diameter) commonly have hollow centers. These cavities may be lined with quartz crystals, minute calcite crystals, or botryoidal masses of opal and chalcedony (fig. 39). Agate, the banded form of chalcedony, is rare, although it is found in a few localities. It is most common on the flats about 1 mile north of the northern end of the Thomas Range. Collectors have dug a number of pits in this area (see prospect symbols on plat e 1),· and a brief article on collecting has been written by I ves (1946b, p. 411-415).
In some parts of the glass, conspicuous flow structures are brought out by the alinement of microlites or crystallites or by color-layering in the glass (fig.-1:0). The color layers range from microscopic size to several feet in width. Some bands may be entirely spherulitic material, and in other s spherulites are absent.
Small open pockets or cavities, as much as 4 inches across, are plentiful and commonly contain rectangular crystals of tridymite or, in a few places, wedge-shaped twins. Cristobalite is also found in some of the cavities.
Few phenocrysts are visible in hand specimens, but in thin sections (fig. 40) it can be seen that small phenocrysts of quartz and sanidine commonly make up from 10 to 25 percent of the rock. Some of this rock contains larger and more conspicuous phenocrysts than the typical green glass, though the phenocrysts are of the same kinds and about equal in average abundance in the two kinds of rock. Quartz in subhedral to euhedral crystals, from 0.08 to 1.5 mm across, is very scarce in some specimens but makes up as much as 20 percent of others. Sanidine in subhedral to euhedral crystals, from 0.2 to 2.25 mm long, has about the same range in abundance. The embayment of both quartz and sanidine crystals in places by glass shows that they were formed before the glass solidified. Plagioclase phenocrysts form at most about 3 percent of the rock. They are generally smaller than those of quartz or sanidine, and some are enclosed in sanidine. Their composition ranges from An 20 to An 33 and averages about An 26 • Small more or less ragged crystals of biotite occur in almost every specimen, but nowhere make up more than 1 percent of the rock. Magnetite, in small rounded anhedral, generally occurs only in minute quantity. Some thin sections contain a very few small euhedral crystals of sphene and zircon.
Chemical composition.-A chemical analysis of one specimen of typical dark greenish-gray spheroidal
102 GEOLOGY AND MINERAL DEPOSITS, THOMAS AND. DUGWAY RANGES, UTAH
glass collected about 1,000 feet northwest of Colored Pass (table 15, no. 14) shows that this rock has a rhyolitic composition and that it falls into the same category in Rittmann's chemical classification (1952, p. 95) as most of the rhyolites of the younger volcanic group. The analysis reveals, however, that the green glass is lower in Si02 , Al2 0 3 , and Na2 0 than any rhyolite of the younger group. Nevertheless, the norm of the green glass is higher in quartz than that of about half of the rhyolites of the younger group and higher in albite than the norms of most of the rhyolitic rocks in the older group. The biggest difference in composition between this rock and the other rocks of rhyolitic composition is in the water content.
In chemical analyses the water content. is divided into H 2 0- and H 2 0+ with H 2 0- .representing the water driven off by heating the rock to about 105°. H2O- would ordinarily represent water held in pore spaces, but some of the H 2 0- in this glass, which has the second highest H 2 0- content of all the analyzed rocks in this district and little pore space, may represent water dissolved in the glass itself. The H2O+ content of the green glass (4.05 percent) is the highest found in any rock of this district and is at least six times as great as that found in any of the rhyolites except the obsidian (2.34 percent) (table 15, no. 8).
The specific gravity and index of refraction of two selected specimens of clean dark greenish-gray spheroidal glass of similar appearance, in which some crystallites were visible, were determined, with the results shown in table 10.
TABLE 10.-Specific gravity and index of refraction of two specimens of green glass from the Thomas Range, Utah
[Specific gravity determinations were made on a Berman balance and are the average
of three fragments from the same specimen. Refractive index determinations were made in white light and have an accuracy of ±0.003]
Specimen Location Description Specific gravity Reflective Index
---
MH8-8-54 _____ 1,000 feet north of Dark greenish-gray 2. 30 1. 485 Colored Pass. glass containing some crystallites.
WJC-12-54 ____ Top of south end of Dark greenish-gray 2. 36 1.50 Antelope Ridge. glass containing abundant crystallites.
The specimen from north of Colored Pass ( MHS-8-54) is from the sample on which the chemical analysis (table 15, no. 14) was made. Inasmuch as the refractive indices of these two specimens of green glass are similar, the difference in specific gravity is probably due to the presence of compounds other than H 2 0. The difference in specific gravity is considerably greater than that found between any two specimens of obsidian (table 9). The second specimen of green glass (WJC-12-54) is similar in specific gravity and refractive index to some of the obsidian specimens, and therefore presumably resembles them In chemical composition. If it does, both specimens of green glass are rhyolitic but differ considerably In composition. This difference may be due either to differences in their parent magmas or to the incorporation and remelting of chemically different wall rocks.