Coarse-grained Quartz Research Articles

Considerações sobre a origem dos grãos "olhos-de-quartzo" nas rochas porfiroidais do distrito de Western Mines, Ilha de Vancouver, Canadá

Quartz-eye bearing porphyroidal occur in straight association with the vent ore, in the vent zone, of the base metal massive sulphide deposits of the Western Mines District. Microscopic observations on these indicated two stages of development, which are part of a continuous process of rock alternation. In the earlier stage (first generation quartz) there was a total or quasi-total replacement of chloritecalcite spherulites or a filling of vesicles by quartz. It was accompanied by development of strong vertical foliation which superimposed to the primary bedding. In the final stage (second generation quartz) the quartz overprinted the vertical schistosity, and deformation was minimum. Quartz-eye typical of the earlier stage, termed first generation quartz, is surrounded by a bowed micaceous matrix; this is not the case when it is examined in a section perpendicular to the foliation. Composite grains and recrystallized single round grains may be in association with deformed or broken grains of original quartz and with latter or second generation grains. A preserved nucleus of calcite is occasionally observed. Hypidiomorphic and idiomorphic coarse grains of quartz are typical of the final fase of the vent alteration process, they are termed second generation quartz. Overgrowth of second generation quartz without optical character of continuity upon first generation quartz is commonly observed. In addition most of the round rims of quartz represent the outer zone of the crystallized second generation quartz. It becomes distinguishable from the rest of the quartz grain because of the abundance of microinclusions. Since both stages of quartz generation affected either mafic-intermediate or intermediate-acidic rocks, it is suggested that the term quartz-eye bearing porphyroidal rocks might be better regarded as a textural terminology than a compositional one.

CO2-rich fluid inclusions along planar elements of quartz in basement rocks of the Vredefort Dome, South Africa

Along a NW-SE profile through the basement core, starting below the sedimentary unconformity and ending in the center of the nearly circular structure, the constituent quartz grains and their fluid inclusions exhibit the following characteristics: In the NW, fluid inclusions composed of CO2 and occasionally up to 50 Vol.% H2O occur along shock-induced planar elements following predominently {0 0 0 1} of coarse, largely unrecrystallized quartz grains. The planar elements are partly still open microcracks, partly they are healed, the fluid inclusions decorating the former sites of the cracks. Along these planar elements recrystallization into fine grained new quartz fabrics starts, this process increasing decidedly towards the southeast; nevertheless fluid inclusions are still retained. — Near and within the center of the dome the formerly coarse quartz grains are completely recrystallized to medium grained annealing fabrics, in which — surprizingly — the fluid inclusions have often retained their original positions relative to the old grains, so that their planar alignment now traverses the new grain boundaries. Here the enclosed fluid is pure CO2 as far as can be determined. On the basis of the homogenization temperatures of the fluid inclusions measured, and of independent petrologic geothermometry of the basement rocks near the center, the fluids trapped after the shock event had exhibited partial pressures of CO2 as high as 3 kbars at temperatures around 850° C. The derivation of these CO2-rich, post-shock fluids is either through release of older fluid inclusions from the lower crustal granulites affected by the catastrophic shattering event, or it is from a direct mantle source that might be genetically connected with the Vredefort event itself.

A Textural Inversion Phenomenon Within Lower Jurassic Red Beds of the Ardon Formation, Makhtesh Ramon, (Israel)

ABSTRACT Textural inversion occurs in a red bed horizon of the Lower Jurassic Ardon Formation at Makhtesh Ramon. This texture consists of highly spherical, well rounded very well-sorted, coarse quartz grains, dispersed within an argillaceous silty micrite. The former is indicative of high energy sedimentation while the latter is believed to represent a low energy environment of deposition. Negative skewness values (-0.172), together with low values of standard deviation, (0.345 ) suggest a beach environment for the dispersed quartz gram phase. Surface textural features of these grains, studied under the scanning electron microscope suggest the influence of aeolian activity in a previous cycle of sedimentation. Mixing of the two texturally d ffering quartz grain populations was demonstrated by the bimodality of grain size within the suspension sub-population occurring at 2.25 . A bimodility in grain shape, determined by electron microscopy in the 2.25 grain size fraction, further highlights the mixing of the two quartz populations. Mixing of barrier beach sediments by storm and high wave activity into free grained lagoonal sediments is postulated for the origin of the textural inversion.

Geochronology of Some Precambrian Rocks of the Southern Front Range, Colorado

Rb-Sr analyses of whole-rock samples from Precambrian coarse-grained quartz monzonite and fine-grained granite exposed in the Blue Ridge and Royal Gorge areas in the southern part of the Front Range, Colorado, yield ages of 1,781 ± 66 and 1,722 ± 50 m.y., respectively [λ (Rb 87 ) = 1.39 × 10 −11 yr −1 ]. These ages correlate with the oldest recognized igneous and metamorphic events of the Front Range in Colorado. Analyses of minerals separated from two samples of the quartz monzonite define a metamorphic isochron with an age of 1,430 ± 50 m.y. Analyses of minerals from pegmatite in the Blue Ridge area indicate an age of 1,450 ± 100 m.y., with some geological uncertainty in interpretation of the data. These ages correlate with widespread 1,450-m.y.-old igneous and metamorphic activity recognized throughout the Front Range. Field relations indicate that metasedimentary rocks in the region have been intruded by the quartz monzonite and therefore are older than 1,781 ± 66 m.y. No evidence of igneous or metamorphic activity corresponding to the age of the Pikes Peak Granite (1,040 m.y.) was found.

Source and dispersal of reservoir sands in El Morgan field, gulf of Suez, Egypt

More than 1,500 samples of cuttings from 17 wells in El Morgan Field were studied for grainsize distribution, degree of roundness, and feldspar content. The sands belong to the Kareem Formation of the Miocene and could be described as arkose or subarkose with feldspar content ranging from 5–40%. Percentage maps clearly indicate a source area to the south and southwest of the field. The presence of high feldspar content and the angularity of the very coarse quartz and feldspar grains, in addition to the presence of granitic rock fragments, indicate a plutonic source for the sands. The source is believed to be the nearby Zeit Range. Minor contributions from a Nubian sandstone source from both the southwest and northeast directions may have taken place at certain intervals. The lack of feldspar alteration is attributed to rapid erosion as a result of the high relief attained by large-scale block faulting in pre-Miocene and Early Miocene times and to conditions of extreme airdity. The bottom topography of the Miocene Sea controlled the movement of water currents and determined the size, shape, and distribution of the sands.

The Lakeview Mountains Pluton, Southern California Batholith Part I: Petrology and Structure

The Lakeview Mountains pluton, a part of the southern California batholith, is a steep-walled body with a tear-shaped ground plan, exposed discontinuously over an area of 100 to 130 km 2 . It is located at an abrupt local deflection of the regional structural grain and is concordant with planar structures in the enclosing rocks. These are gabbro, granitic rocks, mixed rock composed of granitic and metamorphic rocks, and regionally metamorphosed rocks of the cordierite-amphibolite facies. Most of the adjacent granitic rocks are older than the Lakeview Mountains pluton, although some may be of the same age. The pluton is almost entirely coarse-grained hornblende-biotite quartz diorite that lacks potassium feldspar. Schlieren are ubiquitous, and small lenticular inclusions, common throughout the pluton, parallel the schlieren. The schlieren, which make the rock extremely inhomogeneous on a small scale, geometrically fall into three orientation groups. One group, the most pronounced, is concordant to the outline of the pluton and planar structures in the adjacent rocks; the other two, discordant with respect to the shape of the pluton, strike northeast and northwest. Some outcrops exhibit schlieren intersecting at a large angle, and mesoscopic fabric analysis indicates that mineral orientation of much of the massive-appearing rocks is complex on the scale of an outcrop. Granitic pegmatite dikes, masses of hypersthene gabbro, and both mafic and leucocratic quartz diorite are concentrated in the geometrically and structurally deduced center of the pluton. It is concluded that the concordant schlieren resulted from outward growth of the pluton and the discordant sets in response to regional forces. The concordance between the outline of the pluton and its wall rocks and the abrupt deflection of the regional structural grain resulted from the outward growth of the pluton. The pegmatite represents the fugitive constituents of the pluton-forming magma, and the sites of pegmatite concentration in the wider part of the tear-shaped body mark the last portions of the pluton to crystallize.

The origin of jasperoid in limestone

The name jasperoid has been applied to rocks that consist mainly of silica and that have formed by replacement. This paper considers only those jasperoids formed by replacement of limestone. Major problems involved in the origin of such jasperoid include: source of the silica; nature of solutions that dissolve, transport, and precipitate silica; and the mechanism of replacement of limestone by silica. The answers to these problems are of practical as well as scientific interest because many jasperoid bodies are closely related to mineralization. Silica may be derived from: juvenile silica of magmatic origin; silica leached from underlying rocks by hydrothermal solutions; silica locally derived from enclosing rocks by circulating solutions; and silica carried downward in ground water from the weathering of overlying rocks. The nature and the concentration of other substances in solutions influence,in a complex manner, the ability of these solutions to dissolve, transport, and precipitate silica. Nevertheless, the following generalizations can be made. The solubility and rate of solution of silica in water at moderate pressure increase slowly with temperature up to 200 degrees C.; from 200 degrees to 360 degrees C. they increase rapidly; above 360 degrees C. solubility is pressure dependent, increasing steadily at high pressure and decreasing slightly at moderate pressure due to the formation of a vapor phase. The pH has slight effect on the ionic solubility of silica in the range from pH 1 to ph 9 at low temperature. The effect of other components on the solubility of silica is probably subordinate to that of temperature above 200 degrees C., but becomes increasingly important as the temperature falls below that point. Most jasperoid bodies form by both replacement and silica deposition in voids, with replacement dominant during the early phase, and precipitation dominant during later phases. Replacement of limestone by silica is favored by relatively low temperature acid solutions, and the presence of CO 2 . As limestone dissolves, Ca ions are released to promote the precipitation of colloidal silica. Acid solutions then diffuse through this gelatinous film to continue dissolving limestone behind it; the Ca ions diffusing outward cause the precipitation of more colloidal silica at the solution-gel interface. As the gel mass ages, it shrinks, hardens, and ruptures. More silica is then deposited in the fractures. Eventually the gel crystallizes to a dense mass of aphanitic quartz and chalcedony, with shrinkage cracks and vugs filled or coated with younger coarse-grained quartz and other minerals that have been deposited directly from solution. The theory that relatively low temperature favors the formation of jasperoid replacement bodies in carbonate rocks, and high temperature inhibits their formation, offers an explanation for the gap that is observed in some districts between contact metasomatic lime silicates and siliceous replacement of limestone. This gap is characterized by the lack of any reaction between limestone and silica-bearing solutions moving through it.

Stratigraphic Relations of Hoskinnini Member (Triassic?) of Moenkopi Formation on Colorado Plateau

Recent work on strata of Permian and Triassic age on the Colorado Plateau indicates that the unit originally defined as the Hoskinnini tongue of the Cutler formation extends into parts of southeastern Utah where it had not been previously reported. In addition, the Hoskinnini is correlated with part or all of the unit defined as the Tenderfoot member of the Moenkopi formation in the salt anticline region of east-central Utah and west-central Colorado. As a result of this work, the Hoskinnini is redefined as the Hoskinnini member of the Moenkopi formation. The Hoskinnini and laterally continuous strata in the Tenderfoot member are exposed within a north-northeast oriented area about 180 miles long and 50 miles wide extending from the Monument Valley area, in northeastern A izona and southeastern Utah, to west-central Colorado. Previously the Hoskinnini had been recognized only in the Monument Valley area. The Hoskinnini member and laterally continuous strata in the Tenderfoot member are pale reddish brown sandy siltstone grading to silty very fine-grained sandstone and contains disseminated fine, medium, and coarse quartz grains. The Hoskinnini is composed of horizontal beds, which generally range from 1 to 2 feet in thickness. Individual beds are marked by indistinct discontinuous wavy laminae bounded by grayish red claystone or siltstone films. The Hoskinnini is 50-120 feet thick in most areas. The combination of fine to coarse sand grains in a silt or very fine-grained asnd matrix and of discontinuous wavy laminae serves to differentiate the Hoskinnini member from the underlying and overlying strata. These features also make possible the correlation of the member with rocks not previously correlated with the Hoskinnini in southeastern Utah and adjoining parts of Colorado.

Microstructural Behaviour of Dissolution of Quartz Grains into Feldspathic Fusions

In his previous paper, the writer has reported a quantitative investigation on the behaviours of dissolution of coarse quartz gains into feldspathic fusions, one of the most important reactions occurring in porcelain bodies during firing (this Journal 63 [712] 432 (1955)). The present one gives the results of a study on the same subject from a microstructural point of view.By firing the small tablets consisting of 97% fine ground potash-, or soda-feldspar and 3% coarse quartz grains (Table 1), one fired at 1150°-1450°C, with heating rates of 60°, 120°C, and 180°C per hr., and the other with 0.5-4 hrs. soaking at 1300°, 1350°, or 1400°C. They were studied with aid of polarization microscope, and occasionally by means of X-ray.The results obtained were summarized as follows:(1) The reaction between quartz and feldspar started from a little lower temperatures than 1150°C, with the formation of very thin reaction layers between them.(2) Refractive index distributions of glasses formed in the fired bodies were measured. From the results obtained, it is inferred that the uniformity of the melting phases formed in the bodies of quartz-potash-feldspar system were raised with the increase of firing temperatures and holding times (Fig. 4), and that in the case of the bodies of quartz-soda-feldspar system, unhomogeneity of the phases was, on the contrary, rather increased with their SiO2 content being also gradually increased with temperature and time (Fig. 5). These differences were mostly due to the difference of diffusion velocity of SiO2 which entered into the melting phases accompanied with dissolution of quartz grains.(3) In soda-feldspar fusions, the diffusion layers formed around the quartz grains temporarily showed double construction layers (Fig. 1-9), and, with soaking time, these layers gradually changed into continuous ones with the increase of their thickness. The temporary formation of these double diffusion layers was due to the fusion of soda-feldspar itself, whose fusion behaviour showed a marked change at a certain temperature range.(4) Quartz grains kept their phases metastablly until they dissolved at 1150°-1450°C., and converted to neither cristobalite nor tridymite, though a weak pseudo biaxial interferrence figures which were perhaps owing to the stresses formed with rapid cooling, were observed (Table 2). In the diffusin layers and the glassy phases, cristobalite or tridymite were also not found.

Dissolution of Quartz Grains in Feldspathic Fusions

The purpose of this paper is to study the behaviour of melting of coarse quartz gains in feldspathic fusions, one of the most important reactions occurring in porcelain bodies during firing. Small tablets consisting of 97% fine ground potash-or soda-feldspar and 3% definitely sized quartz graines (mean intersection diameter, 150.4 μ) were prepared. Some of them were fired at 1150°-1450°C with different rates of 60°, 120°, and 180°C per hr., and the others were maintained for 0.5 hr. -4hr. at 1300°, 1330°, 1350°, or 1400°C in anelectric furnace, and thence abruptly cooled in air. Intersection diameters of 100-300survived quartz grains scattered within glassy phases were measured through thin sections by polarization microscope, and, then, the mean values were calculated. Fromthe values obtained, melting behaviour of quartz grains in feldspathic fusions was inferred as follows:(1) The dissolution of quartz grains into feldspathic fusions was markedly slower than had been usually considered. In general, coarse quartz grains were more easily dissolved in potash-feldspathic fusions than in soda-feldspathic ones.(2) Jander's equation was applied approximately to the dissolution of quartz grains in feldspathic fusions, as treated in this paper.(3) It seems that Arrhenius' equation did not strictly hold for the dissolution of quartz grains into the feldspathic fusions.(4) The following relationship was obtained between firing temperatures and dissolution of quartz grains in the fusions:1-(1-x)1/3=Be-C/Twhere x: reaction ratio in volume, T: firing temperature, (°K), B and C: constants.(5) In the system soda-feldspar-quartz, it was found that the dissolution behaviour of quartz grains might show a marked change at some temperatures within 1250°-1300°CIt was also certified that the microscopical method was very advantageous and precise for the quantitative study on the dissolution of solid crystalline grains into fusion.

PETROGRAPHY, STRUCTURES, AND PETROFABRICS OF THE PINCKNEYVILLE QUARTZ DIORITE, ALABAMA

The Pinckneyville quartz diorite complex underlies an area in eastern Alabama extending from the Coosa River in northwest Elmore County northeast through Coosa and Tallapoosa counties into Clay County. Dark-gray, coarse-grained quartz diorite gneiss constitutes the major part of the complex, but there are smaller amounts of granodiorite and granite gneiss. Hornblende-biotite gneisses also occur and are considered to represent metamorphosed basic sills and dikes intruded before the quartz diorite complex. Schist septa lie within the complex and are usually parallel to the regional strike. The sequence of intrusion as interpreted from inclusions and petrographic data is (1) quartz diorite, (2) granodiorite, (3) granite. Small but persistent amounts of allanite and epidote are characteristic of the rocks of the complex. The texture is subporphyritic and shows evidence of protoclastic deformation. A northeast-southwest trend of inclusions, foliation, lineation, and contacts dominates. Primary flow structures are shown in the orientation of inclusions and micas. Secondary structures were superposed on the original flow structures shortly after intrusion and prior to complete consolidation. A prominent secondary structure is a lineation in b. Stretching in b is indicated by boudinage structures, the lineation, and small displacements of dikes. The intensity of the planar structures in the gneiss grades from strong to indistinct or lacking from east to west. Structures in schist inclusions indicate that some folding had taken place prior to intrusion. Thin pegmatite, granodiorite, and granite dikes are common. Most of them are essentially parallel to the regional strike. The petrographic boundary between gneiss and schist is always sharp. On a large scale, the gneiss structures are essentially parallel to the contacts, but slight to marked discordant relations occur at many localities. The schist structures usually parallel the contacts. Petrofabric analysis confirms the structural field observations. The mica fabrics are ac -girdles and are most complete where the foliation is weakest. Microscopic s -planes do not always coincide with the observed foliation. The quartz fabrics are also ac -girdles but are not so well developed as the mica girdles. Quartz orientations cannot always be related to known s -planes. It is concluded that the Pinckneyville quartz diorite complex represents a syntectonic intrusion—that is, that intrusion took place while external or regional forces were still active. The shape of the intrusion was controlled largely by the wall-rock structures which had developed earlier. The rocks show both igneous and metamorphic characteristics because the intrusion took place under regional metamorphic conditions.

Australian Origin of Red Rain in New Zealand

THE sample of red rain dust collected at Ben Lomond, near Queenstown in southern New Zealand, by Dr. Kidson, which the Editor has kindly sent me for inspection, agrees in its microscopic characters with falls previously described. It contains some coarse quartz grains, doubtless of local origin, but the bulk is a fine clay and silt such as is common on the dry lake beds of the interior of Australia; it contains some diatoms and shreds of algae, which look like those that grow in the pools in such localities.

On Some Mammalian Teeth from the Wealden of Hastings

S ince the late Prof. Marsh's discovery of mammalian teeth and bones in the coarse grits of the Laramie Formation in Wyoming (U.S.A.), Mr. Charles Dawson, F.G.S., has persistently searched for similar fossils in deposits of the same nature in the Wealden formation of Sussex. These beds consist chiefly of coarse quartz-grains mingled with teeth of fishes and fragments of bones, and were originally noticed by Mantell under the name of ‘Tilgate Grit.’ According to Mr. Dawson's observations, however, they form only small lenticular deposits, and occur at several horizons in the Wealden Series. The first result of this search was the discovery of a molar of Plagiaulax , which I described and named Pl. dawsoni in 1891 More recent work, in which Mr. Dawson has been helped by Messrs. P. Teilhard de Chardin and Félix Pelletier, has led to the finding of three additional specimens—two probably belonging to Plagiaulax itself, the third to a distinct though related genus. The first-described tooth was met with in the Wadhurst Clay behind St. Leonards, while the three new teeth were obtained from the Ashdown Sands of the Fairlight Cliffs near Hastings. The two new molars which seem to belong to Plagiaulax are unfortunately very imperfect. In one the crown is seen to be closely similar to that of the original tooth of Pl. dawsoni . In the second specimen most of the crown has decayed, but the two divergent roots are well displayed, the one somewhat stouter than the other. The third specimen is especially