K-feldspar Research Articles

Classification of K-feldspar polymorphs by X-ray means

Classification of K-feldspar polymorphs by X-ray means

Late Cenozoic Transitional Alkali Olivine–Tholeiitic Basalt and Andesite from the Margin of the Great Basin, Southwest Utah

Late Cenozoic lava flows of southwest Utah comprise olivine basalt, with subordinate hypersthene-bearing basalt and andesite. Some basic flows containing normative nepheline may be classified chemically and mineralogically as alkali olivine basalt, others as tholeiitic on the basis of the presence of a reaction relation between olivine and Ca-poor pyroxene. Most of the basic lava flows, however, are transitional between typical alkaline and typical tholeiitic basalt and contain both olivine and hypersthene in the norm. The andesites are mineralogically similar to calc-alkaline andesites but are chemically richer in K 2 O. Most of the rocks are only mildly alkaline yet are lacking any clear trend of iron enrichment. Sr and Ba are the most abundant trace elements and are strongly enriched in andesite relative to basalt. Feldspar compositions vary from plagioclase of An 70 to potash feldspar of Or 70 , and olivine shows a considerable compositional range, except in basalt which contains groundmass hypersthene. Clinopyroxene is only weakly zoned and the behavior of its Al z is different according to whether the host rock is chemically alkaline or tholeiitic. Coexisting titano-magnetite and ilmenite in the groundmass of some of the basalts indicate consolidation temperatures in the range of 1040° to 800°C and oxygen fugacities ( f o 2 ) between 10 −10.3 and 10 −13.6 . The basaltic rocks probably originated by partial melting in the mantle, and their diversity is thought to reflect events which took place in the mantle rather than near the Earth9s surface. It is unlikely that the andesitic magmas were produced by crystal fractionation of basaltic magma, particularly if this involved removal of plagioclase. Sr 87 /Sr 86 ratios ranging from 0.7037 to 0.7054 place a limit on the possible amount of contamination of basalt by old, radiogenic crustal material. Thus the andesite may be the result of contamination of basaltic magma by young or nonradiogenic crustal material, or it may represent liquid produced by partial melting in the mantle. In either case, material rich in Sr and Ba seems to have been involved.

Oil Shale - An Impending Energy Source

Recent political, technological, and economic developments lead to optimism for the commercial exploitation of oil slide as a source of energy. Although presently the production of oil by mining or aboveground retorting seems feasible, the production of oil by in-situ techniques or of pipeline gas by either aboveground or underground gasification is less probable for a while. Introduction Where is oil shale going now? After many years of research, during which enthusiasm for development has periodically waxed and waned, the climate for development seems once again favorable. This report attempts to tell why. It includes a review of the size, location, and characteristics of the deposits, an assessment of the status of the processing technology, and a description of a planning program for oil-shale development. An attempt to weigh positive and negative factors to develop a current outlook results in a moderate optimism for the near-term development of an oil-shale industry. The Oil-Shale Deposits Oil shales are widely distributed throughout the U. S., but only those of the Green River formation of Colorado, Utah, and Wyoming have received appreciable attention as a possible source of fuels and chemicals. The location of the Green River deposits is indicated on Fig. 1. They underlie an area of about 17,000 sq miles in four principal basins: the Piceance Creek basin of Colorado, the Uinta basin Piceance Creek basin of Colorado, the Uinta basin of Utah, and the Washakie and Green River basins of Wyoming. Their energy potential has been estimated at more than 1 trillion bbl of oil, with about 600 billion bbl represented by the richer deposits those that assay greater than 25 gal of oil per ton in continuous sections of shale 10 ft or greater in thickness. The shale deposits vary in accessibility from those with no overburden (outcrops) to very deeply buried shales in the western Uinta basin. The outcrop (called the Mahogany ledge because of its color) along the cliffs above the Colorado River at the southern edge of the Piceance Creek basin has been utilized to study mining and retorting processes. In this Colorado basin, continuous sections of shale several hundred feet thick occur. Oil-yield assays average more than 25 gal/ton.: In Utah and Wyoming the sections of rich shale are not so thick, and in Wyoming they are often interspersed with alternating beds of lean shale. Mineralogically, the oil shales vary in composition from rocks composed of illite clay to those containing dawsonite, nahcolite, and potash feldspar as the principal minerals or to those made up of calcite and principal minerals or to those made up of calcite and dolomite. The minerals often occur as small crystals. When the organic material is present in significant quantity, it seems to be the cementing agent that holds the rock together, so that removal of the organic material from a rich shale leaves a residue with little structural stability. A typical oil shale from the stratum called the Mahogany zone in Utah and Colorado has the following composition: Organic matter, percent of shale: 13.9 Elemental composition, weight percent Carbon 80.5 Hydrogen 10.3 Nitrogen 2.4 Sulfur 1.0 Oxygen 5.8.

Experimental studies on the formation of inclusions in plagioclases from metatonalites, Hohe Tauern, Austria (lower temperature stability limit of the paragenesis anorthite plus potash feldspar)

New experimental data are presented at stability conditions of paragenesis in the system K2O-CaO-Al2O3-SiO2-H2O. These results are used to estimate the pressure temperature conditions under which minute inclusions, mostly consisting of zoisite/clinozoisite and muscovite, have crystallized in calcic plagioclases from metatonalites and metadiorites (Hohe Tauern, Austria). In the pressure region 1.5–8 kb the following reactions were observed: zoisite+muscovite+quartz=anorthite+potash feldspar+water (1) grossularite+muscovite+quartz=anorthite+potash feldspar+water (2) zoisite+quartz=anorthite+grossularite+water (3) natural plagioclase with its inclusions (zoisite/clinozoisite and muscovite) (4) =more basic plagioclase without inclusions.

Myrmekite-like intergrowths in larvikite feldspars

Electron microprobe analyses of feldspars and nepheline from a number of larvikite specimens are presented. Nepheline occurs as primary grains, as vermicular grains in mrrmekite-like intergrowths with potash feldspar, as small blebs in potash feldspar lamellae and as larger grains surrounded by irregular, K-rich feldspar domains. It is believed that the different forms of nepheline associated with potash feldspar are formed by exsolution in the solid state.

Myrmekites of Exsolution and Replacement Origins

SummaryMyrmekite in a two-feldspar migmatite suite has two distinct modes of occurrence: (i) marginally embaying potash feldspar, and (ii) in replacement aggregates after that mineral, the other major constituent of the pseudomorphs being a muscovite-quartz intergrowth. Electron-probe data give quartz proportionalities agreeing with the predictions of existing theories. It is shown that these theories, which refer to either exsolution or small-scale metasomatism, share a common foundation of kinetic impediment to AlSi diffusion. In neither case need one assume the presence of ‘Schwantke's molecule’ as a component of potash feldspar. The two myrmekite occurrences here described are interpreted in terms of essentially simultaneous exsolution and hydrative replacement processes operating as regional metamorphic temperatures began to decline from their climactic values. The existence of exsolution myrmekites of ideal quartz proportionality is not in itself evidence for the existence of ‘Schwantke's molecule’.

Progressive Metamorphism from Prehnite-Pumpellyite to Greenschist Facies in the Dansey Pass Area, Otago, New Zealand

Mesozoic graywackes on the northern margin of the Otago schist belt show a progressive metamorphic increase from prehnite-pumpellyite facies, through pumpellyite-actinolite to greenschist facies. Corresponding, but only crudely coincident, progressive textural modification ranges from massive, nonfoliated graywacke, through schistose graywacke, semischist, to thorough-going laminated quartzo feldspathic schist. Isograds marking the disappearance of prehnite and clastic potash feldspar, and the replacement of pumpellyite by clino-zoisitic epidote have been mapped, and probable reactions occurring at them are explored by modal analyses of chemically similar rocks. The thickness of the pumpellyite-actinolite facies is about 9 km. The great width of this and adjacent facies zones are not readily reconcilable with conventional concepts of geothermal gradients and PT fields of individual metamorphic facies. Tectonic disturbance of geothermal gradients during synmetamorphic deformation appears to be a possible explanation.

Clay Mineral Formation In A Deeply Weathered Boulder Conglomerate In North-East Scotland

AbstractThe clay minerals formed in a deeply weathered boulder conglomerate of Middle Old Red Sandstone (Devonian) age in north-east Scotland have been studied by a variety of physical and chemical techniques. The granite and granulite boulders in this deposit are completely weathered. With the exception of microcline, all the feldspars in these rocks—orthoclase feldspar, orthoclase-microperthite, albite, and oligoclase—weather to a Cheto-type montmorillonite, poor in iron. Electron and optical microscopy indicate that the weathering transformation is a direct one, without the intervention of any intermediate crystalline or well-defined amorphous phase. Structural control of the primary mineral over the formation of the montmorillonite seems to have been a minimal factor and the evidence suggests that the clay mineral crystallized from the soluble or colloidal products arising from the decomposed feldspars. Smaller amounts of kaolinite also formed during weathering but largely from the weathering of muscovite. The environment in which these changes occurred seems to have been alkaline in a relatively closed system. Chemical analyses of related cores and weathered shells of granite and granulite bounders show only a slight decrease of silica and an increase in magnesia. Judging from the extent of alteration to secondary clay minerals, the order of resistance towards weathering of the primary minerals in these rocks is plagioclase = orthoclase < muscovite < biotite < microcline < quartz.

Mineralogy and Textures of some cordierite–sillimanite rocks from Labrador, Canada

SummaryThe cordierite–sillimanite rocks first reported from Joan Lake Area, Labrador, show some unusual mineral associations and textures. Two myremkitic intergrowths, one among cordierite, potash feldspar and quartz, and the other between potash feldspar and plagioclase feldspar, are observed. The presence of microcline–microperthite and the textures described in the sequel indicate that these rocks were recrystallized at high temperature.

Myrmekite from the Charnockitic Rocks of the Eastern Ghats, India

SummaryThe myrmekite occurring in acid to basic charnockites of the area around Garbham and Shikaruganji, Srikakulam district, Andhra Pradesh is described. Myrmekite, whenever present, occurs only at the contact of plagioclase and potash feldspar. The textures and paragenesis of minerals suggest that potash feldspar reacted with plagioclase giving rise to myrmekite under favourable stress condition, higher stress favouring the formation of myrmekite. The difference in the stress factor or the microstrain pattern in the rock can explain the presence of myrmekite at some contacts of plagioclase–potash feldspar and absence in others.

A Study of Fenitization Around the Alkaline Carbonatite Complex at Callander Bay, Ontario, Canada

Fenitization in granitoid rocks is first noticeable in the form of hematite veinlets, accompanied by increased ordering of potash feldspar. With increasing fenitization, plagioclase is converted to albite plus calcite, and K-feldspar progressively converted toward maximum microcline. Acmitic pyroxenes become increasingly abundant. The chemical changes producing these effects consist of the substitution of sodium plus another cafemic ion for silicon in such a way as to maintain charge balance. The fenitizing fluid was essentially an NaCl brine, roughly 1 molal in alkali chlorides, and 0.01 molal total acidity. The content of iron, calcium, and magnesium was high. The oxygen fugacity was near 10−17 bars (hematite–magnetite), and the sulfur fugacity near 10−12 bars. Fenitization may have occurred at temperatures as high as 700°, but the main fenitization took place at temperatures slightly above 500° and fenitization ceased at about 450°. Pressures were low, probably less than 500 bars. The source of the fenitizing solutions was carbonatite, which in turn was derived from nephelinitic magma.

Lunar antiperthites

Antiperthitic intergrowths have been recognised during single-crystal X-ray studies of some plagioclases from lunar rock 12013,10. Host plagioclase crystals can be in the labradorite-bytownite range contrasting with the less calcium-rich feldspars normally associated with such textures in terrestrial rocks. The nature of the included phase has not finally been established but it is certainly a potash feldspar of some kind. The occurrence of such intergrowths implies that some plagioclase crystallised at high temperature in a potassium-rich environment.

A Study of Potash Fenitization around the Brent Crater, Ontario,—A Paleozoic Alkaline Complex

Fractured and brecciated granitoid gneisses around and within the Brent Crater display progressive chemical and mineralogical changes due to metasomatism, culminating in potassic, silica-undersaturated metasomatic rocks. In the metasomatized rocks, potash feldspar increases in amount and degree of triclinicity with increasing metasomatism, while quartz and plagioclase decline in amount. Calcite of carbonatitic isotopic character is found in some of the breccia matrices. Alkaline ultrabasic dikes, identical in chemistry, petrography, and radiometric age to those of the Nippissing alkaline petrographic province, cut the breccia. Potassic trachyte, which appears to form dikes and lenses within the crater, may be the result of anatexis of potassic metasomatites. The geometric form, petrography, and chemical trends of the metasomatized rocks are virtually identical to those of the fenite aureole of the Callander Bay alkaline carbonatite complex, 42 miles (67.6 km) to the west, suggesting that they result from a fenitization process in which potassium rather than sodium is enriched.

Concentrations of Li, Rb, and Cs in potash feldspar and muscovite as criteria for assessing the rare-metal mineralization in granite pegmatites

(1971). Concentrations of Li, Rb, and Cs in potash feldspar and muscovite as criteria for assessing the rare-metal mineralization in granite pegmatites. International Geology Review: Vol. 13, No. 2, pp. 134-142.

The composition of sands from the Fiskenæsset region, South-West Greenland, and its bearing on the bedrock geology of the area

A study of sand samples has shown that most of the material of the sands has only undergone short transportation and that their composition gives a good impression of the composition of the rocks in the area. Therefore, the study of sands may be used for prospecting and for regional geochemical studies in this part of Greenland. Eighty-six sand samples were studied. Most of the sands were taken from river beds and minor streams and contain 45-60 vol. % of plagioclase, 20-35 vol. % of quartz, 5-20 vol. % of dark minerals and less than 15 vol. % of potash feldspar. Hypersthene occurs especially in the northern part of the area, and the amount gradually decreases towards the south. The bulk composition of the rocks seems to be much the same throughout the whole area.

Thermoluminescence of quartz, plagioclase, and potash feldspar from granitoids of northeastern Transbaykalia

Thermoluminescence of quartz, plagioclase, and potash feldspar from granitoids of northeastern Transbaykalia

Precambrian alkaline-ultramafic/carbonatite volcanism at Qagssiarssuk, South Greenland

An outburst of alkaline-ultramafic/carbonatitic volcanic activity took place at Qagssiarssuk in the early Gardar period (about 1300 m.y.). High level volcanic structures which have been preserved include the remnants of pyroclastic cones and the uppermost parts of diatremes, some of which did not penetrate to the surface. There are many features which recall the carbonatite volcanoes of East Africa and Zambia, including the presence of flows of carbonatitic material. As a consequence of faulting and subsequent erosion, crystalline basement rocks are exposed nearby. They are penetrated by tuffisite diatremes and other minor intrusions, comagmatic with the effusive rocks, but corresponding to a depth of at least half a kilometre. The igneous rock types belong to three main groups: (i) monchiquite – alnöite – mica-peridotite, (ii) melilite-rock (a high-level equivalent of uncompahgrite) and (iii) mica-pyroxenite. In addition, a concealed body of carbonatite is directly indicated by blocks of sovite, and other material, in the pyroclastics. An episode of calcitic carbonatization was followed by ankeritic carbonatization. Carbonatization of the melilite-rock was particularly intense. Barytes and fluorite were also introduced. The volcanism was characterized by an abundance of volatiles and it is probable that fluidized systems were developed. Granitic basement rocks adjacent to the intrusions underwent progressive potash feldspathization with the ultimate production of almost pure potash feldspar rock, locally intrusive (Na2O = 0.50 %; K,O = 13.50 %).

Plagioclase Feldspar As An Indicator Of Provenance In Sedimentary Rocks

ABSTRACT A technique utilizing a petrographic microscope with a flat stage for analysis of provenance has been developed. Procedure is to concentrate feldspar in the medium or coarser sand range by flotation techniques, then to separate this concentrate by means of heavy liquids into potash feldspar, albite, oligoclase-andesine, and calcic plagioclase. Thin sections of these plagioclase separations are examined for twin characteristics (C-twins, A-twins, and untwinned), and zoning characteristics (progressive, oscillatory, and unzoned). This leads to inferences about the presence of metamorphic, plutonic igneous, or volcanic-hypabyssal rocks in the source area. Only the predominant rock type usually can be detected. An exception to this is volcanic and hypabyssal rocks, which commonly yield pl gioclase with diagnostic properties.

Paleocurrents and Origin of Huronian Lorrain Formation, Ontario and Quebec: ABSTRACT

The Lorrain Formation crops out in 3 areas between Sault Ste. Marie, Ontario, and Ville-Marie, Quebec--Bruce Mines, Whitefish Falls, and Cobalt. It conformably overlies the Gowganda Formation in each area; in the Cobalt area it also locally overlies Archean granite. The Lorrain is overlain by the Gordon Lake Formation. In ascending order, the Lorrain Formation consists of arkose, subarkose, jasper-bearing orthoconglomerate, and orthoquartzite. In the Bruce Mines area the Lorrain is 8,300 ft thick and is divided into 5 members (A-E); in the eastern areas the Lorrain is only 5,000 ft thick and is divided into 3 members (lower, middle, and upper). In each area, mineralogic and textural maturity increases upward; grains in immature arkose at the base are subrounded and 44% feldspar, and grains in supermature orthoquartzite at the top are well rounded and 95-100% quartz. Chronologically, the depositional environments suggested are shallow-water marine, lacustrine, delta fringe, overshelved beach, and high-energy beach. Currents flowing south and southeast deposited most of the sediments; however, in jasper-bearing conglomerate of the Bruce Mines area, cross-bedding and pebble composition indicate a 90° variance in transport direction, and an additional source on the east. Abundant potash feldspar and lack of metamorphic and sedimentary rock fragments in the lower members suggest a plutonic provenance. Subsequent deposition of jasper, chert, and quartz-rich detritus indicates erosion and reworking of Precambrian jaspilites and igneous source rocks similar to those now exposed in the Canadian shield. End_of_Article - Last_Page 850------------

Dikes of the McClure mountain — Iron mountain alkalic complex, Fremont county, Colorado, U.S.A.

The McClure Mountain — Iron Mountain alkalic complex, the largest of three Precambrian alkalic complexes in south-central Colorado, is an irregularly ovoid body about 8×6.5 kilometers in plan. It contains major petrologic units (in age sequence): 1) peridotite and ilmenite-magnetite lenses, 2) gabbro, 3) hornblende syenite, 4) ijolite and 5) nepheline syenite, whose internal arrangement is irregular. Dikes are numerous and varied both within the complex and in the surrounding Precambrian metasediments and meta-volcanics (Idaho Springs formation), granitoids and migmatites to a distance of 27 kilometers from the complex. Fenitization has only locally affected these wall rocks at their contacts with the complex. Dikes within the complex are chiefly light colored and of syenitic or nepheline syenitic composition. Texturally they range from aphanitic, through porphyritic with an aphanitic to fine-grained matrix, to uniformly medium-grained. Most of the intracomplex dikes (save the few carbonatites) are represented by compositionally equivalent major units within the complex. In contrast, the extracomplex dikes are chiefly lamprophyres and carbonatites, none of which has a compositionally similar unit within the complex. The carbonatite dikes are younger than the lamprophyres; where the two types co-occupy the same fracture, the lamprophyre has been carbonatized. The lamprophyres are represented by an extraordinary number of textural varieties, involving phenocrysts and phenocryst combinations of augite, barkevikite, olivine and plagioclase in varying matrix combinations of olivine, augite, barkevikite, biotite, magnetite, apatite, plagioclase and orthoclase. Many of the dikes can be classed as camptonites or olivine camptonites. The Goldie lamprophyres contain local marginal phases spectacularly marked by concentrically zoned ovoids. The Cabin dike is studded by megaphenocrysts of augite. The carbonatites are of three main types: Many carbonatites and Th-RE veins emit, when broken, a fetid gas consisting of a mixture of fluorinated hydrocarbons of the C5 and C6 types along with F2, HF, and F2O. The fluorine has been derived from structurally degraded radioactive fluorite; the hydrocarbon gases appear to represent primary inclusions. Within the dike halo are two small breccia pipes consisting of fenitized, locally derived Precambrian gneiss blocks with minor fragments of transported biotitized lamprophyre in a subordinate calcitic matrix also containing aegirine, crocidolite, potash feldspar and hematite. The abundance of the lamprophyres, the great width of the dike halo, the lack of ring structure within the complex, the restriction of carbonatite to dikes, and the subordinate fenitization effects all combine to suggest that the McClure Mountain — Iron Mountain complex is now exposed at a relatively deep niveau, possibly near the mesozone — catazone boundary.