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Showing papers on "Petrography published in 1977"


01 Jan 1977
TL;DR: In this article, it is shown that the interdisciplinary study of non-mare lunar rocks based on trace element, major element, and isotopic data plus petrographic evidence can succeed in amassing a large suite of demonstrably pristine rocks, and that the relative numbers of these rocks are not in accord with statistics amassed on soil fragments and glasses.
Abstract: It is shown that the interdisciplinary study of the nonmare lunar rocks based on trace element, major element, and isotopic data plus petrographic evidence can succeed in amassing a large suite of demonstrably pristine rocks, and that the relative numbers of these rocks are not in accord with statistics amassed on soil fragments and glasses. The term 'pristine' is taken to mean rocks with primary compositions (albeit not necessarily textures) produced by lunar endogenous igneous processes. Melt rocks and crystalline matrix breccias produced by impact processes are excluded. A petrographic synonym for pristine would be 'unremelted, monomict'. It is found that anorthositic norites and noritic anorthosites were rare as primary nonmare rocks. Mechanical mixing appears to have been the dominant petrogenetic process on the highlands.

139 citations



Book ChapterDOI
01 Jan 1977
TL;DR: The basic rules for their recognition and interpretation are almost as old as petrography, as they were clearly formulated by Sorby in 1858, but only in recent times have decisive advances in technology (microthermometry) and in the knowledge of mineral reactions helped to understand their importance as possible samples of the volatile phase during many petrogenetic processes as discussed by the authors.
Abstract: At high magnification, quartz and many other non-opaque minerals show the presence of small (a few μm) inclusions filled with various fluids. These are mainly H2O and CO2, and more rarely hydrocarbons. The basic rules for their recognition and interpretation are almost as old as petrography, as they were clearly formulated by Sorby in 1858, but only in recent times have decisive advances in technology (microthermometry) and in the knowledge of mineral reactions helped to make it possible to understand their importance as possible samples of the volatile phase during many petrogenetic processes.

102 citations


Journal ArticleDOI
TL;DR: A review of the state of knowledge concerning a chronology for some alkalic rocks of eastern and central United States can be found in this paper, where the authors focus on several petrographically cohcrent groups of ignioric rocks, or provinces.
Abstract: Recent efforts to understand the ongm of igneous rocks have emphasized the association between petrographic suites and geologic environment. Plate tectonic theory has been particularly useful in relating certain types of calc-alkalic rocks to their position near active plate boundaries. In contrast, alkalic rocks are often emplaced after cessation of orogenic forces or in completely anorogenic settings of the craton. The role of plate tectonics in dictating such magmatism is not readily understandable. One obviou s prerequisite to any effort at iden tification of under­ lying causes is the accurate dating of the igneou s rocks. My objective in this paper is to review our prescnt state of knowledge concerning a chronology for some alkalic rocks of eastern and central United States. Although'representing less than five percent of the volume of all igneous rocks, the alkalic clan includes a variety of interesting petrographic types, such as syenite, alkalic granite, alkalic gabbro, lamprophyre, and mica peridotite. These rocks typically occupy discordant stocks, plugs, dikes, sills, and diatremes at relatively shallow levels in the crust; and they may also include eruptive counterparts. The literature abounds with contributions regarding their structure, petrography, mineralogy, and geochemistry; and I do not stray too far from my intended objective into these areas. My working definition of an alkalic rock corresponds essentially to that of Sorensen (1974), but I also include certain u ltram afic rock s (Wyllie 1967) that are often found in petrologic and spatial association. Generally, rock n ames are accepted as the original authors used them without defending the specific genetic implication sometimes att�ched to the name. This paper focuses upon several petrographically cohcrent groups of alkalic rocks, or provinces, that occur within the eastern and central U nited States. Although each province contains some felsic and some mafic members, two rather different

84 citations


Journal ArticleDOI
01 Dec 1977-Geoderma
TL;DR: In the Ndias Massif (western Senegal), a detailed study of ferricrete profiles developed on kaolinic sandstones showed that these ferricretes (iron crusts) are composed of a succession of different horizons or layers, the arrangement of which is both chronologic and genetic as mentioned in this paper.

77 citations


Journal ArticleDOI
TL;DR: Petrography of compaction features within chert masses indicates that chertification occurred after some burial as mentioned in this paper, based on stratigraphic reconstruction this burial depth was a maximum of about 215 m.
Abstract: Chert distribution in the Lake Valley rocks is selective to mud-supported facies; it is not related to proximity to unconformities. The facies selectivity of the chertification is believed to be a function of the depositional distribution of indigenous silica as sponge spicules, an interpretation that is supported by high positive qualitative correlation of chert with spiculitic rocks. Petrography indicates that the spicules were all originally siliceous, and that they all went through a moldic stage during which many molds were compactively destroyed and distorted. Remaining molds were subsequently cemented by calcite or chalcedony. Chert distribution and spicule petrography argue for an intraformational source for much of the silica. Chert micro-fabrics are dominated by microquartz, a replacement of grains and lime mud; length-fast chalcedony, a pore-filling cement; and megaquartz, a post-chalcedony pore-filling cement. Petrography of compaction features within chert masses indicates that chertification occurred after some burial. Based on stratigraphic reconstruction this burial depth was a maximum of about 215 m. and was most likely a few metres to a few tens of metres. Petrography of chert-calcite cement relationships indicates that chertification occurred before and during first generation, pre-Pennsylvanian non-ferroan calcite cementation, and was completed before late-stage, post-Mississippian ferroan calcite precipitation. Petrography of chert clasts in basal Rancheria (Meramecian) and basal Pennsylvanian conglomerates proves these clasts derived from the Lake Valley Formation and were chertified before redeposition. Thus, some cherts in the Lake Valley are pre-Meramecian in age, but all are pre-Pennsylvanian in age. Furthermore, association of the cherts with the non-ferroan cements indicates the cherts were probably precipitated in meteoric phreatic lens established beneath the pre-Meramecian and pre-Pennsylvanian subaerial unconformities.

60 citations


Journal ArticleDOI
R. Vollmer1
TL;DR: Pb isotopic composition and concentrations were determined for whole-rock samples from the Pliocene-Pleistocene, acid, Tuscan Igneous Province, Italy, in order to clarify their relation to the Cenozoic alkaline volcanism of Central Italy as mentioned in this paper.
Abstract: Pb isotopic composition and concentrations were determined for whole-rock samples from the Pliocene-Pleistocene, acid, Tuscan Igneous Province, Italy, in order to clarify their relation to the Cenozoic alkaline volcanism of Central Italy. The Pb of the Tuscan rocks is fairly uniform in its isotopic composition, despite variable and high 87Sr/86Sr ratios (0.711–0.725; Barberi et al, 1971). A comparison of the Pb isotopic composition in the Tuscan and the potassic alkaline rocks strongly suggests a continuous mixing relation between both rock series, with partial melts of mantle material and argillaceous sedimentary rocks as likely end-members. Such an interrelation could also provide a simple explanation for the range in the Sr isotopic ratio within the Tuscan rocks. The favoured model envisages a similar history for both rock-series; it is suggested that their main petrographic differences result from a lower portion of mantle-derived material in the parental material for the Tuscan rocks, and from the mineral phases present in the residue during partial melting of the source region.

59 citations


Journal ArticleDOI
TL;DR: Trace element analyses of the various textural and mineralogic rock types in all cases substantiate the genetic conclusions of earlier workers based on field and petrographic observations as discussed by the authors.
Abstract: Rare earth and other trace element data are presented for samples of the Sokoman Iron Formation, Labrador, and its associated sediments. The results show that the slates associated with the iron formation are typical in trace element contents compared to other argillaceous sediments except for the large Eu depletion characteristic of slates of their age. The iron formation, however, is fundamentally different in its trace element concentrations and patterns from those of the associated rocks. It is relatively enriched in the heavy REE and Eu and both the REE and Co, Cr, Sc, and Th concentrations bear no relationship to those of the slates and the dolomite.Trace element analyses of the various textural and mineralogic rock types in all cases substantiate the genetic conclusions of earlier workers based on field and petrographic observations. Silicate–carbonate facies samples show constant REE, Co, Sc, and Th distributions which are compatible with an origin as crystalline precipitates in equilibrium with s...

45 citations


Journal ArticleDOI
TL;DR: The Taif area occupies 170 km 2 at the western margin of the Saudi Arabian Hijaz plateau, where granitic rocks, which form about 55% of the total outcrop, have been intruded into amphibolites and quartzo-feldspathic gneisses as discussed by the authors.
Abstract: The Taif area occupies 170 km 2 at the western margin of the Saudi Arabian Hijaz plateau, where granitic rocks, which form about 55% of the total outcrop, have been intruded into amphibolites and quartzo-feldspathic gneisses. The host rocks have been metamorphosed to the almandine-amphibolite facies and have petrographic and geochemical features that suggest that they were originally igneous rocks of calc-alkalic affinity. The granitic intrusions have been divided into four groups (G 1 , G 2 , G 3 , and G 4 ) on the basis of field relations, chemistry, and textures; G 1 through G 3 are synkinematic, and G 4 is postkinematic. The granitic rocks are calc-alkalic. Isotopic data suggest that they were emplaced between 595 and 525 m.y. ago; initial 87 Sr/ 86 Sr ratios (0.703 to 0.710) indicate an origin by partial fusion of the mantle or anatexis of the lower crust. Field, geochemical, and petrographic studies suggest emplacement in an island-arc environment. This is compatible with other investigations that have led to the conclusion that the crystalline basement of Arabia and northeast Africa is the product of cratonization of island arcs over a period of 600 m.y. (from 1,100 to 500 m.y. ago). The Taif granitic rocks represent final phases in the episodic cratonization process.

44 citations


Journal ArticleDOI
TL;DR: In this article, the authors examined thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis of acid volcanic rocks at Tolfa.
Abstract: A B S TRA C T : Alunite-kaolinite deposits occur in argillized zones in acid volcanic rocks at Tolfa. Over seventy samples were examined by one or more of the following methods: thin section, DTA, X-ray, IR, TG, SEM, chemical, and hydrogen and oxygen isotope analysis. Kaolinite and dickite, of high structural order, are the dominant clays with micamontmorillonite and halloysite subordinate. In many samples kaolinite and dickite coexist; their relative amounts are variable but the dickite content tends to increase with depth. SEM studies show that dickite crystals--up to 30/tm--are bigger than kaolinite. The D/H and x80/160 ratios of six days, a whole rock, two biotites from fresh host rock, two chalcedony veins and three local meteoric waters indicate that the clays formed in a meteoric-hydrothermal environment of acid hot spring type at about 80~ The fresh volcanic host rocks are strongly enriched in 180 relative to 'normal' igneous rocks due to derivation from or exchange with 1SO-rich sedimentary rocks at depth prior to hydrothermal and solfataric activity. In the absence of present-day geothermal activity the life of this hot spring system was less than 4 m.y. Northwest of Rome (Fig. 1) extensive outcrops of acid volcanics of Late-Pliocene to Quaternary age (Devoto & Lombardi, 1977) are locally intensely altered to clay minerals and sulphates of the alunite group. The igneous rocks which occur in three separate districts: Tolfa, Cerite and Manziate, intrude and overlie the Cretaceous-Palae ogene flysch sediments that constitute the local basement and, in places, overlie the psammiticpelitic facies of the Mio-Pliocene rocks. The volcanic rocks are ignimbrites, in places cut and uplifted by lava domes of typical morphology. Petrographically most of them are liparites, trachyliparites and quartz latites, with latites and trachyandesites restricted to a few limited outcrops. Tufts sensu stricto are absent, whilst in the Tolfa district there are some related hypabyssal rocks in the sediments. Pyroclastic rocks related to the Quaternary Sabatini volcanic group and Quaternary sediments locally overlie these volcanics.

35 citations



01 Jan 1977
TL;DR: In this article, it is suggested that most of the chemical variation within the suite could result from fractionation of olivine equivalent to the composition of phenocrysts in vitrophyre 12008 which serves well as a parental magma for the other textural-chemical variants.
Abstract: Mineral chemistry data and petrographic interpretations as well as fractionation modeling determined independently indicate that the bulk chemical variations in the ilmenite suite can be reconciled with differentiation within a single cooling unit Olivine porphyritic basalts, medium-grained basalts with evolved compositions, and coarse-grained olivine cumulates are found in the suite It is suggested that most of the chemical variation within the suite could result from fractionation of olivine equivalent to the composition of phenocrysts in vitrophyre 12008 which serves well as a parental magma for the other textural-chemical variants The relative cooling rate sequence within all three textural-chemical subgroups was determined, and systematic correlations between the degree of olivine addition or subtraction and the relative cooling rate is indicated

Journal ArticleDOI
TL;DR: The Harp olivine diabase dikes, of Neohelikian age, form an east-northeast trending swarm that cuts rocks of the Harp Lake Complex in central Labrador.
Abstract: The Harp olivine diabase dikes, of Neohelikian age, form an east-northeast trending swarm that cuts rocks of the Harp Lake Complex in central Labrador. The petrography, and rock and mineral chemist...

01 Jan 1977
TL;DR: In this paper, the upper core 60010 of the double drive core 60009/60010 was analyzed for grain size distribution, petrographic particle types and abundances, magnetic fraction, and ferromagnetic resonance.
Abstract: Six samples from the upper core 60010 of the double drive core 60009/60010 were analyzed for grain size distribution, petrographic particle types and abundances, magnetic fraction, and ferromagnetic resonance. It was found that the magnetic fraction of these soils is not equivalent to the agglutinate fraction, and magnetic separation partitions high-FeO agglutinate from low-FeO agglutinates. Various maturity indices are in essential agreement. The deposition of most or all of the core could have occurred in a single event. If that is the case, the core units have no time-stratigraphic significance and do not reflect individual cratering events.

Journal ArticleDOI
15 Oct 1977-Lithos
TL;DR: In this paper, rare earth element (REE) concentrations have been determined for 27 plutonic rocks in the Permian Oslo rift, including kjelsasite/larvikites, lardalites, nordmarkites, ekerites and Drammen granites.

Journal ArticleDOI
15 Jul 1977-Lithos
TL;DR: The Quaternary island Bouvetoya is situated near the triple junction on the South Atlantic Ridge as discussed by the authors, and it is the top of a volcano of about 2000 km 8, evolved beyond the stage of caldera collapse.

01 Feb 1977
TL;DR: The basic petrographic, chemical, and age data for a representative suite of igneous rocks gathered during the six Apollo missions are presented in this paper, where the textural-mineralogic name assigned each sample is included.
Abstract: The basic petrographic, chemical, and age data is presented for a representative suite of igneous rocks gathered during the six Apollo missions. Tables are given for 69 samples: 32 igneous rocks and 37 impactites (breccias). A description is given of 26 basalts, four plutonic rocks, and two pyroclastic samples. The textural-mineralogic name assigned each sample is included.

ReportDOI
01 Aug 1977
TL;DR: In this paper, 27 cores from GT-2 and EE-1 were collected from the Fenton Hill site for petrographic and trace-element analyses, and they were used to identify four major Precambrian units.
Abstract: During the drilling of GT-2 and EE-1, 27 cores totaling about 35 m were collected from the Precambrian section. Samples of each different lithology in each core were taken for petrographic and whole-rock major- and trace-element analyses. Whole-rock analyses are now completed on 37 samples. From these data four major Precambrian units were identified at the Fenton Hill site. Geophysical logs and cuttings were used to extrapolate between cores. The most abundant rock type is an extremely variable gneissic unit comprising about 75% of the rock penetrated. This rock is strongly foliated and may range compositionally from syenogranitic to tonalitic over a few centimeters. The bulk of the unit falls within the monzogranite field. Interlayered with the gneiss is a ferrohastingsite-biotite schist which compositionally resembles a basaltic andesite. A fault contact between the schist and gneiss was observed in one core. Intrusive into this metamorphic complex are two igneous rocks. A leucocratic monzogranite occurs as at least two 15-m-thick dikes, and a biotite-granodiorite body was intercepted by 338 m of drill hole. Both rocks are unfoliated and equigranular. The biotite granodiorite is very homogeneous and is characterized by high modal contents of biotite and sphene and by high K/sub 2/O,more » TiO/sub 2/, and P/sub 2/O/sub 5/ contents. Although all of the cores examined show fractures, most of these are tightly sealed or healed. Calcite is the most abundant fracture filling mineral, but epidote, quartz, chlorite, clays or sulfides have also been observed. The degree of alteration of the essential minerals normally increases as these fractures are approached. The homogeneity of the biotite granodiorite at the bottom of GT-2 and the high degree of fracture filling ensure an ideal setting for the Hot Dry Rock Experiment.« less

Journal ArticleDOI
TL;DR: In this article, a comparison of petrographic data from several Holocene and ancient basins suggests that the relative importance of any one factor (e.g., abrasion, selective sorting, etc.) is a function of overall conditions that exist within the basin.
Abstract: The Upper Devonian Sonyea Group and laterally equivalent sedimentary rocks in New York provide an ideal setting to study the effects of transport and environment on sediment composition and texture under miogeosynclinal conditions. Sandstones are lithic arenites to lithic wackes near the primary metamorphic source terrain and they grade into quartz wackes at greater distances into the basin. Near the source monocrystalline and polycrystalline quartz grains in subequal amounts (averaging 34 and 39% respectively) and fine-grain foliated metamorphic rock fragments (averaging 16%) are the chief detrital constitients. At distances of about 100 miles (160.9 km) from the source monocrystalline quartz grains and matrix (averaging 30 and 63% respectively) remained as the principal detrital com onents. Despite the limited modifying effects of diagenesis, destruction of unstable rock fragments in river channels appears to have been one of the most significant factors increasing sediment maturity near the source. Chemical weathering, dilution, and selective sorting of sand and mud during sediment transport were probably less important factors contributing to observed changes in composition and texture. Environmental processes such as winnowing and sorting were only moderately effective in altering sediment maturity in the low to moderate energy coastal environments that existed in this area in the late Devonian. Farther from the source, environmental factors were ineffective in modifying composition and texture because of the fine-grained nature (coarse silt) and mineralogical ma urity of the marine sediments which reached these deeper marine environments. A comparison of petrographic data from several Holocene and ancient basins suggests that the relative importance of any one factor (e.g. abrasion, selective sorting, etc.) is a function of the overall conditions that exist within the basin.


Dissertation
01 Jan 1977
TL;DR: In this article, a 1,962m-deep geothermal well (HGP-A) in the east rift zone of Kilauea Volcano, Hawaii, reveals three zones of hydrothermal alteration beneath a zone of unaltered lavas.
Abstract: Mineralogical examination of basaltic rocks obtained during drilling of a successful 1,962-m-deep geothermal well (HGP-A) in the east rift zone of Kilauea Volcano, Hawaii, reveals three zones of hydrothermal alteration beneath a zone of unaltered lavas. Each alteration zone is characterized by the dominance of a particular mineral: zone 1, montmorillonite; zone 2, chlorite; zone 3, actinolite. Three zones of relative permeability can be tentatively identified on the basis of filled versus partly filled vesicles and fractures. Because the well has not returned to thermal equilibrium as of this writing, stability relations between secondary minerals and temperatures cannot be calculated with accuracy. However, the latest downhole temperatures measured at the boundaries of alteration zones, compared with similar data from high-temperature geothermal areas in Iceland, indicate that HGP-A temperatures are considerably higher than those encountered for the same alteration-zone boundaries in Icelandic wells. This indicates that the present-day thermal regime in HGP-A is relatively young and that the hydrothermal minerals probably have not reached equilibrium with existing conditions.

ReportDOI
25 Apr 1977
TL;DR: In this article, the authors examined drill cuttings and core samples from the Magmamax Nos. 2 and 3 and Woolsey No. 1 wells and found that the sequence of sedimentary rocks in the Salton Sea geothermal field from the surface to below 4000 ft can be divided into three categories: cap rock, unaltered reservoir rocks, and hydrothermally altered reservoir rocks.
Abstract: The examination of drill cuttings and core samples from the Magmamax Nos. 2 and 3 and Woolsey No. 1 wells indicate that the sequence of sedimentary rocks in the Salton Sea geothermal field from the surface to below 4000 ft can be divided into three categories: cap rock, unaltered reservoir rocks, and hydrothermally altered reservoir rocks. The cap rock extends from the surface to a depth of approximately 1100 ft in all three wells. There is evidence to suggest that the cap rock has undergone self-sealing through time as a result of the circulation of hot brine through the rocks. Essentially unaltered reservoir rocks extend from a depth of 1100 ft to approximately 3000 ft. The mineralogical and textural changes that occur at depth can be attributed to the process of hydrothermal alteration. Alteration has occurred in a chemically open system and the important variables in the alteration scheme have been temperature, permeability, brine composition, and rock composition. The transition from unaltered to altered reservoir rocks is marked by the replacement of calcite by epidote. The first appearance of epidote correlates reasonably well with the top of the alteration zone as determined in other studies by electric log analysis. Biotite more » and chlorite, potential indicators of alteration zones, are considered to be of detrital origin rather than hydrothermal origin. The primary effect of hydrothermal alteration on the reservoir rocks in the Salton Sea geothermal field has been the reduction of porosity and permeability with depth. Petrographic analysis indicates that porosity and permeability in the field is enhanced by the presence of fractures in shales. The geologic picture that emerges from spontaneous potential (SP) log correlation is that of a structural basin whose axis lies to the northwest of Magmamax No. 2. The data suggest that unaltered reservoir rocks on the periphery of the field offer good production possibilities. « less

Journal ArticleDOI
TL;DR: In this article, the chemistry, petrography, mineralogy, K/Ar isotopic age and remanent magnetism of the Wolf Rock, Cornwall, were investigated.
Abstract: By systematic sampling (including the drilling of orientated cores) new data are presented on the chemistry, petrography, mineralogy, K/Ar isotopic age and remanent magnetism of the Wolf Rock, Cornwall. The overall homogeneity of the Rock is confirmed as a fine-grained fluxioned microporphyritic phonolite. Major joints strike NNW–SSE and NNE–SSW and there is a pronounced floor jointing. New whole-rock and probe analyses of the constituent minerals are given. Nosean, previously recorded, is found to be replaced by analcime, cancrinite and other minerals. K–Ar determinations on nepheline and K-feldspar give mean ages of 112±2, and 130±6 Ma respectively. The mean pole position of 70° N, 6° E, suggests that the 130 Ma pole for stable Europe may have been somewhat closer to Europe than previously supposed. The significance of this Lower Cretaceous magmatism in the context of Mesozoic volcanism in Southern England and European alkaline provinces is discussed.

Journal ArticleDOI
TL;DR: In this article, a geochemical and petrographic examination of onland phosphorites from the southwestern Cape is made to elucidate their origin and to compare them with the known offshore occurrences.
Abstract: Summary A geochemical and petrographic examination of onland phosphorites from the south-western Cape is made to elucidate their origin and to compare them with the known offshore occurrences. The investigation shows that some of the massive phosphorites and phosphatic pebbles on land have probably formed as phoscretes in a similar manner as calcretes, whereas some of the pelletal deposits originated by disaggregation of phosphorite rock, which was derived by interstitial precipitation. This mechanism is in contrast to the mode of formation suggested for the offshore phosphatic deposits where replacement of calcium carbonate by calcium phosphate is the major diagenetic process. A model which takes these two modes of formation into account is discussed briefly.

Journal ArticleDOI
01 Nov 1977
TL;DR: A thermal and mechanical history of the gabbroic anorthosite 15418 (1140g) has been deduced from petrographic examination of both exterior and interior thin sections and electron microprobe analysis and transmission electron microscopy of interior thin segments as discussed by the authors.
Abstract: A thermal and mechanical history of lunar gabbroic anorthosite 15418 (1140g) has been deduced from petrographic examination of both exterior and interior thin sections and electron microprobe analysis and transmission electron microscopy of interior thin sections. We suggest that the rock underwent two major shock events - an early brecciation and annealing that produced a recrystallized breccia, followed by a second shock event that melted the surface of the rock, vitrified the interior plagioclase and heavily deformed the mafic phases. This latter shock even was also followed by annealing which crystallized the shock-produced glass and promoted recovery and recrystallization of the deformed crystalline phases. The complex mechanical and thermal history of 15418 compared with other ANT suite rocks at Spur Crater suggests that it had a different provenance.



Book ChapterDOI
01 Jan 1977
TL;DR: Precambrian stromatolites are widely distributed both in Peninsular and Extra-Peninsular India (Valdiya, 1969; Schnitzer, 1971).
Abstract: Precambrian stromatolites are widely distributed both in Peninsular and Extra-Peninsular India (Valdiya, 1969; Schnitzer, 1971). Although these bio-sedimentary structures have been used for stratigraphic correlation of various Precambrian sedimentary basins in India, their petrographic and chemical features have been hardly touched.


17 Aug 1977
TL;DR: In this paper, three hundred and forty-three samples, representing weathering profiles or sequences of some twenty-three different rock types collected in the field in Brazil, have been analyzed by wet-chemical, physical and x-ray diffraction methods for the purpose of understanding the processes involved in the decomposition of rocks by weathering and in the formation of clay minerals.
Abstract: : Three hundred and forty-three samples, representing weathering profiles or sequences of some twenty-three different rock types collected in the field in Brazil, have been analyzed by wet-chemical, physical and x-ray diffraction methods for the purpose of understanding the processes involved in the decomposition of rocks by weathering and in the formation of clay minerals. Evidence indicates that there is a strong relationship between (a) the presence or absence of quartz in the parent rock and the type of decomposition product formed, and (b) the width of the 'transition zone' between fresh rock and completely weathered saprolite. Quartz-rich rock types exhibit wide, gradational weathered zones and usually form kaolinite or halloysite in well-drained, or perhaps smectite in poorly-drained, environments. Quartz-free rocks exhibit extremely narrow, sharply-defined weathered zones, and weather to gibbsite (plus or minus iron oxides) in well-drained, and smectite in poorly-drained, environments. Kaolinite found in the vicinity of quartz-free rocks is either formed by re-silication of gibbsite, or is of secondary origin (transported). Texture of the rock (aphanitic vs. phaneric) has little or no effect on weathering product formed, each producing identical materials.