Vernon J. Hurst

Visual estimation of iron in saprolite

Secondary iron compounds are the foremost coloring agents in subtropical and tropical saprolites. Amorphous Fe(OH) 3 and goethite are yellow in submicron particles, and coarse goethite is brown. Submicron hematite is red, whereas coarse hematite is gray to black. At most outcrops the color of saprolite is due to the secondary ferric compounds: the hue of the color relates to the mineralogy and particle size of the ferric pigments; value and chroma vary systematically with the proportion of pigment. The color of the saprolite determined by visual comparison with a Munsell Soil Color Chart and referred to our diagram yields a rapid estimate of total iron, as well as ancillary information about particle size and hydration state of the ferric compounds.

Accurate quantification of quartz and other phases by powder X-ray diffractometry

Abstract The 33 parameters that affect accuracy of quantitative analysis by X-ray powder diffractometry can be grouped as 1. (1) Instrumental or systematic, 2. (2) Inherent properties of the analyte, or 3. (3) Parameters related to preparation and mounting of powders. The effect of each on diffraction intensity is summarized. An optimal value or range is given for instrumental parameters. Evaluation of inherent parameters of the analyte and optimization of those related to preparation and mounting of powders are discussed. Published methods are briefly reviewed. Their reported detection limits for crystalline silica are well below what can be reliably determined in natural and industrial products if one or more critical parameters are neglected, as the size and shape of coherent diffraction domains. An addendum illustrates practical consideration of major parameters during routine analysis for quartz.

Dehydroxylation, rehydroxylation, and stability of kaolinite

From hydrothermal experiments three pressure-temperature-time curves have been refined for the system Al2O3−SiO2−H2O and reversal temperatures established for two of the principal reactions involving kaolinite. The temperatures of three isobaric invariant points enable the Gibbs free energy of formation of diaspore and pyrophyllite to be refined and the stability field of kaolinite to be calculated. The maximal temperature of stable kaolinite decreases from 296°C at 2 kb water pressure to 284°C at water’s liquid/vapor pressure, and decreases rapidly at lower pressures. On an isobaric plot of [H4SiO4] vs. °K-1, kaolinite has a wedge-shaped stability field which broadens toward lower temperature to include much of the [H4SiO4] range of near-surface environments. If [H4SiO4] is above kaolinite’s stability field and the temperature is < 100°C, halloysite forms rather than pyrophyllite, an uncommon pedogenic mineral. Pyrophyllite forms readily instead of kaolinite above 150°C if [H4SiO4] is controlled by cristobalite or noncrystalline silica.Kaolinite and a common precursor, halloysite, are characteristic products of weathering and hydro-thermal alteration. In sediments, relatively little halloysite has survived due to its low dehydration temperature and instability at low water pressure, but kaolinite commonly has survived since the Devonian Period. In buried sediments, the water pressure and [H4SiO4] requisite for stable kaolinite generally are maintained. In oxidized sediments and in pyritic reduced sediments, kaolinite commonly has survived, but where alkalies, alkaline earths, or aqueous iron has concentrated in the pore fluid, kaolinite has tended to transform to illite, zeolites, berthierine, or other minerals.РезюмеНа основе гидротермальных экспериментов были усовершенствованы три кривые давление-температура-время для системы Al2O3-SiO2-H2O и были определены реверсные температуры для двух из числа основных реакций, включающих каолинит. Величины температуры трех изобарных инвариантных точек позволили усовершенствовать величину свободной энергии Гиббса образования диаспора и пирофиллита, а также рассчитать поле стабильности каолинитов. Максимальная температура стабильного каолинита уменьшается от 296°С при давлении воды 2 кбар до 284°С при давлении жидкость/пар (для воды) и уменьшается быстро при низших давлениях. На изобарной кривой зависимости [H4SiO4] от °K-1, каолинит имеет клинообразное поле стабильности, которое расширяется по направлению к низшим температурам, чтобы включить большую часть [H4SiO4] области близких к поверхности сред. Если [H4SiO4] больше, чем для поля стабильности каолинита и температура < 100°С, галлуазит образуется вместо пирофиллита, необычного педогенического материала. Пирофиллит легко образуется вместо каолинита при температуре свыше 150°С, если [H4SiO4] контролируется кристобалитом или некристаллическим кремнеземом.Каолинит и обычний предшественник, галлуазит, являются характерными продуктами выветривания и гидротермальных изменений пород. В осадочных отложениях сохранилось сравнительно небольшое количество галлуазита вследствие его низкой температуры дегидратации и нестабильности при низких давлениях воды, тогда как каолинит обычно сохраняется со времени девонского периода. В захороненных осадочных отложениях необходимые для стабильного каолинита давление воды и количество [H4SiO4] в основном поддерживаются. В окисленных отложениях и в отложениях с уменьшенным количеством пирита каолинит обычно сохраняется, но каолинит стремится видоизмениться в иллит, цеолит, бертьерин или другие минералы там, где в жидкости пор сосредотачиваются щелочи, щелочные почвы или осадочное железо. [E.G.]ResümeeAus hydrothermalen Experimenten wurden drei Druck-Temperatur-Zeit-Kurven für das System Al2O3-SiO2-H2O bestimmt, und die Temperaturen für zwei der wichtigsten Kaolinitreaktionen gewonnen. Die Temperaturen von drei isobar invarianten Punkten ermöglichen die Bestimmung der Gibbs’schen Freien Energie für die Bildung von Diaspor und Pyrophyllit und die Berechnung des Stabilitätsfeldes von Kaolinit. Die maximale Temperatur für stabilen Kaolinit nimmt von 296°C bei 2 kBar Wasserdampfdruck auf 284°C bei gesättigtem Wasserdampfdruck ab und verringert sich sehr schnell bei niedrigeren Drucken. Auf einem isobaren Diagramm, in dem [H4SiO4] gegen °K-1 aufgetragen ist, hat Kaolinit ein keilförmiges Stabilitätsfeld, das sich gegen niedrigere Temperaturen hin verbreitert, um viel von [H4SiO4]-Bereich der Oberflächenzone mit einzuschließen. Wenn [H4SiO4] über dem Kaolinitstabilitätsfeld liegt, und die Temperatur unter 100°C ist, dann bildet sich eher Halloysit als Pyrophyllit, ein unübliches Bodenmineral. Pyrophyllit bildet sich sehr leicht anstelle von Kaolinit bei Temperaturen über 150°C,wenn [H4SiO4] durch Cristobalit oder nichtkristallisiertes SiO2 kontrolliert wird.Kaolinit und eine häufige Übergangsphase, Halloysit, sind typische Produkte der Verwitterung und hydrothermalen Umwandlung. In Sedimenten ist relativ wenig Halloysit aufgrund seiner niedrigen Dehydratationstemperatur und seiner Instabilität bei niedrigem H2O-Druck zu finden, während Kaolinit im allgemeinen seit dem Devon überlebt hat. In Versenkungssedimenten bleiben der für stabilen Kaolinit geforderte H2O-Druck und die notwendige [H4SiO4]-Aktivität im allgemeinen erhalten. In oxidierten Sedimenten und in pyritisch reduzierten Sedimenten bleibt Kaolinit gewöhnlich erhalten. Wenn jedoch Alkalien, Erdalkalien oder hydratisiertes Eisen in den Porenlösungen konzentriert sind, dann wandelt sich Kaolinit leicht in Illit, Zeolithe, Berthierit und anderen Minerale um. [U.W.]RésuméA partir d’expériences hydrothermiques, 3 courbes pression-température-temps ont été rafinées pour le système Al2O3-SiO2-H2O et des températures de revers ont été établies pour deux des réactions principales impliquant la kaolinite. Les températures de trois points invariants isobariques permet le rafinement de l’énergie libre de Gibbs de formation de la diaspore et de la pyrophyllite et le calcul du champ de stabilité de la kaolinite. La température maximale de kaolinite stable décroit de 296°C a 2 kb de pression d’eau a 284°C à la pression liquide/vapeur d’eau, et décroit rapidement à des pressions plus basses. Sur un diagramme isobarique de [H4SiO4] vs. °K-1, la kaolinite a un champ de stabilité éffilé à trois coins qui s’élargit vers la température plus basse pour inclure une grande partie de la gamme [H4SiO4] d’environements proches de la surface. Si [H4SiO4] est au delà du champ de stabilité de la kaolinite et la température est < 100°C, l’halloysite est formée plutôt que la pyrophyllite, un minéral pédogénique peu commun. La pyrophyllite est formée promptement à la place de la kaolinite au delá de 150°C si [H4SiO4] est contrôlée par la cristobalite ou par la silice non cristalline.La kaolinite et un précurseur commun, l’halloysite, sont des produits caractéristiques de l’altération à l’air et hydrothermique. Dans des sédiments, relativement peu d’halloysite a survécu à cause de sa température de déshydratation basse et de son instabilité à de basses pressions d’eau, mais la kaolinite a communément survécu depuis la période dévonienne. Dans des sédiments ensevelis, la pression d’eau et l’[H4SiO4] nécéssaires pour la kaolinite stable sont généralement maintenues. Dans des sédiments oxidés et dans des sédiments pyritiques réduits, la kaolinite a communément survécu, mais là où des alkalins, des terres alkalines, ou du fer aqueux a été concentré dans les fluides de pores, la kaolinite a eu tendance à se transformer en illite, zéolite, berthierine ou en d’autres minéraux. [D.J.]

Origin and classification of coastal plain kaolins, Southeastern USA, and the role of groundwater and microbial action

Along the inner Coastal Plain, kaolinite-metahalloysite-rich, neritic muds of Cretaceous-Eocene age have undergone intense postdepositional alteration in the recharge area of the regional groundwater system. Weathering processes have had the following profound effects on the original sediments: 1) strong compositional and textural modification of both clay and non-clay minerals; 2) whitening of the originally darker sediments by partial removal of organic matter, Fe and Mn; and 3) recrystallization of kaolinite and metahalloysite, most conspicuous where there are coarse stacks and vermiforms. Where the combination of initial sediment composition and alteration intensity was most favorable, these changes have produced important deposits of commercial quality, which now sustain the world’s largest kaolin production district. The earliest change was partial sequestration of iron as sulfide and concurrent destruction of some organic matter, mediated by sulfate-reducing bacteria. Subsequent weathering resulted in gradual leaching of alkalies, alkaline earths, iron and silica, and attendant nucleation and growth of minerals compatible with the compositional changes. The existence of several closely spaced erosional unconformities, separated by neritic sediments, is proof that weathering conditions commonly changed at a given site, in response to changes in thickness or lithology of the overlying rocks. Dsyoxic → ← oxic reversals modified both the rate and kind of alteration. (“Dysoxic” refers to molecular oxygen concentration too low to be toxic to anaerobes or cause abiotic oxidation; less extreme than “anoxic”.) Kaolins were produced partly by slower dysoxic weathering in saturated groundwater zones but mainly by more rapid oxic weathering in unsaturated zones, where bauxites also locally formed. Gradual transformation of some sediments to kaolin rarely began and ended in the same epoch. At several places most of the kaolinization (see “Definitions”) took place during Recent time, tens of millions of years after deposition of the sediments. Since the kaolins resulted from postdepositional alteration rather than sedimentary processes, they are better referred to as “Coastal Plain” rather than “sedimentary” kaolins.

Patination of Cultural Flints: Flint artifacts can be dated by cortical changes in mineralogy and texture

All flints containing unstable impurities are susceptible to patination. The rate of patination varies with many factors: (i) the texture and microstructure of the flint; (ii) its permeability; (iii) the kind, proportion and distribution of impurities; and (iv) environmental factors, such as temperature and soil chemistry. The thickness of the patina varies also with time. Two contrasting types of patina can develop: a chalky white patina and a ferruginous brown patina. Both types are observable primarily as a color change, and study of these types is facilitated by a clear understanding of the causes of color in flint. The color of most flints is the result of repeated refraction and reflection of light at numerous intergranular surfaces, whereby part of the light is internally absorbed and part is reflected back to the observer. The ratio of reflected to absorbed light governs the lightness of the color, or its value. The preferential absorption of certain wavelengths by natural pigments (such as iron oxide and hydrous iron oxide) disseminated through the flint determine the hue of the color. The color changes produced during patination relate to changes in texture and impurity content occasioned by the attack of weathering agents on impurities in the flint. The creation of voids by the dissolution and leaching of carbonates, the loosening of quartz crystallites, and the dispersal of clays all modify the reflectivity of the flint. Chemical changes involving the epigments, their dispersal along intergranular surfaces, or removal by leachig modify both reflectivity and capacity to preferentially absorb. Attempts to correlate patina thickness with age, and thus to use flint patinae chronometrically, have proven unsatisfactory because other factors, whose importance in some cases exceeds that of age, have not been taken into account. The texture and microstructure of flint, its permeability, and the kind, proportion, and distribution of impurities can be evaluated by regular petrographic techniques. Environmental factors can be assumed constant for artifacts from the same types of soil in a given climatic region. Only after allowances have been made for these additional variables does the age-dependence of flint patination become clear.

Effects of secondary iron phases on kaolinite 27Al MAS NMR spectra

Eight kaolinite and 2 halloysite samples were analyzed using 27Al magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, chemical analysis and magnetic susceptibility to understand the effect of isomorphously substituted Fe3+ and secondary Fe phases on the NMR signal. Known additions of goethite and hematite were made to determine the response of kaolinite 27Al MAS NMR spectra and sample magnetic susceptibilities.Results from high field (11.7 T) NMR studies show positive correlations between 1) Fe content, 2) magnetic susceptibility and 3) relative intensity of the spinning side band (SSB) to central band (CB) ratio. No correlation is observed between the mass-corrected NMR spectral intensity and Fe content. Comparative high/low field (11.7 T/8.46 T) NMR studies show a decrease in the relative ratio of line broadening with increasing Fe content. Projected trends of known additions of hematite and goethite versus magnetic susceptibility extrapolate back to zero y intercepts that have Fe concentrations higher than actually measured.Absolute intensity observations have negative implications for the use of 27Al MAS NMR spectroscopy in assessing Fe-ordering in kaolinites. First, high-energy, short (1/6 of π/2 solutions) pulse sequences do not produce reliable quantitative data needed to assess paramagnetic line-broadening affects caused by different Fe-ordering clustering scenarios. The lack of perfect correlation between SSB/CB, Fe content and magnetic susceptibility indicates that differences exist with respect to 1) the amount of isomorphously substituted Fe, 2) the ordering of the Fe within kaolinite, 3) the concentration of secondary Fe phases and 4) magnetic susceptibility of the secondary Fe assemblage. Variability of line-width ratios at different field strengths indicates an increasing second-order quadrupole effect (SOQE) with increasing Fe. Finally, the difference between the observed Fe content and that predicted from magnetic susceptibility measurements suggest that magnetic domain properties of secondary Fe phases behave differently from Fe domains bound in kaolinite.

Nickeliferous nontronite, a 15Aa garnierite at Niquelandia, Goias, Brazil

Garnierite from the Tocantins Complex at Niquelandia, Brazil, is a 15Å, dioctahedral clay mineral, nickeliferous nontronite. The principal octahedral cations are Fe3+, Al and Ni. The ferric state of the iron has been verified by ESCA. Ni occupies both the octahedral site and an exchange site. The garnierite formed (and is still forming) by the weathering of nickeliferous pyroxenite. Although the garnierite is a secondary product of weathering, it undergoes further change as weathering progresses: Ni and silica decrease, Fe3+ and Al increase, and the color changes from bright yellow green to red brown. Eventual breakdown of the garnierite leaves mainly hydrated oxides of iron and aluminum.

Accumulation of 2,8 Dihydroxyadenine in Bovine Liver, Kidneys, and Lymph Nodes

A variety of tissues from 20 cattle slaughtered at federally inspected facilities contained abundant light green to greenish-yellow material. Gross lesions were most common in the liver and hepatic lymph nodes. Less frequent lesions were present in the mediastinal, renal, intercostal, and gastric lymph nodes. The material was most prominent in the portal triads, and in the medullary sinuses of the lymph nodes, at times occupying up to one half of the nodal mass. Renal calculi were present in one animal. Histologically, the condition was characterized by the intracytoplasmic accumulation of innumerable brown, acicular crystals in hepatocytes, macrophages, and renal tubular epithelial cells. Less frequent large aggregates of extracellular crystals were found in the lumens of renal tubules and in portal triads. Crystals were highly birefringent when examined using polarized light. The crystals were identified as 2,8 dihydroxyadenine using X-ray diffraction, electron diffraction, infrared spectroscopy, and mass spectrometry. In mammals, adenine is normally converted to adenylate by the enzyme adenine phosphoribosyltransferase. When adenine phosphoribosyltransferase is absent, deficient, or inhibited, adenine is oxidized to 2,8 dihydroxyadenine, which is extremely insoluble at physiological pH. In human beings, an autosomal recessive disease known as 2,8 dihydroxyadeninuria is caused by a deficiency of adenine phosphoribosyltransferase.

Bacterial trace fossils in eocene kaolin of the huber formation of Georgia; phylloderma microsphaeroides, n. ichnogen., n. ichnosp.

An Eocene kaolin deposit in Georgia contains microspheres of tangentially oriented kaolinite enclosing authigenic cristobalite/tridymite overgrown with kaolinite. These structures resemble cutans of clay minerals that surround polysaccharide‐covered bacteria in recent soils and sediments. Bacteria actively synthesize anionic mucopolysaccharides as adhesives for attachment to surfaces. Empty cutans are common; these polymer‐bound clay mineral microspheres persist following cell death and lysis. Even though the void space thus created and associated organic material influence subsequent diagenetic changes, sedimentary microfabrics attributable to bacterial activity are preserved.

Origin of Amphibolites in the Cartersville–Villa Rica Area, Georgia

The Cartersville–Villa Rica amphibolites are of extrusive volcanic origin. Relict amygdaloidal textures are widespread. Pillow structures are well preserved at some localities. Relict porphyritic textures characteristic of volcanic rocks are readily recognized in interlayered gneisses. These rocks are in the upper part of the Ashland group, and probably are correlative with the Ashe Formation of North Carolina and the Mount Rogers volcanic series.