Climatic signals in lacustrine deposits of the Upper Yacoraite Formation,Western Argentina: Evidence from clay minerals,analcime, dolomite and fibrous calcite

Authigenic clays are an important control on reservoir quality in lacustrine carbonates but remain challenging to predict. Lacustrine depositional systems respond to climatic variations in rainfall, surface runoff and groundwater input, and evaporation, and result in rapid and frequent changes in lake volume; this is expressed through changing water depth and shoreline position. In the upper portion of the Early Palaeocene Yacoraite Formation of the Salta Basin in Argentina, extensive lacustrine deposits were deposited during the sag phase of rifting. Prior high-resolution stratigraphic studies have suggested that climatic factors control microbial carbonate sequences within a ‘balanced fill’ lake, with variation in the lake level having a major influence on facies association changes. This study characterizes the evolution of facies and mineralogy within the Yacoraite Formation, focusing on the distribution of clay minerals, making a link between the high, medium and low-frequency sequence stratigraphic cycles. The low-frequency transgressive hemicycle of the upper portion of the Yacoraite Formation is comprised of abundant siliciclastic facies, suggesting a wetter period. Microbialites occurring in this interval are coarse-grained and agglutinated. Detrital clay minerals such as illite and chlorite and associated siliciclastic sediments were input to the lake during high-frequency transgressive periods. During high-frequency regressive hemicycles, sedimentation was dominated by carbonate facies with Ca-rich dolomite and the authigenic clays are comprised of chlorite/smectite mixed-layers. By contrast, the low frequency regressive hemicycle records fine-grained agglutinated microbialite with horizons of fibrous calcite, more stoichiometric dolomite, barite and authigenic magnesian smectite. This indicates elevated ion concentrations in the lake under intense evaporation during an arid period. Understanding the conditions that are favourable for formation and preservation of authigenic clays within the lacustrine environment can improve understanding of reservoir quality in comparable economically important deposits.     

Vibrational states of H<Subscript>2</Subscript>O in beryl: physical aspects

Single-crystal polarized Raman spectra (3,000–4,000 cm⁻¹ at 3 ≤ T ≤ 300 K) were measured for synthetic alkali-free and natural beryl, Be₂Al₃Si₆O₁₈·xH₂O, to determine the behavior of H₂O molecules of both Type I and Type II in the cavities. At low temperature, the H₂O molecules of Type I displace from the center of cavity and give rise to very weak hydrogen bonding with the host lattice. The H₂O Type I translational motion is characterized by substantial anharmonicity and looks like a motion of “a particle in the box” with a frequency of 6.3 cm⁻¹. Water Type II is characterized by a free rotation with respect to the C ₂ molecule axis, and it makes possible the water nuclear isomers (i.e. ortho- and para-) to be observed at low temperature.

Clinopyroxene chemistry in Tertiary alkaline volcanic rocks from Taleghan,central Alborz,Iran: Implications for two parental melts

In the Taleghan area of Alborz mountain range, two distinct units of alkaline rocks are identified. Whole rock major and trace element data does not provide a clue as to their common or different parental melts. However, major and trace element geochemistry of their included clinopyroxene phenocrysts, combined with the whole rock data, helped to infer that two parental magmas were involved in the evolution of volcanic succession in the Taleghan area.     

IR spectroscopic characterization of NH4-analcime

 The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH₄ ⁺ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν₄-bending mode; appearance of the ν₁-stretching mode which is predicted to become IR-active when the ideal T _d symmetry of NH₄ ⁺ ion is violated, and the low-frequency shift of the ν₁- and ν₃-stretching modes. The absence of absorption lines in the 1800–2400-cm⁻¹ region indicates that hydrogen bonding between the framework and the NH⁺ ₄ ion is very weak. The three-component splitting of the ν₄-bending mode indicates that the symmetry of NH⁺ ₄ ion is lower than C_3v. This implies that at least two N–H bonds of the NH⁺ ₄ ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH⁺ ₄ molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν₄ triplet frequencies for C₁ symmetry implies a deviation of the valent angle ∠H–N–H from ideal T _d symmetry of around ±2.5°. The factors governing the behaviour of the NH⁺ ₄ ion in the analcime structure are discussed. The geometry of the nearest environment of the NH⁺ ₄ ion in the analcime structure is analyzed with respect to the present IR data. Received: 2 January 2002 / Accepted: 26 June 2002 Acknowledgements We thank Dr. D. Harlov and an anonymous reviewer for their helpful comments, as well as Dr. I.A. Belitsky and Dr. S.V. Goryainov for discussion of the material. This work is supported by RFBR grants 01-05-65414, 00-05-65305 and 02-05-65313.     

Ordered distribution of aluminum atoms in analcime

Based on the connectivity relations among tetrahedron sites and the NMR spectra, we propose an ordered distribution of aluminum atoms in the framework of analcime with space groups of Pbcb and Pcaa. The symmetries of the ordered distributions are formally the same as Pcca. These symmetries are much lower than those reported by others such as Ia3d, I4₁/acd, Ibca, and I2/a. The lowering of symmetry results apparently from the fact that aluminum atoms in the framework of analcime were strictly discriminated from silicon atoms. Dependence of the populations of Si(nAl)s upon Al content was estimated from ²⁹Si MAS NMR, and can be successfully simulated by the ordered distributions. In the simulation, a small number of defects in the Al distribution were introduced into the ordered distribution mainly to adjust the deviation of Al content per unit cell from the typical composition. Analysis of the powder X-ray diffraction patterns of analcime was carried out, which suggests that the space groups proposed in the present work offer a better fit than the ones reported previously. Received: 5 April 1997 / Revised, accepted: 12 March 1998     

方沸石-白榴石系列矿物的制备与应用

方沸石属小孔沸石,既是天然矿物,又可人工合成。在Na2O-Al2O3-SiO2-H2O水热体系,方沸石结构稳定,其生成受热力学因素控制,多种含SiO2、Al2O3组分的原料均可合成方沸石,且易于实现。方沸石通过离子替代可形成多种方沸石-白榴石系列矿物,离子交换反应存在2种形式,与交换离子尺寸相关。在农业上,负载有K、N等营养元素的方沸石兼具土壤调理和养分缓释/控释功能,是一种新型生态型肥料;在废水处理方面,方沸石可吸附多种金属离子,并对放射性元素封存有独特效果;在石油化工领域,方沸石限于对特定小分子的催化氧化作用。从绿色生态角度考虑,水热处理固体废弃物并用于有害离子处理、利用富钾岩石在制取钾盐的同时制备方沸石并用作土壤调理剂及新型缓释肥料是此类矿物资源综合利用的有效途径。     

Na and K distribution in agpaitic pegmatites

Composition and zoning of amphibole in agpaitic pegmatites of the 1.16 Ga Ilímaussaq complex, South Greenland record the chemical evolution of the final stages of an already extremely fractionated melt. Our results show that the general differentiation trends found in the earlier rocks of the complex are continued in the pegmatites, albeit with some significant modifications: the dominating exchange mechanism of Na + Si Ca + Al in the amphiboles of the magmatic stage changes to K + Si Ca + Al and K Na in some pegmatitic samples. Na/K ratios in amphiboles, which generally increase in the course of the Ilímaussaq fractionation, partly display a reversal during the crystallization of the most differentiated amphiboles.

The alkali trends are probably related to the buffering of Na⁺and K⁺activity by the co-crystallization of albite and microcline. This buffering favors Na⁺in cooling fluids. This mechanism is lost when analcime replaces feldspar as a stable phase in the late stages of crystallization, e.g. due to locally elevated H₂O activity. Analcime does not incorporate significant amounts of K and accordingly, amphibole incorporates more K in analcime-bearing assemblages. The Na–K variation in amphiboles in the Ilímaussaq pegmatites allow a detailed view into the late-stage evolutionary trends of a textbook agpaitic complex. The transition from silicate melt to aqueous fluid is recorded by the change of the dominant alkali ion in the pegmatitic amphiboles from Na to K.

Only in the very latest stage, virtually K-free mineral assemblages in analcime–aegirine veins support the existence of a Na-dominated aqueous fluid.