Evaporite Mineralogy Research Articles

Editorial for Special Issue “Mineralogy, Petrology, and Geochemistry of Evaporites”

In his excellent and complete compendium “Evaporites” [...]

Yarzhemskiite, K[B5O7(OH)2]⋅H2O, a new mineral from the Chelkar salt dome, Western Kazakhstan

AbstractThe new mineral yarzhemskiite, K[B5O7(OH)2]⋅H2O, was found in a halite–sylvite evaporite rock at the Chelkar salt dome, Western Kazakhstan Region, Kazakhstan. It is also associated with carnallite, polyhalite, gypsum, strontioginorite, satimolite and quartz. Yarzhemskiite occurs as separate thick tabular, short prismatic or equant crystals up to 0.5 mm × 0.7 mm × 1 mm and grains having irregular outlines up to 1 mm × 1.5 mm × 2 mm. The mineral is transparent, colourless, with vitreous lustre. It is brittle, the Mohs’ hardness is ca 2½. Cleavage is perfect on {100}. Dmeas is 2.13(1) and Dcalc is 2.112 g cm–3. Yarzhemskiite is optically biaxial (+), α = 1.484(2), β = 1.508(2), γ = 1.546(2), 2Vmeas = 75(10)° and 2Vcalc = 80°. Chemical composition (wt.%, electron microprobe, H2O was calculated by stoichiometry) is: Na2O 0.01, K2O 17.84, CaO 0.07, B2O3 67.21, H2Ocalc 13.91, total 99.04. The empirical formula based on 10 O atoms per formula unit is K0.98B5.005O7(OH)2⋅H2O. Yarzhemskiite is monoclinic, P21/c, a = 9.47340(18), b = 7.52030(16), c = 11.4205(2) Å, β = 97.3002(17)°, V = 807.03(3) Å3 and Z = 4. The strongest reflections of the powder XRD pattern [d,Å(I,%)(hkl)] are: 9.39(86)(100), 4.696(41)(200), 3.296(18)($\bar{1}$13), 3.130(19)(022, 300), 2.935(42)(220), 2.898(100)($\bar{3}$02, $\bar{2}$21, 310), 2.832(56)(004) and 1.867(18)($\bar{2}$25). The crystal structure was solved based on single-crystal X-ray diffraction data, R1 = 3.36%. The structure contains infinite chains built by boron-centred polyhedra. The basic structural unit of the chain is a double ring B5O7(OH)2 consisting of one BO4 tetrahedron and four BO3 triangles. K+ cations centre ten-fold polyhedra which form, together with the borate chains [B5O7(OH)2]–∝, layers linked with each other only via H bonds. The mineral is named in honour of the Russian geologist, petrologist and mineralogist Yakov Yakovlevich Yarzhemskii (1901–?), a specialist in petrology of evaporite rocks and mineralogy and genesis of boron deposits related to evaporites.

Минералогия современных эвапоритов урочища Талое озеро (Республика Хакасия)

Минералогия современных эвапоритов урочища Талое озеро (Республика Хакасия)

Mineral and Physicochemical Systems of Evaporites: Geochemical and Thermodynamic Aspects

The distribution of mineral species by various systems in evaporites from Inder Lake (Kazakhstan) and Searles Lake (United States) is compared, and these localities are compared in terms of the distribution of species-defining elements. Ca, K, Na, Cl, B, and S are excess elements whose contents are high in both localities, whereas Si and Al are deficient. It is shown that comprehensive analysis of the evaporite mineralogy with selection of species-defining elements (mineral systems) can be used as an objective base for constructing quantitative physicochemical models on the formation of various types of evaporites. In addition, this approach allows a better understanding of the formation conditions of certain solid phases (including crystalline hydrates), which have been synthesized in aqueous–salt systems but have not yet been found in nature.

Mineralogy and formation of evaporite deposits from the Lewis Cliff ice tongue, Antarctica

AbstractThe mineralogy of evaporites from the Lewis Cliff ice tongue (LCIT), Antarctica, and their mineral stabilities and transformation behaviours under different temperature and relative humidity (RH) conditions have been evaluated to elucidate formation mechanism(s). A variety of sodium (Na)-rich evaporite minerals were documented using RH-controlled powder X-ray diffraction (XRD) methods including Na-sulfates (mirabilite and thenardite), Na-carbonate/bicarbonates (nahcolite, occasional trona and natron) and Na-borates (qilianshanite and borax). Mirabilite begins to dehydrate to thenardite, and natron to trona and natrite when exposed to room temperature, even when maintained at RH values similar to those measured at the LCIT (50–70%). The boron-mineral qilianshanite was discovered for the first time in Antarctica within the evaporite mounds. The mirabilite-rich mounds are deduced to have formed via a freezing/sublimation process that occurred in glacial or subglacial bodies of water supplied by glacial tills containing microbially oxidized sulfate ions. The needle-like nahcolite crystals growing on the exteriors of the mounds suggest a dissolution/precipitation process involving atmospheric CO2 and water. The co-existence of nahcolite and boron-bearing minerals indicates the presence of a Na+-, HCO3-- and boron-bearing alkaline brine, which produces qilianshanite as a secondary mineral by reaction of nahcolite and borax in atmospheric CO2 and H2O.

Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry

Research Article| February 01, 2014 Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry Natalie Spear; Natalie Spear 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA Search for other works by this author on: GSW Google Scholar H.D. Holland; H.D. Holland 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA †Deceased Search for other works by this author on: GSW Google Scholar Javier Garcia-Veígas; Javier Garcia-Veígas 2Scientific and Technological Centers, University of Barcelona, Barcelona 08028, Spain Search for other works by this author on: GSW Google Scholar T.K. Lowenstein; T.K. Lowenstein 3Department of Geological Sciences, State University of New York at Binghamton, Binghamton, New York 13902, USA Search for other works by this author on: GSW Google Scholar Robert Giegengack; Robert Giegengack 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA Search for other works by this author on: GSW Google Scholar Heide Peters Heide Peters 4Leibniz Institute for Science and Mathematics Education, University of Kiel, D-24118 Kiel, Germany Search for other works by this author on: GSW Google Scholar Author and Article Information Natalie Spear 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA H.D. Holland †Deceased 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA Javier Garcia-Veígas 2Scientific and Technological Centers, University of Barcelona, Barcelona 08028, Spain T.K. Lowenstein 3Department of Geological Sciences, State University of New York at Binghamton, Binghamton, New York 13902, USA Robert Giegengack 1Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19014, USA Heide Peters 4Leibniz Institute for Science and Mathematics Education, University of Kiel, D-24118 Kiel, Germany Publisher: Geological Society of America Received: 07 Jul 2013 Revision Received: 08 Oct 2013 Accepted: 09 Oct 2013 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2014 Geological Society of America Geology (2014) 42 (2): 103–106. https://doi.org/10.1130/G34913.1 Article history Received: 07 Jul 2013 Revision Received: 08 Oct 2013 Accepted: 09 Oct 2013 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Natalie Spear, H.D. Holland, Javier Garcia-Veígas, T.K. Lowenstein, Robert Giegengack, Heide Peters; Analyses of fluid inclusions in Neoproterozoic marine halite provide oldest measurement of seawater chemistry. Geology 2014;; 42 (2): 103–106. doi: https://doi.org/10.1130/G34913.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract We analyzed primary fluid inclusions in halite from marine evaporites in the ca. 830 Ma Browne Formation of the Officer Basin in Western Australia using the cryogenic scanning electron microscopy–energy dispersive X-ray spectrometry analysis technique. The concentrations of the major ions, except K+ and possibly SO42−, fall within the range of Phanerozoic seawater. This is the first direct measurement of the composition of mid-Neoproterozoic seawater, and extends present-day knowledge of seawater chemistry by ∼300 m.y. Our estimates suggest that mid-Neoproterozoic marine sulfate concentrations were lower (∼90%) than modern values. By the terminal Neoproterozoic, fluid inclusions in halite and evaporite mineralogy indicate seawater sulfate levels rose significantly, to 50%–80% of modern concentrations, which parallels increases in atmospheric and oceanic oxygen. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Origin and deformation of intra-salt sulphate layers: an example from the Dutch Zechstein (Late Permian)

From salt mine galleries and well data it is known that thick rock salt layers can contain anhydrite and carbonate layers with thicknesses on the millimetre to tens of metre scale. The relatively thick Zechstein 3 anhydrite–carbonate layer in the northern Netherlands has been studied previously using 3-D seismic data. Observations from geophysical well logs in this study reveal the presence of thin sulphate layers on the sub-seismic scale imbedded in the Zechstein 2 (Z2) salt. Core samples, thin sections, seismic data and geochemical measurements were used to determine the mineralogy and origin of one of these Z2 sulphate layers. Bromine analyses show that they mark a freshening event in the Z2 salt, which can be correlated over large distances in the northern Netherlands. Their core-calibrated log signature indicates that the Z2 sulphate layers consist either of pure anhydrite or of anhydrite and polyhalite. The mineralogy and thickness of the sulphate layers are interpreted to vary between synsedimentary morphologic lows (thin anhydrite–polyhalite couplets) and highs (thicker anhydrite layers). Such a combination of core observations and well log analysis is a powerful tool to detect lateral trends in evaporite mineralogy and to reconstruct the environmental setting of their formation. Salt internal geometries can further be used to distinguish between different deformation mechanisms. In our study area, the distribution of sulphate layers within the Z2 salt indicates that subjacent salt dissolution was not the dominant process leading to salt-related deformation.

Evaporite mineralogy and major element geochemistry as tools for palaeoclimatic investigations in arid regions: A synthesis

This paper presents a synthesis of the applications of evaporite mineralogy and the relationship between major elements for the palaeoclimatological research of arid regions, with examples from Playa Oum el Krialate in Tunisia, Wadi Natron in Egypt, East African Rift Valley, etc. The numerous evaporite minerals serving as indicators of palaeo-drylands (salinity and evaporation) include carbonates, sulfates, and Na, K, Ca, and Mg chlorides. The occurrence of double salts, such as glauberite, carnallite, kainite, gaylussite, pirssonite, burkeite, etc., suggests disequilibrium conditions. Apart from that, the presence of very rare Fe-sulfates, such as rozenite and szomolnokite, indicates anoxic conditions with higher salinity. The formation of Na-silicates, such as magadiite and kenyaite, implies a decrease in pH of a highly alkaline Na concentrated brine. The Mg-silicates (palygorskite, Mg-montmorillonite and talc) form quickly and then re-dissolve when conditions change. Identification of fulgurites in the Sahara has been related to palaeo-lightning. We have also discussed a simple geochemical approach of using the ratios of soluble/insoluble elements to identify palaeo-arid events with examples from loess-soil sequences from Feiran Oasis in the Sinai Desert (Egypt) and salty silt lacustrine sequences from Thar Desert (India).

Experimental constraints on the evaporation of partially oxidized acid-sulfate waters at the martian surface

Isothermal evaporation experiments were carried out on an acidic (pH 2), partially oxidized (Fe 2+/Fe T ∼0.5) brine with a cation composition consistent with derivation from the chemical weathering of martian basalt. During evaporation, the brine composition evolved to a highly acidic (un-scaled pH −1.3) Mg–Fe–SO 4–Cl brine depleted in Ca, Al and K. Evaporite minerals identified throughout the course of the experiment include (in order of crystallization): gypsum, Mg-rich voltaite, (Mg 0.7, Fe 2 + 0.3 )SO 4·7H 2O and rhomboclase. The solid solution compositions of voltaite and (Mg 0.7, Fe 2 + 0.3 )SO 4·7H 2O, although uncommon in analogous environments on Earth, result from the distinct chemistry of evaporating martian surface fluids. Analysis of brine compositions with available thermodynamic models indicates that, although gypsum and rhomboclase precipitate at equilibrium saturation, kinetic controls on the precipitation of copiapite-group minerals affect the subsequent sulfate mineralogy and evolving chemistry of the entire system. In addition, geochemical simulations of the experimental evaporation process suggest that the appearance of voltaite and rhomboclase indicate a “metastable” evaporation pathway for martian fluids whereby bilinite and copiapite-group minerals did not form despite thermodynamic saturation. Comparison of the experimentally-produced assemblage to available observations of saline minerals at the martian surface represents a step toward systematically characterizing evaporite mineralogy as a function of Fe-oxidation in the initially dilute fluid. Deconvolving the complexity of Fe-sulfate formation in martian environments ultimately will help to exploit the sensitivity of these minerals to pH and redox conditions present at the ancient martian surface.

Geochemical modelling of hardpan formation in an iron slag dump

The geochemical processes underlying hardpan formation in an alkaline iron slag dump containing up to 30 cm thick hardpans were investigated. A modelling approach has been developed to simulate mineral precipitation sequences upon pore water evaporation with PHRQPITZ and PHREEQC. This modelling approach follows the thermodynamic reaction path in which reaction products, except for calcite, may react again with the aqueous phase and form new products as evaporation proceeds. Phases that were allowed to precipitate include am-SiO 2, am-Al(OH) 3, ferrrihydrite, calcite, and salts that are known to occur in natural evaporites. The predicted results were compared with the observed mineralogy of evaporites and hardpans as analysed by XRD, FTIR-spectroscopy, and/or SEM–EDS. The evaporites precipitating from relatively unweathered iron slag were found to consist mainly of sodium salts (mirabilite, trona, glaserite) and to a smaller extent of calcite and gel-like phases of in particularly Si, but also Al and Fe. After 10 years of weathering, amorphous Si-gels and calcite were found to be most abundant. These results are based on model predictions as well as on observations. This silica gel phase, which actually consists of heterogeneous Ca- and Fe-rich silica gels, covers particles and fills micropores and -cracks in the hardpan. Hardpans formed by gel-like phases may hold rain water by their swelling properties.

Geochemical zones and reconstruction of late Holocene environments from shallow core sediments of the Pachapadra paleo-lake, Thar Desert, India

Lithostratigraphy, mineralogy, major and trace element concentrations, carbon and sulphur contents are investigated from a shallow depth profile from Pachapadra paleo-lake, Thar Desert, north-western India, to understand the phases of paleo-hydrology and paleo-limnology. Based on the geochemical proxies (Na/Al, Si/Al, Zr/Al and Ca/Mg) and evaporite mineralogy, the depth profile is divided into three geochemical zones of variable sediment–water interaction, evaporation and aeolian activity. The sub-recent zone (I) enriched in halite (NaCl) indicates low chemical weathering and higher aeolian input. The intermediate relatively humid zone II is enriched in major elements, trace elements and calcite (CaCO 3) and reflects higher chemical weathering in the catchments. Zone III is enriched in gypsum (CaSO 4·2H 2O) and characterised by lower chemical weathering, higher aeolian activity and evaporation.

Chemical divides and evaporite assemblages on Mars

Assemblages of evaporite minerals record detailed physical and chemical characteristics of ancient surficial environments. Accordingly, newly discovered regions of saline minerals on Mars are high priority targets for exploration. The chemical divide concept of evaporite mineral formation is used successfully to predict evaporite mineralogy and brine evolution on Earth. However, basaltic weathering largely controls fluid compositions on Mars and the robust predictive capabilities of terrestrial chemical divides cannot be used to interpret Martian evaporites. Here we present a new chemical divide system that predicts evaporite assemblages identified in SNC-type meteorites, ancient evaporites discovered on Meridiani Planum by the Opportunity rover, and Mars Express OMEGA data. We suggest that a common fluid type that has been buffered to different pH levels by basaltic weathering controls the variability among Martian evaporite assemblages and that evaporite mineralogy and brine evolution is essentially established by the initial composition of the dilute evaporating fluid.

Chemical composition of seawater in Neoproterozoic: Results of fluid inclusion study of halite from Salt Range (Pakistan) and Amadeus Basin (Australia)

Data on chemical composition of brines in primary inclusions of marine halites and on mineralogy of marine evaporites and carbonates lead to the conclusion that during the Phanerozoic two long-term cycles of chemical composition of seawater existed. During each of those cycles, seawater dominantly a Na-K-Mg-Ca-Cl (Ca-rich) type changed to a Na-K-Mg-Cl-SO 4 (SO 4-rich) type. Recrystallised halite from the uppermost Neoproterozoic Salt Range Formation (ca. 545 Ma) in Pakistan, contains solitary inclusions indicating SO 4-rich brines. This supports the concept derived from the study on primary fluid inclusions from the Neoproterozoic Ara Formation of Oman; SO 4-rich seawater existed during latest Neoproterozoic time (ca. 545 Ma). In contrast, samples of recrystallised halite from the Bitter Springs Formation (840–830 Ma) in Australia contain inclusion brines that are entirely Ca-rich, indicating that basin brines and seawater were Ca-rich during deposition of central Australian evaporites. These combined data supported by the timing of aragonite and calcite seas suggest that during the Proterozoic, significant oscillations of the chemical composition of marine brines, and seawater, occurred, which are similar to those known to exist during the Phanerozoic. It is suggested that Ca-rich seawater dominated for a substantial period of time (more than 200 Ma), at 650 Ma, this was replaced by SO 4-rich seawater, finally returning to Ca-rich seawater at 530 Ma.

Late Quaternary palaeoclimatic reconstruction from the lacustrine sediments of the Sambhar playa core, Thar Desert margin, India

The Sambhar playa in Rajasthan, India, located at the eastern fringe of the Thar Desert, records multiple phases of clastic- and evaporite-dominated deposition as revealed from a ∼ 23 m deep core drilled in the center of the playa. Seven evaporite facies, identified from the mineralogical data, reflect variable brine chemistry throughout the history of the playa. The complete absence of gypsum-rich facies in the upper 5 m and dominance of carbonate- and gypsum-rich facies in the lower parts of the core reflect fluctuations in salinity level induced by climate change. Stable isotope data on carbonates ( δ 18O values) correlate with the [MgO/(MgO + CaO)] ratio, geochemical ratios (Na/Al, Na/Ti, Na/K) and the evaporite mineralogy. These data, coupled with AMS chronology of the organic fraction of the core sediments, have been integrated to interpret evaporation/inflow ratios which reflect humid and arid climatic conditions. Our data reconstruct the palaeoclimatic fluctuations in the Thar Desert margin for the last 30 ka and show significant spatial variation from the available lacustrine records from the Thar Desert. The Sambhar playa does not show any evidence of complete desiccation throughout its history although we record arid phases during the LGM and between ca. 7500 and 6800 years.

Late Holocene geochemical history inferred from Sambhar and Didwana playa sediments, Thar Desert, India: Comparison and synthesis

The Sambhar and Didwana playas from the eastern margin of the Thar Desert (annual precipitation of 300–450 mm) have been investigated for mineralogy, major and trace element geochemistry of near surface sediments. Geochemical data and evaporite mineralogy of sediments from the playa surface and shallow depths provide insight to the present-day geochemical processes and the different vertical phases of pore water activity. Detrital minerals from both the playas reflect igneous and metamorphic source rocks in their catchments. The zeolite type mineral, analcime (NaAlSi 2O 6·H 2O ) is assumed to be authigenic in origin, a product of chemical reaction between the hypersaline brine and detrital feldspars. The evaporite mineralogy is dominated by halite, calcite and dolomite. Major differences between the evaporite mineralogy of the surface sediments of two playas are the occurrence of trace amounts of the H 2O-bearing carbonate, trona (Na 3H(CO 3) 2 ·2H 2O), in the Didwana and K- and Mg-bearing sulphate and chloride minerals e.g. polyhalite (K 2MgCa 2(SO 4) 4 ·2H 2O), carnallite (KMgCl 3 ·6H 2O), and sylvite (KCl) in the Sambhar. Based on the distribution of major and trace elements, the surface sediments from both playas are divided into three geochemical zones. Fed by river and rainwaters, the playa surfaces show enrichment of insoluble cations and hydrolysates in the outermost zone and soluble oxides and cations in the innermost zone. Further, the shallow depth profiles are also divided into distinct geochemical zones based on the elemental ratios. The sub-recent zone I reflects low chemical weathering and higher aeolian input, the middle zone II indicates relatively higher weathering index and zone III, enriched in evaporites, also shows low chemical weathering. Despite the fact that the Sambhar and Didwana are separated only by ∼50 km, they show significant differences in terms of evaporite mineralogy and inorganic composition of surface sediments. We interpret this to be a manifestation of local variations in precipitation, inflow and aeolian influx.

Secular oscillations in the stratigraphic record—an acute debate

Work on secular oscillations of the ocean-atmosphere system has reached a particularly interesting stage, characterized by competing models that alternately invoke endogenic and celestial drivers. The most widely accepted concept is the icehouse-greenhouse cycle with an approximate length of 300 Ma. The concept holds that in the past 600 Ma the Earth has oscillated between a glacial mode and one of rather warm, equable climate; correlative changes in mineralogy of marine carbonates and evaporites as well as abundance of igneous intrusions point to an endogenic cause, most probably variations in the rates of plate motion and mantle convection that modulate sea level and the level of atmospheric CO2 and thus climate. However, the 300-Ma icehouse-greenhouse cycle leaves important data unexplained. The history of glaciations as well as certain climate indicators, such as sea-surface temperatures derived from oxygen isotopes, indicate oscillations with wave lengths of 130–150 Ma. Such oscillations have been observed in the reconstructed cosmic-ray flux of the past 1000 Ma; they may modulate climate via low-level cloud cover. The debate about long-term climate oscillations poses several distinct questions for sedimentary geologists.

REE geochemistry of the recent playa sediments from the Thar Desert, India: An implication to playa sediment provenance

The playas (saline lakes) situated in the Thar Desert, north-west India, provide prominent examples of alkaline brine and varying assemblages of detrital and evaporite mineralogy. The eastern margin of the desert is relatively semi-arid, whereas the central to western region is arid to hyper-arid in nature. Rare earth elements (REEs) systematics in the sediments of nine different playas of the Thar Desert were studied to understand the provenance of the sediments and the intensity of chemical weathering in the region. Based on the REE patterns, fractionation of light REE (LREE) (La/Sm) N and heavy REE (HREE) (Gd/Yb) N, and Eu anomaly (Eu/Eu*), the upper continental crust normalised playa sediments are divided into two different groups. The eastern margin playa sediments show homogeneous REE contents, relatively positive Eu anomaly and depleted HREE values, whereas the western arid core playa sediments have highly variable REE contents, relatively negative Eu anomaly and similarly fractioned LREE and HREE patterns. The dissimilarity in the degree of HREE fractionations both in the eastern and western playa sediments is attributed to the differential distribution of minerals, depending upon their resistance to chemical weathering. It is believed that the relatively higher abundance of REE bearing heavy minerals and the presence of higher amounts of evaporites influence the large variation of REE distribution and enriched HREE in the western playa sediments. Apart from the relatively higher abundance of heavy minerals, the presence of rock fragments of variable petrographic character and roundness mirror the lower rock–water interaction in the arid western region. The presence of well-rounded metamorphic rock fragments and minerals, sourced from the eastern margin Aravalli mountains, indicates that the playas of the entire desert get the detrital and dissolved material mainly from the Aravalli mountains. Additionally, the western playas receive sediments from their surrounding Proterozoic and Mesozoic formations. This interpretation is supported by the presence of angular rock fragments of basalt, rhyolite and limestone in the western playas.

Palaeoenvironmental history of Bap-Malar and Kanod playas of western Rajasthan, Thar desert

Two playas in the arid core of the western margin of the Thar desert viz., Bap-Malar and Kanod, have been investigated using palynology, geomorphology, archaeology, AMS-radiocarbon dating, stable isotopes, evaporite mineralogy and geoarchaeology. The principal objective was to obtain a reliable lithostratigraphy of the playa sediments. These are about 7 m thick in the Bap-Malar and > 2.5 to 3 m thick in the Kanod. AMS14C dates of > 15 ka BP on pollen from sediment layers indicates that the Bap-Malar playa possibly existed even during the LGM. These playas were full of water during the early Holocene (8 ka BP — 5.5 ka BP) and were ephemeral during the Pleistocene-Holocene transition and early to mid to late Holocene. The playas dried almost 1000 years earlier than those occurring on the eastern margin. Pollen of graminaceae, chenopodiaceae/amaranthaceae, cyperaceae etc. and evaporite minerals like gypsum, halite in the profiles indicate that the playas were surrounded by vegetation dominated by grass and that, they remained brackish to saline even during the mid Holocene, lake full stage. Stable dune surfaces, pediments with regoliths, and gravelly channels of ephemeral streams provided a favorable geomorphic niche for nomadic human activity since ∼ 7 ka BP. Though local ecological factors have played an important role in the evolution of the playas, the winter rains, connected with northwesterly depressions, most likely played a vital role in maintaining these playas

Chemical and environmental implications of visible and near-infrared spectral features of salt crusts formed from different brines.

This work describes a laboratory experiment intended to study the formation and spectral reflectance properties of stratified salt crusts that cause severe environmental degradations to soil and water resources in arid regions. Salt crusts were prepared by evaporating three types of saline solutions consisting of i) NaCl - Na2SO4, ii) Na2SO4 - MgSO4, and iii) NaCl - MgSO4 at an initial concentrations of 50 mmol L(-1). They were examined for evaporite mineralogy using X-ray diffraction, optical and reflected microscopes, and for spectral reflectance with a high-resolution spectroradiometer (GER 3700) in the visible and near-infrared regions (400-2500 nm). The study documented chemical and environmental implications of the spectral properties of salt formed from the studied saline-systems. The reported results can be used to understand remotely sensed signatures of salt crusts and their implications.

Evaporite mineralogy and geochemical evolution of the Sambhar Salt Lake, Rajasthan, India

The Sambhar Lake is the largest playa within the Thar desert of western India. A detailed mineralogical investigation was carried out with bed rock and soil samples collected from the catchment area of the lake and with two deep cores obtained from the lake bed. The clastic fraction of the lake sediment consists of quartz, alkali feldspar, mica, chlorite, amphibole and weathering products such as kaolinite and goethite. The non-clastic evaporite fraction is dominated by calcite and halite. There is a break in evaporite mineralogy at a depth of around 5 m. For example, gypsum is the major sulfate mineral below this depth while in shallower horizons, its place is taken by an assemblage of thenardite, kieserite and polyhalite. Using the principle of chemical divides, such variations in mineralogy have been explained in terms of a change in brine chemistry from K–Na–Ca–Mg–SO 4–Cl to K–Na–CO 3–SO 4–Cl type. It is also suggested that at an earlier stage, the Sambhar Lake brine underwent evaporation under the condition of Ca>alkalinity whereas in more recent times, the evaporite mineralogy has developed with alkalinity>Ca. Dolomitisation of calcite and formation of Mg-clay helped Mg-removal. Presence of K-bearing evaporites in the core sediments suggests that the evaporation of brine exceeded the halite saturation stage. 14C ages from one of the cores indicate that the geochemical evolution of the lake spanned a period of more than 30 ka. This may have important paleoclimatic implications.