Olga A. Kozmenko

Chromatite and its Cr3+- and Cr6+-bearing precursor minerals from the Nabi Musa Mottled Zone complex, Judean Desert

Abstract Chromatite (CaCrO4, tetragonal) is mainly known from Cr6+-contaminated soils associated with chromium ore processing residue. This extremely rare mineral was found at the Nabi Musa locality (Judean Desert, Israel), in a peculiar rock complex of the Mottled Zone. We have explored the possible mechanisms responsible for leaching Cr6+ from natural rocks, by means of field observations, batch experiments, thermodynamic modeling, and mineralogical analyses of the Nabi Musa rocks (XRPD, EMPA, SEM, FTIR, and optical microscopy). A remarkable feature of the Mottled Zone rocks is a broad occurrence of high- and ultrahigh-temperature combustion metamorphic rocks, with Cr3+ accumulated mainly in opaque minerals such as Fe-spinel, brownmillerite, and perovskite. Another feature of the Mottled Zone sequence is abundant Cr3+ (bentorite and volkonskoite) and Cr6+ mineralization (Cr6+-bearing ettringite and baryte-hashemite solid solution) in low-temperature hydrothermal veins. Field, mineralogical, and thermodynamic modeling data suggest that Cr was leached from Cr3+-bearing opaque minerals during late hydrothermal alteration of combustion metamorphic rocks by unusual hyperalkaline waters (pH up to 12). The Cr3+ was then oxidized to Cr6+, and subsequently partially immobilized in Cr6+-bearing ettringite. As a consequence of the highway construction across Nabi Musa hill in 2006, the buried veins filled by Cr6+-substituted ettringite were exhumed and exposed to supergene alteration. The ensuing decomposition of Cr6+-bearing ettringite was followed by Cr6+ release into pore waters in rainy seasons, and then by precipitation of chromatite on the evaporation barrier under the hard desert insolation in dry seasons. The chromatite formation has been due to both unique rock and water chemistry of the Mottled Zone sequence and to the arid climate of the Judean Desert.

Intermediate members of the lime-monteponite solid solutions (Ca1−xCdxO, x = 0.36–0.55): Discovery in natural occurrence

Abstract Lime-monteponite solid solutions [(Ca,Cd)O ss] with 58.5–73.3 wt% CdO were discovered as an accessory phase in medium-temperature combustion metamorphic (CM) spurrite-fluorellestadite/ fluorapatite marbles from central Jordan. The type locality is situated in the northern part of the Siwaqa complex (Tulul Al Hammam area), the largest area of the “Mottled Zone” Formation in the Dead Sea region. The marbles were derived from bitumen-rich calcareous marine sediments of the Muwaqqar Chalk Marl Formation, which have high Cd, Zn, U, and Ni enrichments and contain Cd-rich wurtzite and sphalerite. Oxidative sintering of these sediments at 800–850 °C gave rise to unusual oxide accessories: lime-monteponite solid solutions, Cd-bearing Ca and Zn aluminate—tululite, zincite, and Zn-, Ni-, and Cu-rich periclase. Cadmium incorporation into different oxides was controlled by steric factors, while Cd[6] → Ca[6] was the principal isomorphic substitution. The intermediate members (Ca0.645Cd0.355)O–(Ca0.453Cd0.547)O with a halite-type structure have a cadmium incorporation ratio (KCd = Cdmineral/Cdrock) of about 843 and are the main sites for cadmium in CM marbles. In supergene environments, at low water/rock ratios, (Ca1−xCdx)(OH)2 ss (x ≤ 0.5) constitute the main secondary phase after (Ca,Cd)O ss. At higher water/rock ratios and in the presence of Cl− and F− in the solutions, calcium, and cadmium precipitated as separate phases ﹛fluorite (CaF2) and basic cadmium chloride [Cd(OH)2–xClx)]﹜. A part of cadmium becomes retained in calcium silicate hydrates. The common occurrence of anhydrous (Ca,Cd)O grains in natural rocks, only partly altered to (Ca,Cd)-hydroxide after at least 100 ka exposure to weather and climate, proves that both phases are effective long-term Cd immobilizers.

Testing the snowball Earth hypothesis for the Ediacaran

Ediacaran Siberia was at tropical paleolatitudes when the glacigenic strata of the Goloustnaya Formation (Baikal Group, Siberia) were deposited at sea level. The presence of such deposits (at tropical latitudes) is at the core of the snowball Earth hypothesis, which is generally accepted for the previous Cryogenian glaciations. To test this hypothesis for the Ediacaran Period, we determined concentrations of platinum group elements (PGE) in the transitional unit between glacigenic conglomerates and postglacial cap carbonates of the Goloustnaya Formation. We speculate that if oceans were completely covered by ice during the glaciation, the ice prevented accumulation of PGE-rich cosmic dust and micrometeorites during that period, i.e., the snowball Earth stage. Such particles would have accumulated rapidly on the ocean floor at the ice-melting event, providing a geochemical signal; however, unlike the previous Cryogenian glaciations, this signal is at a background level, and we conclude that either the Ediacaran glaciation did not reach the snowball stage, or it was of very short duration.

Clastic dikes of the Hatrurim basin (western flank of the Dead Sea) as natural analogues of alkaline concretes: Mineralogy, solution chemistry, and durability

This study shows that the mineral assemblages from clastic dikes in areas adjacent to the Dead Sea graben may be considered as natural analogues of alkaline concretes. The main infilling material of the clastic dikes is composed of well-sorted and well-rounded quartz sand. The cement of these hard rocks contains hydroxylapophyllite, tacharanite, calcium silicate hydrates, opal, calcite, and zeolite-like phases, which is indicative of a similarity of the natural cementation processes and industrial alkaline concrete production from quartz sands and industrial alkaline cements. The quartz grains exhibit a variety of reaction textures reflecting the interaction with alkaline solutions (opal and calcium hydrosilicate overgrowths; full replacement with apophyllite or thomsonite + apophyllite). The physicochemical analysis and reconstruction of the chemical composition of peralkaline Ca, Na, and K solutions that formed these assemblages reveal that the solutions evolved toward a more stable composition of zeolite-like phases, which are more resistant to long-term chemical weathering and atmospheric corrosion. The 40Ar/39Ar age of 6.2 ± 0.7 Ma obtained for apophyllite provides conclusive evidence for the high corrosion resistance of the assemblages consisting of apophyllite and zeolite-like phases.

PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia)

The results of the first study of the PGE distribution in deformed lherzolites of the Udachnaya kimberlite pipe (Yakutia) are presented here. The complex character of evolution of the PGE composition in the Deformed lherzolites is assumed to be the result of silicate metasomatism. At the first stage, growth in the amount of clinopyroxene and garnet in the rock is accompanied by a decrease in the concentration of the compatible PGE (Os, Ir). During the final stage, the rock is enriched with incompatible PGE (Pt, Pd) and Re possible due to precipitation of submicron-sized particles of sulfides in the interstitial space of these mantle rocks.