Hatten S. Yoder

Synthetic and natural muscovites

Abstract The syntheses of randomly stacked one-layer monoclinic (IMd), one-layer monoclinic (1M), and twolayer monoclinic (2M) muscovite, have been accomplished. The upper stability limit of muscovite is represented by a curve passing through the points 625°C, 5000 p.s.i. water pressure; 665°C, 15,000 p.s.i.; and 715°C, 30,000 p.s.i. Immediately above this curve sanidine + corundum + vapour are the stable phases. The stability ranges of the muscovite polymorphs named could not be fixed accurately because of the sluggish nature of the transformations; however, the transformation of 1Md → 1M → 2M was effected. It is believed that this sequence obtains in the progressive metamorphism of a sediment. The first transformation is probably dependent on factors affecting reaction rate. The second transformation may be related to a univariant curve of equilibrium (i.e., an isograd). The growth of a three-layer trigonal muscovite (3T) was suspected in only one run with 2M muscovite, and its synthesis is, therefore, not substantiated. Study of natural moscovites and muscovite-like materials indicates that the hitherto unrecognized 1Md and 1M muscovites are common. Their relation to materials called illite, high-silica sericite, hydromuscovite as well as other dioctahedral minerals, is described. The complete destruction of the muscovite structure by grinding is demonstrated by selected area electron diffraction studies. The alleged resynthesis of muscovite by further grinding is shown to be suspect. A comparison of the maximum upper stability curve of muscovite with the minimum melting curve of the “granite” system suggests that muscovite may form in granitic magmas above approximately 1500 atmospheres water pressure, and in the solid state below that pressure in granitic rocks when the water pressure equals the total pressure. These relations may account for the apparent two generations of muscovite in some granites. A limitation on the amount of muscovite in igneous granites is believed to be imposed by the character of the liquidus surfaces. The upper stability curve of muscovite marks the maximum possible conditions of the second sillimanite or orthoclase isograd in metamorphic rocks. The nature of the processes of weathering, abrasion, and diagenesis of muscovite in sediments is outlined.

Effect of Water on the Melting of Enstatite

Melting relations for the MgSiO 3 composition have been studied at pressures between 5 and 30 kb under hydrous conditions. The beginning of melting of enstatite is about 1360°, 1280°, and 1270° C at 10, 20, and 30 kb, respectively. The incongruent melting behavior of enstatite, which disappears at pressures higher than about 5 kb under anhydrous conditions, persists to at least 30 kb water pressure. The melting interval where forsterite, liquid, and vapor co-exist is 30° to 50° C in the pressure range 10 to 30 kb, compared with about 20° at 1 atm. Below the solidus the assemblage enstatite + forsterite + vapor is obtained from the MgSiO 3 composition, because the vapor probably contained silica in excess of that in stoichiometric MgSiO 3 . The maximum content of H 2 O that dissolves in the initial H 2 O-saturated melt, determined by the phase-assemblage method, is about 11 weight percent at 10 kb. It is suggested that silica-saturated magmas such as a quartz tholeiite magma may be generated at considerable depths (at least up to 100 km) under hydrous conditions, either by direct partial melting of peridotites or by fractional crystallization of olivine tholeiitic magmas. Even if the H 2 O content is small, its effect cannot be disregarded.

Abiotic nitrogen reduction on the early Earth

The production of organic precursors to life depends critically onthe form of the reactants. In particular, an environment dominated by N2 is far less efficient in synthesizing nitrogen-bearing organics than a reducing environment rich in ammonia (refs 1, 2). Relatively reducing lithospheric conditions on the early Earth have been presumed to favour the generation of an ammonia-rich atmosphere, but this hypothesis has not been studied experimentally. Here we demonstrate mineral-catalysed reduction of N2, NO2− and NO3− to ammonia at temperatures between 300 and 800 °C and pressures of 0.1–0.4 GPa — conditions typical of crustal and oceanic hydrothermal systems. We also show that only N2 is stable above 800 °C, thus precluding significant atmospheric ammonia formation during hot accretion. We conclude that mineral-catalysed N2 reduction might have provided a significant source of ammonia to the Hadean ocean. These results also suggest that, whereas nitrogen in the Earths early atmosphere was present predominantly as N2, exchange with oceanic, hydrothermally derived ammonia could have provided a significant amount of the atmospheric ammonia necessary to resolve the early-faint-Sun paradox.

Phlogopite synthesis and stability range

Abstract A hydroxyl-bearing phlogopite has been synthesized and its upper stability limit determined up to 75,000 psi water vapour pressure. Above the upper stability limit the stable phases are forsterite + leucite + orthorhombic KAlSiO4 + vapour. At the lowest temperatures investigated the mica grows quickly as randomly stacked one-layer monoclinic phlogopite (1Md). At higher temperatures or with longer runs the mica orders to either a one-layer monoclinic (1M) or a three-layer trigonal (3T) polymorph. It is not possible to distinguish the 1M or the 3T polymorphs by means of powder X-ray diffraction patterns; however, the two-layer monoclinic (2M) mica is easily identified. The small size of the synthetic crystals prohibited identification of their polymorphic form by optical methods or single-crystal studies. The upper stability range of two natural phlogopites is in accord with that of synthetic phlogopite. The transformation of a natural 2M to a 1M or a 3T was accomplished; however, the stability ranges of the natural phlogopite polymorphs could not be fixed because of the sluggish nature of the transformations and the character of the recrystallization. The behaviour of phlogopite in igneous rocks high or low in silica is accounted for by the relation of its stability curve to the minimum melting curves of “granite” and “basalt.” The trioctahedral clay micas are identified structurally as IMd micas. The completely disordered phlogopite produced synthetically and its subsequent ordering well illustrate the observed stages of authigenic growth or, conversely, a weathering of the natural micas.

Geochemical roots of autotrophic carbon fixation: Hydrothermal experiments in the system citric acid, H2O-(′FeS)-(′NiS)

Recent theories have proposed that life arose from primitive hydrothermal environments employ- ing chemical reactions analogous to the reductive citrate cycle (RCC) as the primary pathway for carbon fixation. This chemistry is presumed to have developed as a natural consequence of the intrinsic geochemistry of the young, prebiotic, Earth. There has been no experimental evidence, however, demonstrating that there exists a natural pathway into such a cycle. Toward this end, the results of hydrothermal experiments involving citric acid are used as a method of deducing such a pathway. Homocatalytic reactions observed in the citric acid-H2O experiments encompass many of the reactions found in modern metabolic systems, i.e., hydration- dehydration, retro-Aldol, decarboxylation, hydrogenation, and isomerization reactions. Three principal de- composition pathways operate to degrade citric acid under thermal and aquathermal conditions. It is concluded that the acid catalyzed decarboxylation pathway, leading ultimately to propene and CO2, may provide the most promise for reaction network reversal under natural hydrothermal conditions. Increased pressure is shown to accelerate the principal decarboxylation reactions under strictly hydrothermal conditions. The effect of forcing the pH via the addition of NaOH reveals that the decarboxylation pathway operates even up to intermediate pH levels. The potential for network reversal (the conversion of propene and CO2 up to a tricarboxylic acid) is demonstrated via the Koch (hydrocarboxylation) reaction promoted heterocatalytically with NiS in the presence of a source of CO. Specifically, an olefin (1-nonene) is converted to a monocar- boxylic acid; methacrylic acid is converted to the dicarboxylic acid, methylsuccinic acid; and the dicarboxylic acid, itaconic acid, is converted into the tricarboxylic acid, hydroaconitic acid. A number of interesting sulfur-containing products are also formed that may provide for additional reaction. The intrinsic catalytic qualities of FeS and NiS are also explored in the absence of CO. It was shown that the addition of NiS has a minimal effect in the product distribution, whereas the addition of FeS leads to the formation of hydrogenated and sulfur-containing products (thioethers). These results point to a simple hydrothermal redox pathway for citric acid synthesis that may have provided a geochemical ignition point for the reductive citrate cycle. Copyright

Raman study of some melilites in crystalline and glassy states

Abstract Raman spectra are reported for crystalline akermanite (Ak, Ca2MgSi2O7), hardystonite (Har, Ca2ZnSi2O7), gehlenite [Geh, Ca2Al(AlSi)O7], sodium melilite (SM, CaNaAlSi2O7) and for glasses of corresponding compositions. The spectra of melilites are dominated by the vibrational modes of pyrosilicate (T2O7) units (T = Si or Al) and not by the sheet-like structure formed by interconnected TMO4 tetrahedra (M = Mg, Zn and Al). The frequency of vs(T-O-T), t symmetric stretching mode of the bridging oxygen in the pyrosilicate unit, is directly related to the angle of the T-O-T linkage. The symmetric stretching bands of nonbridging oxygens vs(T-O−) appear in the spectral range characteristic of T-O− stretching in pyrosilicate units. The intensity of the vs(T-O−) band is, however, affected by th presence of A13+ in tetrahedral sites adjacent to the pyrosilicate units. The lowering of the intensities of nonbridging oxygen stretching bands in the spectra of Geh and SM is attributed to a change in the degree of covalency of T-O− bonds in the pyrosilicate unit resulting from substitution of A13+ for Mg2+ in the adjacent tetrahedral sites. Comparison of the spectra of crystals with glasses indicates that most Al3+ ions act as network-forming cations, whereas Mg2+, Zn2+ and Ca2+ retain their role as network modifiers. Spectra of glasses of Ak and Har composition show SiO4−4 and Si2O6−7 bands, indicating a redistribution of silicate species among monomer, dimer, trimer and tetramer chains. Glasses of Geh and SM compositions are more highly polymerized than their respective crystalline counterparts because of the role of Al3+ as network-forming cations in the glass structures.

High pressure and the origin of life

Three lines of experimental research suggest that high pressure may have played a significant role in the origin of life. Discoveries of abundant life in high-pressure environments, including deep oceans, hydrothermal vents, and crustal rocks, point to the adaptation of life to a variety of aqueous habitats. Cultures of microbes at high pressure display both barotolerant and barophilic behaviour. And studies of high-pressure hydrothermal organic synthesis reveal unexpectedly facile reaction mechanisms for the production of a variety of requisite biomolecules.

Inorganic Nitrogen Reduction and Stability under Simulated Hydrothermal Conditions

Availability of reduced nitrogen is considered a prerequisite for the genesis of life from prebiotic precursors. Most atmospheric and oceanic models for the Hadean Earth predict a mildly oxidizing environment that is conducive to the formation and stability of only oxidized forms of nitrogen. A possible environment where reduction of oxidized nitrogen to ammonium has been speculated to occur is aqueous hydrothermal systems. We examined a suite of transition metal oxides and sulfides for their ability to reduce nitrate and nitrite, as well as oxidize ammonia, under hot (300 degrees C) high-pressure (50-500 MPa) aqueous conditions. In general, iron sulfides exhibited the most rapid and complete conversion noted, followed by nickel and copper sulfides to a much lower degree. Of the oxides examined, only magnetite exhibited any ability to reduce NO(3)(-) or NO(2)(-). Ammonium was stable or exhibited small losses (<20%) in contact with all the mineral phases and conditions tested. The results support the idea that hydrothermal systems could have provided significant amounts of reduced nitrogen to their immediate environments. The enhanced availability of reduced nitrogen in hydrothermal systems also has important implications for prebiotic metabolic pathways where nitrogen availability is critical to the production of amino acids and other nitrogenous compounds.

Experimental Studies on Micas: A Synthesis

The principal end members of the micas believed to be common in sediments have been synthesized and some of their stability relations determined. The polymorphs of muscovite and paragonite, the principal dioctahedral end members, obtained were 1Md, 1M and 2M1, and those of phlogopite and annite, the principal trioctahedral end members, 1Md and 1M or 3T. The range of stability of each of the polymorphs could not be fixed accurately because of the slow rate of transformation; however, the transformations 1Md → 1M → 2M1 were effected for muscovite and paragonite and 1M → 1Md or 3T and 2M1 → 1M or 3T for phlogopite. The growth characteristics of these micas in the laboratory are believed to be analogous to the formation of micas in sediments. Knowledge of the synthetic mica.s contributes greatly to an understanding of the natural materials called illite, hydromica, and high-silica sericite. The dioctahedral members of these materials and related minerals may be delineated accurately in the system muscovite—Al-celadonite—pyrophyllite and their iron analogues. The trioctahedral members of some of the same materials may be outlined in the system phlogopite— eastonite—tale and their iron analogues. The postulated substitution schemes in these systems are mainly MgSi ⇌ AlVIAlIV, KAl ⇌ Si, and H3O ⇌ K. In materials intermediate between these systems, such as most biotites and vormiculites, the substitution of 3Mg ⇌ 2AlVI is of major importance. The mixed-layer structures involving micas are elucidated.

Petrologic implications of plate tectonics.

Petrologists can make significant contributions to the plate tectonic concept. Fixing the stability fields of the principal rock types involved will provide the limits of pressure and temperature of the various environments. Experimental determination of the partition coefficients of the trace elements will be helpful. Studies of the partial melting behavior of possible parental materials in the absence and presence of water, especially the undersaturated region, will contribute to the understanding of magma production. Experimental observations on the rheological properties of the peridotites below and just above the solidus will lead to a better evaluation of the convective mechanism. Measurement of the fundamental properties of rocks, such as the density of solids and liquids at high pressures and temperatures, would contribute to understanding the concepts of diapiric rise, magma segregation, and the low-velocity zone. Broader rock sampling of the oceanic areas of all environments will do much to define the petrologic provinces. The field petrologist specializing in the Paleozoic regions and Precambrian shields can contribute by examining those regions for old plate boundaries and devising new criteria for their recognition.