Introduction

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Of all natural environments where chemical reactions occur that produce organic molecules, the dense hard matrix of igneous minerals may appear as the most unlikely place. Yet, our earlier research has shown that a suite of medium- to long-chain fatty acids, C₆---C₁₂, can be identified among the organics extracted from crushed olivine single crystals from the CO/CO₂/H₂O-laden high temperature, high pressure environment of the upper mantle (1). Freshly crushed olivine single crystals, when heated in vacuum, were found to release a range of organic molecules, including aromatic compounds (2). Solvent extraction of crushed MgO crystals, grown in the laboratory at 1 bar from a CO/CO₂/H₂O-saturated MgO melt (3), produced short-chain carboxylic and dicarboxylic acids up to C₄ (4).

When minerals grow either in the laboratory or in nature, their environments are always "contaminated" and often saturated with CO₂ and H₂O. The presence of CO₂ and H₂O introduces the low-z elements carbon and hydrogen as "impurities" into the mineral matrix. As will be shown in this report, solute C and H₂ participate in reactions that lead to the precipitation of protomolecular C_x entities and formation of C---H bonds inside the hard, dense mineral matrix. These solid state reactions are different from the reactions that lead to the synthesis of lipids under hydrothermal conditions by Fischer-Tropsch-type reactions (5) or to the reduction of CO₂ during serpentinization of olivine and the production of organics with the help of catalysts such as magnetite (6) or to any other abiogenic reaction that has been considered for the early Earth (7-9).

	Dissolution of H2O and CO2 in Mineral Matrices

Top Abstract Introduction Dissolution of H2O and... Experimental Procedures Results Discussion Conclusions References

H₂O becomes incorporated into the matrix of minerals that crystallize in H₂O-laden environments, even of minerals that are nominally anhydrous. The basic reaction controlling the uptake of "impurity" H₂O can be described as a proton transfer from H₂O onto an O² of the host oxide/silicate matrix:

[ 1 ]

A large body of literature exists about OH in nominally anhydrous minerals from various geological settings (10, 11). The most widely used method of analysis is IR spectroscopy. If transparent crystals are available, IR can detect very small amounts of OH by way of their O---H stretching bands, _OH, in the range of 3000-3700 cm¹. Because the _OH bands lie at relatively high wavenumbers and are decoupled from the lower frequency lattice modes, they are generally sharp and unambiguously identifiable.

The amounts of OH in olivine (Mg,Fe)₂SiO₄, the dominant upper mantle mineral, range from 10-1000 H/10⁶ Si (ppm) (11). Similar but generally low OH concentrations have been found in other petrologically important minerals (10). Under the assumption that Eq. 1 completely describes the dissolution reaction of H₂O, the OH concentrations determined by IR have been used to calculate the total amount of chemically bound "water."

CO₂ is the dominant gas in many volcanoes and the dominant gas/fluid component in the magmas that feed them. The important role that CO₂ plays in the petrogenesis of igneous rocks has also long been recognized. At high pressures, it can lower the melting points of mineral assemblies, i.e., the temperatures at which partial melts form, by hundreds of degrees (12). Whereas CO₂ is known to dissolve in quenched high-pressure silicate melts, maybe in form of carbonate anions, CO (13, 14), some researchers maintain that C or CO₂ will not enter as "impurity" into the solid matrix of minerals (15, 16). The formation of a carbonate anion can be described as the merging of a CO₂ molecule with a lattice O² without changing the oxidation state of the carbon:

[ 2 ]

At closer inspection, dissolution of H₂O and CO₂ in solid matrix turns out to be more complicated than Eqs. 1 and 2 might suggest. This finding is exemplified by Fig. 1, which displays the IR spectrum of a nominally high purity MgO single crystal,^¶ grown from an MgO melt equilibrated with an atmosphere of CO/CO₂ plus H₂O and N₂ at 1 bar pressure (3). In the center, a group of _OH bands is seen that consists of (i) a relatively strong band or group of bands around 3300 cm¹, (ii) a weaker band at 3560 cm¹, and (iii) a very broad band extending from below 3000 cm¹ to 3700 cm¹. Because MgO crystallizes in the simple, face-centered cubic rocksalt structure, the _OH bands i-iii have been assigned to single OH adjacent to an Mg²⁺ vacancy site, to OH pairs adjacent to an Mg²⁺ vacancy site, and to interstitial OH, i.e., H⁺ associated with O² at regular O² sites, respectively (17).

View larger version (29K): [in this window] [in a new window]   Fig. 1.   Infrared spectrum of a nominally high purity MgO crystal, grown from a CO/CO2/H2O-laden melt, in the 2800-4200 cm1 range covering the H---H, O---H, and C---H stretching frequencies.

Fig. 1 also shows a weak but distinct band on the left, at 4152 cm¹ (enlarged in the Inset). It has the characteristic signature of a _HH band arising from the H---H stretching mode of lattice-bound H₂ molecules similar to the _HH band of H₂ in noble gas matrices (18). Because the _HH band is intrinsically weak, the fact that it can be observed in the MgO crystal under study suggests a high concentration of H₂ molecules.

This _HH band and the _CH bands on the right of Fig. 1, between 2800-3000 cm¹, jointly point at the complexity of the solid state dissolution of H₂O and CO₂. The _CH bands suggest that some form of C---H entities exist in the MgO crystal, providing an "organic" signature. These C---H entities do not come from surface contamination as has been suggested (16, 19, 20) but are associated with C in the bulk that remains detectable even on heating in ultrahigh vacuum up to 700-900°C (21-23).

The main focus of this study will be to address the nature this "organic" signature and how the presence of C---H entities is linked to the dissolution mechanism of H₂O and CO₂ in mineral matrices.

	Experimental Procedures

Top Abstract Introduction Dissolution of H2O and... Experimental Procedures Results Discussion Conclusions References

We chose MgO crystals for the basic study because MgO crystallizes in the simple, face-centered cubic rocksalt-type (NaCl) structure, consisting of a close packing of O² anions with the Mg²⁺ cations occupying all available octahedral sites. A further advantage of MgO is that large single crystals can be grown from the melt in high purity grades. The structure of olivine (Mg,Fe)₂SiO₄, although orthorhombic, is similarly dense, deriving from a hexagonal close packing of O² anions with Mg²⁺ and Fe²⁺ cations in two differently distorted octahedral sites and Si⁴⁺ in tetrahedral sites. Both MgO and olivine tend not to develop internal cleavage planes as some other minerals do, in particular, pyroxenes.

The MgO crystals used in this study were grown at 1 bar pressure from a CO/CO₂/H₂O-saturated melt (3). Nominally, i.e., with respect to metal impurities, these MgO crystals were of 99.9% purity, colorless, with some turbidity because of micrometer- and sub-micrometer-sized cavities that decorate a dense network of subgrain boundaries and dislocations. They were available in form of large cubes, 20-30 mm in size, reflecting the perfect cleavage of MgO along (100). The olivine single crystals used in this study came from Afghanistan. They were recovered from peridotite nodules brought up by volcanic eruptions (24). The crystals were 20-30 mm in size, irregular in shape, olive-green and partly turbid because of a decorated network of subgrain boundaries and dislocations. A selected large olivine crystal was cut with a low-speed diamond saw to a rectangular shape of about 20 × 10 × 6 mm. This olivine crystal and a similarly sized MgO crystal were cut into several identical pieces, about 5 × 10 × 6 mm, so that the study to be described below could be done with pieces of the same single crystals. The cut surfaces were left "as is", i.e., without further grinding or polishing.

The single crystal pieces were cleaned with organic solvents. They were mounted in Al blocks that fit into the sample holder of the Nicolet Nexus 670 Fourier transform (FT)-IR spectrometer. The MgO and olivine crystals were heated for 12 h to 400°C and 45 min to 300°C in a stream of high purity N₂ gas, respectively. Previously it had been shown by gas chromatographic techniques (1, 4) that, after drying, no solvents are retained, even on finely crushed single crystal powders. By measuring the _CH intensities from a thick MgO crystal and then cutting the same crystal into several slices, it had been shown earlier (17) that the _CH band strength correlates with the length of the optical path through the bulk, not with the number of surfaces, indicating that the signal came from C---H entities in the bulk. All IR spectra (before and after heating) were recorded at 30°C, acquiring data during 20 scans over the range 400-4000 cm¹.

The heat treatment pyrolyzed their C---H entities in the MgO and olivine crystals, causing their _CH bands to disappear or nearly disappear as determined from the IR spectra recorded immediately after cooling to room temperature. For the next 32 days, sets of these MgO and olivine crystals (wrapped in Al foil) were stored in air at 70°C, whereas one control set was stored at 24°C. A run started at 45°C was lost because of a malfunction of the temperature controller. The IR spectra of each sample were recorded, first in daily intervals, later in weekly intervals. In the case of MgO, the background in the _CH region was fitted linearly between 2785 cm¹ and 3025 cm¹. In the case of olivine, a best-fit polynomial was used to compensate for the more steeply sloping background.

Dislocations in MgO were modeled by using the CRYSTALMAKER 4.0 program by David C. Palmer (Crystalmaker Software, Bicester, U.K.) modified in such a way as to allow the introduction of defects into the perfect structure. No lattice relaxation around the dislocation cores was taken into account.

	Results

Top Abstract Introduction Dissolution of H2O and... Experimental Procedures Results Discussion Conclusions References

Fig. 2 shows the _CH bands in the "as received" MgO and olivine crystals, i.e., before heating. In both cases, the bands are similar in number and with respect to their position and relative intensities. The two strongest _CH bands lie at 2926 and 2855 cm¹ in MgO and at 2922 and 2852 cm¹ in olivine. Minor bands occur at 2955 and 2870 and a shoulder at 2895 cm¹. MgO exhibits an additional weak band at 3008 cm¹, which the spectrum of olivine does not show. In MgO, all _CH bands are slightly broader than in olivine.

View larger version (47K): [in this window] [in a new window]   Fig. 2.   C---H stretching bands, CH, in the synthetic MgO and an upper mantle derived olivine single crystal recorded before heating. The strongest CH bands appear to arise from symmetrical and asymmetrical C---H stretching modes of ---CH2-entities with minor CH bands probably because of ---CH3. The CH bands of MgO are broader than those of olivine.

The different _CH bands may arise from C---H entities in different local environments or from C_x entities in which some C atoms are bonded to two or more H, thereby giving rise to a set of symmetrical and asymmetrical C---H stretching modes. Indeed, the two strongest bands at 2926 and 2855 cm¹ in MgO and at 2922 and 2852 cm¹ in olivine agree with the symmetrical and asymmetrical C---H stretching modes of ---CH₂ units in aliphatic hydrocarbon chains such as in polyethylene (25). The weak band at 2955 cm¹ and a companion at 2870 cm¹ agree with the symmetrical and asymmetrical C---H stretching modes of ---CH₃ units.

The weakness of the _CH signature does not necessarily mean that the amount of solute C_x entities is small. The strength of the _CH bands solely depends on the number of C---H bonds formed, not on the number of C atoms in the C_x entities. The total C concentration in the laboratory-grown MgO single crystals is probably of the order of 50-100 ppm (21, 26). Similar total C concentrations have been reported for upper mantle-derived olivine crystals (27, 28), and at least 5 ppm C₂---C₆ hydrocarbons.

Heating the MgO crystal to 400°C and the olivine crystal to 350°C caused their _CH bands to nearly completely disappear, because of in situ pyrolysis of the C---H bonds. Fig. 3 shows how the _CH bands reappear in the MgO crystal during annealing at 70°C. The band positions are slightly shifted. At the end of 32 days at 70°C the integral intensity of the _CH bands reached about 10% of the initial intensity. Annealing at 90°C and 24°C caused the _CH bands to reappear faster and slower, respectively. The new _CH bands lie at nearly the same positions as the ones observed before heating. This result suggests that, whereas heating pyrolyzed the C---H bonds, it left the C_x entities intact to which the H atoms had bonded.

View larger version (41K): [in this window] [in a new window]   Fig. 3.   After heating for 14 h to 400°C, the CH bands in MgO vanish nearly completely but reappear during annealing at 70°C over a period of 32 days.

Fig. 4 plots the intensity of the _CH bands in MgO as a function of annealing time at 70°C (solid circles), at 24°C, and 45°C (open circles and solid diamonds), and 90°C after the temperature controller malfunctioned, raising the temperature of the 45°C run to 90°C (open diamonds). The solid and broken lines represent parabolic fits to the data. Obviously, the pyrolysis of the C---H bonds had caused H to disperse in the MgO matrix adjacent to the sites of the C_x entities. We do not know, however, whether hydrogen remains as H after pyrolysis or forms H₂. On annealing, H or H₂ diffuse back to the C_x chains, forming C---H bonds. If this process is controlled by 1-dimensional diffusion, the _CH intensity should increase linearly with the square root of time. Indeed, the parabolic fit to the 70°C data describes rather well the overall increase in the integral intensity of the _CH bands. During the first 32 days at 70°C the _CH bands regain about 10% of their original intensity. Assuming the same diffusion rate, 50% of the original intensity would be reached after 4500 days or 12.5 yr.

View larger version (31K): [in this window] [in a new window]   Fig. 4.   Integrated intensity of the CH bands in the MgO crystal annealed at 70°C and other temperatures, plotted as a function of time. The symbols represent measured data, the lines parabolic fits.

The _CH bands of olivine disappear or nearly disappear on heating to 300°C for 45 min. They reappear on annealing, shifted by about 7 cm¹ to lower wavenumbers, with a similar time constant as in MgO or slightly faster. After 32 days at 70°C the _CH bands of olivine had regained about 15% of their original intensity.

	Discussion

Top Abstract Introduction Dissolution of H2O and... Experimental Procedures Results Discussion Conclusions References

The IR observations presented here can be summarized as follows: (i) the _CH bands in the 2800-3050 cm¹ window arise from C---H entities in the crystal matrix; (ii) nearly identical _CH bands are seen in the IR spectrum of laboratory-grown MgO and natural olivine crystals from the H₂O/CO₂-laden high pressure environment of the upper mantle; (iii) the complexity of the CH bands suggests polyatomic C_x entities with ---CH₂--- and ---CH₃ units; (iv) the _CH bands disappear on heating because of the pyrolysis of the C---H bonds; and (v) the _CH bands reappear relatively rapidly, within a few days and weeks, on annealing at moderate temperatures between room temperature and 70°C.

The presence of the _CH bands, their disappearance on heating, and their reappearance on annealing jointly point at a sequence of physical and chemical processes that occur in solid matrix. The _CH bands are consistent with C_x entities containing ---CH₂--- and ---CH₃. These C_x entities may represent "protomolecules" of those carboxylic, dicarboxylic, and fatty acids that have been extracted from MgO and olivine crystals (1, 4). To understand how such protomolecules form, we review earlier work on the dissolution mechanism of H₂O and CO₂ that has laid the foundation for the study presented here.

Redox Conversion of Solute H₂O and CO₂. Classically, MgO crystallizing in the presence of H₂O and CO₂ can be treated as a three-component system with Mg(OH)₂, MgCO₃, and Mg-hydroxy-carbonates as distinct compounds (29). This result suggests that, even if consideration is given to the possibility that H₂O and CO₂ may enter into solid solution, the only oxyanions will be OH and CO as in Eqs. 1 and 2. However, the presence of H₂ molecules and of C---H entities suggests that the uptake of H₂O and CO₂ into solid solution leads to reactions that change the oxidation state of some of the solutes, producing H₂ and reduced C.

2

2

[ 3 ]

1

1

2

[ 4 ]

[ 5 ]

2

Segregation. Dissolution of H₂O and CO₂ in mineral matrices takes place during crystallization when the gas/fluid components partition between the melt and the growing crystals. The amount of H₂O and CO₂ taken up into solid solution depends on the temperature (T) and partial pressures of the gas/fluid components (12). However, the equilibrium concentrations of the solute H₂O and CO₂ species decreases with decreasing T. This sets up a thermodynamic driving force to segregate the solutes to the surface or to any other sink that might be available inside the bulk (33, 34). The denser the crystal structure, the larger is the driving force. As long as the diffusion of the major cationic and anionic lattice constituents remains activated, i.e., at high T, segregation will simply lead to degassing of H₂O and CO₂. At lower T, as diffusion of cations and anions freezes, only those solutes can respond to the thermodynamic driving force that remain diffusively mobile. If some solutes that derive from H₂O and CO₂ retain diffusive mobility, they will segregate to the surface as well as to dislocations and other defects.

2

2

2

2

2

View larger version (66K): [in this window] [in a new window]   Fig. 5.   (A) An ao/2[100] screw dislocation in MgO showing how the (100) plane is displaced by ao/2 after one turn. The large dark spheres indicate O2 anions, the smaller light spheres Mg2+ cations. (B) Projection of two idealized edge dislocations generated as part of a subgrain boundary in MgO by the insertion of two half-planes of Mg2+ and O2. The arrows point at rows of Mg2+ cations that are under high compressive stress and therefore energetically unfavorable (after Harding et al., ref. 38).

Modeling Dislocations. As part of the work described here, we modeled screw and edge dislocations in MgO and their interaction with CO by decomposing the process into three steps: first, we remove one row of Mg²⁺ cations; second, we convert the two adjacent rows of O² to O, thus providing full charge compensation for the Mg²⁺ vacancies; and third, we allow C to segregate into the dislocation cores and to bond to O.

View larger version (62K): [in this window] [in a new window]   Fig. 6.   (A) Stack of MgO (100) planes with a vertical ao/2[100] screw dislocation in the center, viewed at right angle. The screw dislocation is decorated by one [O2C---C---C---CO2] unit in the energetically favorable trans configuration with the medium size light gray spheres representing O. (B) Oblique view of an edge dislocation in MgO with one row Mg2+ removed and two rows of O2 changed to O (lighter gray spheres). (C) Same edge dislocation with C4 units segregated into its core. Because of mismatch with the surrounding MgO matrix, the [O2C---C---C---CO2] units would have to adopt the energetically less favorable cis configuration.

Stereochemical Control. Dislocations provide for a stereochemically constrained environment that forces the segregating C atoms into C_x chains. Depending on the specific conditions and/or the mineral structure, different chain length C_x entities might form. In other types of lattice defects, solute C atoms may segregate to form cyclic or branched C_x entities. We are here confronted with the possibility that the mineral matrices in which C_x entities precipitate influence or even control the shape and size of these C_x segregates. Hence, when these minerals weather, they will release a set of stereochemically preselected organic molecules. Such a mechanism is expected to produce a smaller number of different compounds than reactions in the gas phase, liquid phase, or by surface catalysis.

1

Prebiotic Evolution. Many pathways are known by which organic matter can be synthesized through gas phase, liquid phase, and gas-solid and liquid-solid reactions. How much could have been produced under early Earth conditions depends on whether or not the atmosphere at that time was reducing. Table 1 gives estimated production rates (8) for two cases: (i) a highly reducing atmosphere rich in methane, hydrogen, and ammonia, and (ii) an intermediate atmosphere, still reducing, with an H₂/CO₂ ratio of 1:10. More likely, however (39), the early atmosphere was non-reducing. In this case almost no organics would have been produced in the atmosphere by the processes indicated.

1

1

1

	Conclusions

Top Abstract Introduction Dissolution of H2O and... Experimental Procedures Results Discussion Conclusions References

The assembly of protomolecules by way of solid state processes shows that "organic" chemistry can take place in the dense, hard matrix of igneous minerals. This finding opens new aspects to the study of stereochemically constrained complex organic molecules, synthesized under prebiotic conditions. Such processes may have provided large amounts of biochemically relevant organics to the early Earth.