The snowpack is a complex photochemical reactor that emits a wide variety of reactive molecules to the atmosphere. In particular, the photolysis of nitrate ions, NO3 −, produces NO, NO2, and HONO, ...which affects the oxidative capacity of the atmosphere. We report measurements in the European High Arctic where we observed for the first time emissions of NO, NO2, and HONO by the seasonal snowpack in winter, in the complete or near-complete absence of sunlight and in the absence of melting. We also detected unusually high concentrations of nitrite ions, NO2 −, in the snow. These results suggest that microbial activity in the snowpack is responsible for the observed emissions. Isotopic analysis of NO2 − and NO3 − in the snow confirm that these ions, at least in part, do not have an atmospheric origin and are most likely produced by the microbial oxidation of NH4 + coming from clay minerals into NO2 − and NO3 −. These metabolic pathways also produce NO. Subsequent dark abiotic reactions lead to NO2 and HONO production. The snow cover is therefore not only an active photochemical reactor but also a biogeochemical reactor active in the cycling of nitrogen and it can affect atmospheric composition all year round.
A single-crystal X-ray diffraction study in a diamond anvil cell up to 5.41 GPa was carried out on a clinochlore monoclinic polytype IIb-2, S.G. C2/m, (Mg9.09Fe2+1.01Mn0.02Ti0.01Cr0.02Al1.80)Σ = ...11.95(Si6.35Al1.65)Σ = 8O20(OH)16 from Val Malenco, Italy. The bulk modulus of monoclinic clinochlore calculated by fitting unit-cell volumes and pressures to a third-order Birch-Murnaghan Equation of State (EoS), is K0 = 71(9) GPa with K' = 8(5). Axial compressibility values were βEoS0a = 3.8(1), βEoS0b = 3.6(1), and βEoS0c = 5.4(5) 10-3 GPa-1, showing that axial anisotropy is much less than that found for other phyllosilicates. Compressibility data are in fair agreement with literature data, which are based on powder neutron and synchrotron diffraction methods. Results were compared with the behavior of the triclinic polytype of similar composition and coexisting in the same rock. Symmetry has little overall influence on compressibility, but compared with the triclinic polytype of similar composition and coexisting in the same hand specimen, the monoclinic polytype is slightly less rigid. Comparison of structural refinements at different pressures showed that structural deformations mainly affect the interlayer region, where hydrogen bonds are important for the structural properties of the phase. The mean decrease in OH-O distances was about 9% in the pressure range 0-5 GPa. Structural behavior was very similar to that found for the triclinic polytype. Although energy differences between polytypes are relatively small, their compressional behavior may have implications in terms of relative stability. A computation of molar volume applying an isothermal EoS shows that the triclinic polytype is lower in volume up to 0.9 GPa, above which the volume of the monoclinic phase is smaller. This fact gives information on the relative stability of the two polytypes and a possible explanation for the greater abundance of the triclinic polytype in low to medium-P environments, as is commonly observed in nature.
The structural behavior of phlogopite was studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals of composition (K0.91 Na0.02 Ba0.03) (Fe2+0.65 Fe3+0.163 ...Al0.123 Mg1.81 Ti0.149)Si2.708 Al1.292 O10 OH1.725 F0.175. Lattice parameters were measured from 0.0001 to 6.5 GPa and fitted with a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V0 = 497.1(1) Å3, K0 = 54(2) GPa-1, and K' = 7(1); a0 = 5.336(1) Å, K0 = 123(9) GPa, and K' = 3(2); b0 = 9.240(3) Å, K0 = 128(15) GPa, and K' = 3(2); c0 = 10.237(6) Å, K0 = 25(2) GPa, and K' = 5(1); the β angle increases linearly with pressure from 100.02(5) to 100.4(1)°. The structural evolution of phlogopite was studied by comparing five structural refinements performed with intensity data collected at 0.0001, 1.2, 3.2, 5.0, and 6.0 GPa. The interlayer site, where K is located, is about 4.5 times more compressible than the T-O-T unit. At the same time, the greater compression of the octahedral layer with respect to the tetrahedral one induces an increase in tetrahedral rotation angle α from 9.29 to 11.9°. Structural evolution with pressure yields a crystallographic rationale for the larger baric stability of K-deficient and Si-rich phlogopites, as observed from natural and experimental data. Combined high-pressure and thermal expansion data yield the approximate EoS: V = V0(1+4.6×10-5 ΔT-0.0167 ΔP), where P is in GPa and T in °C. This equation was used to calculate phase equilibria in lherzolite compositions modeled in the simplified K2O-CaO-MgO-Al2O3-SiO2-H2O (KCMASH) system. The results show that the effect of phlogopite on bulk density is particularly significant at high pressure and temperature since, above the chlorite stability field, phlogopite is the only hydrous phase able to lower bulk density properties by about 0.5% with respect to the K-free bearing system.
We present the results of a single-crystal X-ray diffraction structural study of chlorite in a diamond anvil cell up to 5.47 GPa. The sample is a clinochlore from Val Malenco, Italy, triclinic ...polytype IIb-4, S.G. CFormula: see text, with pseudomonoclinic metric and composition (Mg9.14Fe2+1.02Fe3+0.01 Mn0.01Ti0.01Al1.76)Σ = 11.95(Si6.32Al1.68)Σ = 8O20(OH) 16. Structural refinements were performed at several pressures with intensity data collected on a CCD diffractometer. Unit-cell parameters were accurately measured with the point-detector mounted on the same instrument. The bulk modulus of chlorite fitting data to a third-order Birch-Murnaghan equation of state is K0 = 88(5) GPa with K' = 5(3). Results are in fair agreement with data based on powder neutron and synchrotron diffraction methods. The axial compressibility values were β0aEoS = 3.4(2), β0bEoS = 3.4(1), and β0cEoS = 5.4(2) 10-3 GPa-1. The metric of the lattice remains triclinic in the investigated pressure range. Axial anisotropy is strongly reduced with respect to the axial compressibilities observed in other phyllosilicates. Comparison of structural refinements at different pressures shows that the main structural deformations affect the interlayer region, where the hydrogen bonds are relevant to the structural properties of the phase. The mean decrease of the OH-O distances is about 10% from ambient pressure to approximately 5 GPa. Compressibility data may be combined with those on thermal expansion to formulate an equation of state for clinochlore. Taking into account the thermal expansion coefficient reported in literature for a chlorite with a composition quite similar to that of our sample, we can write the equation: V = V0 (1-1.14 10-2 ΔP + 2.316 10-5 ΔT), where P is in GPa and T in Celsius. Assuming an average rock density of 2.7 g/cm3, this corresponds to an isochoric P-T geothermal gradient of 18°C/km.
Using single-crystal X-ray diffraction from a diamond anvil cell, the compressibility of a synthetic fluorapatite was determined up to about 7 GPa. The compression pattern was anisotropic, with ...greater change along a than c. Unit cell parameters varied linearly with βa=3.32(8) 10−3 and βc=2.40(5) 10−3 GPa−1, giving a ratio βa:βc=1.38:1. Data fitted with a third-order Birch-Murnaghan EOS yielded a bulk modulus of K0=93(4) GPa with K′=5.8(1.8). The evolution of the crystal structure of fluorapatite was analysed using data collected at room pressure, at 3.04 and 4.72 GPa. The bulk modulus of phosphate tetrahedron is about three times greater than the bulk modulus of calcium polyhedra. The values were 270(10), 100(4) and 86(3) GPa for P, Ca1 (nine-coordinated) and Ca2 (seven-coordinated) respectively. While the calcium polyhedra became more regular with pressure, the distortion of the phosphate tetrahedron remained unchanged. The size of the channel extending along the 001 direction represented the most compressible direction. The Ca2–Ca2 distance decreased from 3.982 to 3.897 Å on compression from 0.0001 to 4.72 GPa. The anisotropic compressional pattern may be understood in terms of the greater compressibility of the channel size over the polyhedral units. The reduction of the channel volume was measured by the evolution of the trigonal prism, having the Ca2–Ca2–Ca2 triangle as its base and the c lattice parameter as its height. This prism volume changed from 47.3 Å3 at room pressure to 44.78 Å3 at 4.72 GPa. Its relatively high bulk moduli, 86(3) GPa, indicated that the channel did not collapse with pressure and the apatite structure could remain stable at very high pressure.
Measurements of atmospheric concentrations and fluxes of reactive nitrogen (NO, NO2, HNO3, NO3 (-) (fine) and NO3 (-) (coarse)) above the snow surface were performed from 29 March to 30 April 2010 at ...Ny-lesund, Svalbard. Determinations of chemical and physical properties of snow were also carried out. Both NO and NO2 showed clear diurnal cycles with noontime maxima and nighttime minima. Significant emission fluxes of NO and NO2 were observed, reaching noontime values up to 19.42 and 25.20 pmol/m(2) s, respectively. The snow surface was the source of NO and NO2 but these observed releases were small due to almost alkaline snow environment and chemical forms of snow NO3 (-). Significant deposition fluxes of HNO3, fine and coarse particulate NO3 (-) fluxes were also observed, reaching peak values up to -18.00, -37.80 and -12.50 pmol/m(2) s, respectively, during snowfall events. Measurements of surface snow provided experimental data that the total contribution of dry deposition of these species to the NO3 (-) -N in the snow was about 24 %. However, wet deposition in falling snow seemed to be the major contribution to the nitrate input to the snow.
Nitrogen oxides play a major role in determining the chemical properties of the atmosphere. Measurements of these species in remote areas are rare, although their relevance is well established. This ...is particularly true for polar sites. In order to fill this gap, an annular diffusion denuder system, with an alkaline carbon surface, for the simultaneous collection of nitrogen dioxide (NO
2) and peroxyacetyl nitrate and other organic nitrates has been developed. In the collection stage, nitrogen dioxide (NO
2) and NO
y
yield nitrite (NO
2
−) and nitrate (NO
3
−) ions, respectively. These are extracted after sampling and analysed by ion chromatography. The experimental set-up was tested during two expeditions carried out in sites located in Arctic and Antarctica. The Arctic concentrations were found between 10 and 170 ng
m
−3 for NO
2 and between 100 and 600 ng
m
−3 for NO
y
while in Antarctica concentrations were found between 10 and 300 ng
m
−3 for NO
2 and between 300 and 700 ng
m
−3 for NO
y
.
Results obtained from these expeditions demonstrate that the minor components may be measured at levels as low as a few nanograms per cubic meter in remote atmospheres using the suggested technique. The reported concentrations are to be considered among the first observations of gas phase nitrogen compounds in polar areas.
The crystal structure of a synthetic Rb analog of tetra-ferri-annite (Rb–TFA) 1M with the composition Rb0.99Fe2+3.03(Fe3+1.04 Si2.96)O10.0(OH)2.0 was determined by the single-crystal X-ray ...diffraction method. The structure is homooctahedral (space group C2/m) with M1 and M2 occupied by divalent iron. Its unit cell is larger than that of the common potassium trioctahedral mica, and similar lateral dimensions of the tetrahedral and octahedral sheets allow a small tetrahedral rotation angle α=2.23(6)°. Structure refinements at 0.0001, 1.76, 2.81, 4.75, and 7.2 GPa indicate that in some respects the Rb–TFA behaves like all other micas when pressure increases: the octahedra are more compressible than the tetrahedra and the interlayer is four times more compressible than the 2:1 layer. However, there is a peculiar behavior of the tetrahedral rotation angle α: at lower pressures (0.0001, 1.76, 2.81 GPa), it has positive values that increase with pressure from 2.23(6)° to 6.3(4)° as in other micas, but negative values −7.5(5)° and −8.5(9)° appear at higher pressures, 4.75 and 7.2 GPa, respectively. This structural evidence, together with electrostatic energy calculations, shows that Rb–TFA has a Franzini A-type 2:1 layer up to at least 2.81 GPa that at higher pressure yields to a Franzini B-type layer, as shown by the refinements at 4.75 and 7.2 GPa. The inversion of the α angle is interpreted as a consequence of an isosymmetric displacive phase transition from A-type to B-type structure between 2.81 and 4.75 GPa. The compressibility of the Rb–TFA was also investigated by single-crystal X-ray diffraction up to a maximum pressure of 10 GPa. The lattice parameters reveal a sharp discontinuity between 3.36 and 3.84 GPa, which was associated with the phase transition from Franzini-A to Franzini-B structure.
The structural behavior of phlogopite was studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals of composition (K
Na
Ba
) (Fe
Fe
Al
Mg
Ti
)Si
Al
OH
. ...Lattice parameters were measured from 0.0001 to 6.5 GPa and fitted with a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V
= 497.1(1) Å
, K
= 54(2) GPa
, and K' = 7(1); a
= 5.336(1) Å, K
= 123(9) GPa, and K' = 3(2); b
= 9.240(3) Å, K
= 128(15) GPa, and K' = 3(2); c
= 10.237(6) Å, K
= 25(2) GPa, and K' = 5(1); the β angle increases linearly with pressure from 100.02(5) to 100.4(1)°.
The structural evolution of phlogopite was studied by comparing five structural refinements performed with intensity data collected at 0.0001, 1.2, 3.2, 5.0, and 6.0 GPa. The interlayer site, where K is located, is about 4.5 times more compressible than the T-O-T unit. At the same time, the greater compression of the octahedral layer with respect to the tetrahedral one induces an increase in tetrahedral rotation angle α from 9.29 to 11.9°.
Structural evolution with pressure yields a crystallographic rationale for the larger baric stability of K-deficient and Si-rich phlogopites, as observed from natural and experimental data.
Combined high-pressure and thermal expansion data yield the approximate EoS: V = V
(1 + 4.6 × 10-5 ΔT - 0.0167 ΔP), where P is in GPa and T in °C. This equation was used to calculate phase equilibria in lherzolite compositions modeled in the simplified K
O-CaO-MgO-Al
-SiO
-H
O (KCMASH) system. The results show that the effect of phlogopite on bulk density is particularly significant at high pressure and temperature since, above the chlorite stability field, phlogopite is the only hydrous phase able to lower bulk density properties by about 0.5% with respect to the K-free bearing system.
The response of staurolite to pressure was studied by single crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals with composition: (Fe3.365Zn0.025Li0.114Co0.009Mn0.034) T2, M4 ...(Al2Mg0.307)M3 (Al15.491Fe3+0.104Mg0.394Cr0.004Ti0.07) M1, M2 (Si7.534Al0.466)T1 O48H3. Lattice parameters, measured at various pressure up to 7.264(6) GPa, were fitted using a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V0 = 740.85(7) Å3, K0 = 180(2) GPa and K' = 4.7(6), a0 = 7.8723 (2) Å, K0 = 189(2) GPa, and K'a = 4.1 (6), b0 = 16.62453(1) Å, K0 = 179(2) GPa, K'b = 6.1(6) and c0 = 5.6604 (4) Å, K0 = 179(5) GPa, K'c = 2(1); whereas the angle β remained almost constant with increasing pressure. These data suggest an almost isotropic compressibility. Structural evolution was studied by comparison of structural refinements carried out with data collected at 0.0001, 2.48 4.15, 5.43, 6.84, and 8.74 GPa. All refinements were made in the Ccmm space group. Polyhedral evolution with P is a function of occupancy: whereas the T1 tetrahedron and the M1 and M2 octahedra, occupied by Si and Al, are practically incompressible, the T2 tetrahedron, and the M4 and M3 octahedra, only partially occupied principally by Fe (the first two) and by Al (the last), show larger changes as a function of pressure. As a consequence, the two kyanite and Fe-Al hydroxide layers, which can be used to describe the staurolite structure, have different compressibilities.
