Axel Liebscher

Crystal chemistry of synthetic Ca2Al3Si3O12OH–Sr2Al3Si3O12OH solid-solution series of zoisite and clinozoisite

Abstract Coexisting solid-solution series of synthetic zoisite-(Sr) and clinozoisite-(Sr) were synthesized in a 1 M (Ca,Sr)Cl2 solution at 2.0 GPa, 600 °C for 6 days in a piston cylinder press. Solid solutions were synthesized from XSrZo = Sr/(Ca + Sr) = 0.06 to 1 and XSrCzo = 0.08 to 0.5 in zoisite and clinozoisite, respectively. The products were characterized with SEM, EMP, and powder-XRD. Zoisites form crystals up to 30 μm in size. Lattice parameters of zoisite increase linearly with increasing Sr content. For synthetic zoisite-(Sr) lattice parameters are a = 16.3567(5) Å, b = 5.5992(2) Å, c = 10.2612(5) Å, and V = 939.78(7) Å3 in space group Pnma. Volume of clinozoisite (P21/m) increases with increasing XSrCzo, but the lattice parameter a collapses, and b, c, and β have a discontinuity at XSrCzo ≈ 0.25. The decrease in angle β of clinozoisite results in compression of M3 and T3 polyhedra and increase of the A2 polyhedron. A1-O7 distance of 2.12 Å in clinozoisite is extremely short at XSr Czo ≈ 0.25, but with further Sr incorporation on A2 this distance relaxes quickly to 2.24 Å, combined with a torsion of T3. In zoisite, Sr incorporation leads to an opposite movement of neighboring octahedral chains parallel a and causes changes in the linked T3, and angle O5-T3-O6 increases with XSr from 96.3 to 101°. The intra-crystalline distribution of Sr shows that A2 is the favored position and continuous incorporation on A1-position starts above XSrZo ≈ 0.35 for zoisite and above XSrCzo ≈ 0.45 for clinozoisite.

Crystal chemistry of synthetic lawsonite solid-solution series CaAl2[(OH)2/Si2O7]·H2O–SrAl2[(OH)2/Si2O7]·H2O and the Cmcm–P21/m phase transition

Abstract Crystals of the solid-solution series of (Ca,Sr)-lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P21/m. It is exclusively orthorhombic at low xSrbulk but monoclinic at high xSrbulk; in the range xSrbulk = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between xSrortho ~0.1-0.2 and xSrmono ~0.3-0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017·xSr + 5.841 (Å), b = 0.197·xSr + 8.787 (Å), c = 0.263·xSr + 13.130 (Å), and v = 4.62·xSr + 101.46 (cm3/mol) for orthorhombic lawsonite and a = 0.119·xSr + 5.306 (Å), b = 0.118·xSr + 13.160 (Å), c = 0.025·xSr + 5.833 (Å), β = 0.38·xSr + 124.07 (°), and v = 3.20·xSr + 101.59 (cm3/mol) for monoclinic lawsonite. The data suggest an increasingly negative Δvortho-mono with increasing xSr. In monoclinic lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si2O7-group, whereas in orthorhombic lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic lawsonite with published high-P data yields K0mono = 137(3) GPa (K′ = 4.4). Contrary to the Ca end-member system, the Cmcm-P21/m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing xSr. The Cmcm-P21/m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P-T slope.

The T-X dependence of the isosymmetric displacive phase transition in synthetic Fe3+-Al zoisite: A temperature-dependent infrared spectroscopy study

Abstract We have studied three synthetic Al-Fe3+ zoisites with variable Fe3+-contents [Xps = 0.0, 0.035 (12), and 0.062 (23)] by means of temperature-dependent infrared powder spectroscopy. Spectra were recorded from -150 to 170 °C in steps of 20 °C in the lattice vibrational range 400 to 1200 cm-1. At -150 °C, a total of 34 bands could be identified in all samples. With increasing temperature, the bands broaden and generally shift smoothly to lower wave numbers. The maxima of bands at ~620 and ~909 cm-1 as well as the autocorrelation analysis of the spectral region 820 to 1050 cm-1 display linear but different temperature dependencies of the spectral properties at low and high temperatures. These changes in temperature dependence of the spectral properties marks the isosymmetric displacive phase transition between zoisite I and II. The transition temperatures Ttr are about 45 °C for Xps = 0.0 and = 0.035 (12), and about -35 °C for Xps = 0.062 (23) and suggest a negative correlation between Ttr and Xps. A combination with literature data yields a positive slope of 9.4 ± 2.0 MPa/K and a ΔS of 0.5 ± 0.1 J/(K·mol) for the zoisite I → zoisite II transition for a composition of about Xps = 0.035.

Synthesis of Pb-zoisite and Pb-lawsonite

Hydrothermal syntheses of Pb-zoisite Pb2Al3(SiO4|Si2O7|O|OH) and Pb-lawsonite PbAl2(Si2O7|(OH)2)•H2O were per- formed at high pressure and temperature conditions with standard piston cylinder press experiments. Starting materials were mix- tures of PbAl2O4, SiO2, PbO and H2O. The run products were characterized by single-crystal and powder X-ray diffraction, scanning electron microscopy and electron microprobe analyses. Idiomorphic colourless Pb-zoisite crystals with sizes of 60 × 50 × 120 µm were obtained at 2 GPa and 600 °C, together with Pb- lawsonite and traces of Pb-margarite and plumbotsumite Pb5Si4O8(OH)10. Single-crystal diffraction studies and structure solution of Pb-zoisite yielded space group Pnma (62), Z = 4, a = 16.4529(7) A, b = 5.6432(2) A, c = 10.3631(5) A, V = 962.18 A 3 , R1 = 0.067. Pb-lawsonite was obtained at 3 GPa/600 °C and at 2 GPa/400 °C. Powder-XRD pattern of Pb-lawsonite shows an orthorhombic unit cell. Peaks at (101), (103), (121), (211), (212), (213), (231), (301), (233) suggest space group Pbnm (62) with Z = 4, a = c 5.85 A, b = 9.03 A, c = 13.31 A, V = 703 A 3 , instead of space group Cmcn for Ca-lawsonite and P21/m for Sr-lawsonite. Group-subgroup relations of the lawsonite structure family are presented.

Synthetic Sr–Ca margarite, anorthite and slawsonite solid solutions and solid–fluid Sr–Ca fractionation

Calcium-strontium solid-solutions of margarite, anorthite, slawsonite, calcite and strontianite were synthesized from oxide-hydroxide-fluid mixtures by way of hydrothermal experiments at 400-500 °C and 390-500 MPa. The fractionation of Ca and Sr between the coexisting phases was investigated via electron microprobe and X-ray diffraction analyses of the solids and inductively coupled plasma-optical emission spectrometry of the fluids. A complete solid-solution series of margarite is indicated and the lattice parameters a, b, c, and V increase linearly with increasing Sr content. The lattice parameters of (Ca,Sr)-anorthite show a likewise linear increase with increasing Sr content. The formation of Sr-Ca slawsonite and Sr-Ca strontianite is restricted to high bulk Sr contents. The fractionation of Ca and Sr between minerals and fluid expressed as K mineral-fluid D(Sr-Ca) increases in the following sequence: calcite-fluid < anorthite-fluid < margarite-fluid ≈ 1< strontianite-fluid < slawsonite-fluid. At low X Sr (i.e., for common metamorphic bulk-rock compositions), K calcite-fluid D(Sr-Ca) = 0.09, K anorthite-fluid (Sr-Ca) = 0.51, and K margarite-fluid (Sr-Ca) = 0.56; the Sr/Ca ratio of the fluid is therefore generally higher than that of the coexisting minerals. This suggests that during prograde metamorphism and probably continuous dehydration, the Sr/Ca ratio of the rock decreases continuously with time. But, owing to the roughly equal Sr-Ca mineral-fluid fractionation for margarite and anorthite, which was also determined for zoisite/clinozoisite and lawsonite, this decrease will result in only moderate changes of whole-rock and fluid Sr-composition as long as these phases are the main Sr-bearing minerals.

Crystal structure refinement of synthetic Ca0.43Sr0.57[SiO3]-walstromite and walstromite-fluid Ca-Sr distribution at upper-mantle conditions

(Ca,Sr)-walstromite with the mean composition Ca 0.43 Sr 0.57 [SiO 3 ] was synthesized at 4 GPa/600 °C and X Sr bulk = 0.875 in presence of a 1 molar aqueous (Ca,Sr)Cl 2 fluid together with monoclinic (Ca,Sr)-lawsonite, grossular and strontianite. Intracrystalline as well as walstromite–mineral and walstromite–fluid Ca–Sr distribution was determined by analysing the product fluid with ICP–OES for Ca and Sr concentrations, solids by electron-microprobe analysis, powder XRD-analysis with Rietveld refinement and single-crystal X-ray diffraction. Based on the single-crystal X-ray diffraction data the crystal structure of (Ca,Sr)-walstromite was solved in space group P -1 with refined lattice parameters a = 6.7580(9) A, b = 9.464(3) A, c = 6.7507(16) A, α = 83.22(2)°, β = 76.83(2)°, γ = 70.33(2)°, and V = 395.46(17) A 3 . The data indicate that (Ca,Sr)-walstromite has slightly smaller volume and A-sites than the polymorph (Ca,Sr)-wollastonite-II over the entire compositional range. Coordination numbers and mean bond lengths for the three A-sites in Ca 0.43 Sr 0.57 -walstromite are [VIII] A1–O>= 2.588 A, [VI] A2–O>= 2.369 A, and [VII] A3–O>= 2.660 A. The smaller volume suggests that (Ca,Sr)-walstromite is the high- P /low- T polymorph. The data show extreme intracrystalline fractionation of Sr within the (Ca,Sr)-walstromite solid solution series with exchange coefficients K D (Sr – Ca) site1 – site2 defined as (Sr/Ca) site1 /(Sr/Ca) site2 of K D (Sr–Ca) A3–A1 = 21, K D (Sr–Ca) A3–A2 = 2400, and K D (Sr–Ca) A1–A2 = 114. The walstromite–mineral and walstromite–fluid Ca–Sr distribution indicates notable fractionation of Sr into the coexisting phases lawsonite, strontianite and fluid compared to (Ca,Sr)-walstromite. Grossular has only very minor amounts of Sr. Exchange coefficients K D(Sr–Ca) phase1–phase2 = [(Sr/Ca) phase1 ] n /[(Sr/Ca) phase2 ] m with n and m being the stoichiometric coefficients of the respective exchange reaction are 0.00068 for walstromite–fluid, 0.000011 for walstromite–lawsonite and walstromite–strontianite, and 31.8 for walstromite–grossular.

Crystal structures of synthetic melanotekite (Pb2Fe2Si2O9), kentrolite (Pb2Mn2Si2O9), and the aluminum analogue (Pb2Al2Si2O9)

Abstract Synthetic crystals of melanotekite and kentrolite were obtained at 850 °C from melt. The aluminum analogue of kentrolite Pb2Al2Si2O9 was hydrothermally synthesized at 2 GPa, 650 °C together with zoisite-(Pb) and margarite-(Pb). Synthesis products were characterized by single-crystal diffraction studies and microprobe analysis. The aluminum analogue Pb2Al2Si2O9 was observed in space group Pbcn with lattice parameters a = 6.8981(7) Å, b = 10.6906(15) Å, c = 9.7413(10) Å, and V = 718.37 Å3. Fourier mappings show no irregularities of the Pb site. Melanotekite with lattice parameters a = 6.9786(6) Å, b = 11.0170(11) Å, c = 10.0895(9) Å, and V = 775.71(17) Å3 in space group Pbcn show a slightly deformed Pb-position in Fourier mappings. Kentrolite was observed in space group P21221 with pseudo-symmetry to Pbcn with lattice parameters a = 7.0103(5) Å, b = 11.0729 (7) Å, c = 9.9642(7) Å, and V = 773.47(11) Å3. Fourier mappings of the kentrolite structure show that two different split Pb sites exist, which causes lower symmetry. The unit-cell volume of different members of the kentrolite group is a linear function of trivalent ionic radii in sixfold coordination for the elements Al, Ga, In, and also for Fe and Mn in high spin mode. The structure of Pb2M2Si2O9 (M = Al3+, Fe3+, Mn3+) is built on isolated M-octahedra chains parallel c, M-octahedra sharing alternately trans and skew edges. Each Si2O7-group is linked with their vertices to three octahedra chains. Their Si-O-Si bond angles depend on the size of M-octahedra and are 129.84° in Pb2Al2Si2O9, 131.08° in Pb2Fe2Si2O9, 128.34° and 130.33° in Pb2Mn2Si2O9.

Ca-Sr fractionation between zoisite, lawsonite, and aqueous fluids: An experimental study at 2.0 and 4.0 GPa/400 to 800 °C

Abstract The Ca-Sr fractionation between zoisite and, respectively, lawsonite and an aqueous fluid has been determined by synthesis experiments in the presence of a 1 M (Ca,Sr)Cl2 aqueous fluid at 2.0 GPa/550, 600, and 700 °C and 4.0 GPa/800 °C for zoisite and 2.0 GPa/400 °C and 4.0 GPa/600 °C for lawsonite. Solid run products were characterized by EMP, SEM, and XRD with Rietveld refinement and fluids were analyzed by ICP-OES. Zoisite exhibits notable intracrystalline Ca-Sr fractionation between the A1 and A2 sites and calculated intracrystalline exchange coefficients KD(Sr-Ca)A1-A2 = 1.5 to 26 show strong preference of Sr over Ca for the slightly larger A2 site. Calculated individual site-dependent zoisite/ aqueous fluid (af, in superscripts)-exchange coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are Kzo A1 -af(Sr-Ca)= 3.38 to 41.08 for the A1 site and Kzo A2 -af(Sr-Ca)= 0.45 to 6.51 for the A2 site. Assuming γCaaf = γSraf and a symmetric mixing model, the thermodynamic evaluation of the site-dependent exchange reactions Ca2+(af) + SrA1(M2+)A2Al3[Si3O11(O/OH)] = Sr2+(af) + CaA1(M2+)A2Al3[Si3O11(O/OH)] and Ca2+(af) + (M2+)A1SrA2Al3[Si3O11(O/OH)] = Sr2+(af) + (M2+)A1CaA2Al3[Si3O11(O/OH)] yields Δμ0 = -29 kJ/mol and Wzo A1Sr-Ca = 5.5 kJ/mol for the A1 site and Δμ0 = -1.1 kJ/mol and Wza A2Sr-Ca = 0 kJ/mol for the A2 site at P and T of the experiments. The data indicates ideal Ca-Sr substitution on the A2 site. Lawsonite formed in both the orthorhombic Cmcm and the monoclinic P21/m form. Calculated lawsonite-aqueous fluidexchange coefficients indicate overall preference of Ca over Sr in the solid and are Klaw Cmc-afD(Sr-Ca) = 1.12 to 11.32 for orthorhombic and Klaw P21m-afD(Sr-Ca) = 1.67 to 4.34 for monoclinic lawsonite. Thermodynamic evaluation of the exchange reaction Ca2+(af) + SrAl2Si2O7(OH)2·H2O = Sr2+(af) + CaAl2Si2O7(OH)2·H2O assuming γafCa = γafSr and a symmetric mixing model yields similar values of Δμ0 = -9 kJ/mol and Wlaw CmcmSr-Ca= 10 kJ/mol for orthorhombic and Δμ0 = -10 kJ/mol and Wlaw P21/mSr-Ca = 11 kJ/mol for monoclinic lawsonite. Calculated Nernst distribution coefficients for the studied 1 M (Ca,Sr)Cl2 aqueous fluids are Dzo-afSr = 2.8 ± 0.7 for zoisite at 2 GPa/600 °C and Dlaw Cmcm-af = 0.6 ± 0.2 for orthorhombic lawsonite at 4 GPa/600 °C and show Sr to be compatible in zoisite but incompatible in lawsonite. This opposite mineral-aqueous fluid-fractionation behavior of Sr with respect to zoisite and lawsonite on the one hand and the ideal Ca-Sr substitution on the zoisite A2 site in combination with the strong intracrystalline Ca-Sr fractionation in zoisite on the other hand, make Sr a potential tracer for fluid-rock interactions in zoisite- and lawsonite-bearing rocks. For low Sr-concentrations, xzoSr directly reflects xafSr and allows us to calculate Sr-concentrations in a metamorphic aqueous fluid. During high-pressure aqueous fluid-rock interactions in subduction zone settings the opposite mineral-aqueous fluid-fractionation behavior of Sr results in different aqueous fluid characteristics for lawsonite- vs. zoisite-bearing rocks. Ultimately, subduction zone magmas may trace these different aqueous fluid characteristics and allow distinguishing between cold, lawsonite-bearing vs. warm, zoisite-bearing thermal regimes of the underlying subduction zone.