Kiyotaka Ishida

Infrared and TEM characterization of amphiboles synthesized near the tremolite-pargasite join in the ternary system tremolite-pargasite-cummingtonite

Abstract High-resolution transmission-electron microscopy (HRTEM) and infrared spectroscopy (FTIR) analyses have been done on amphiboles near the join tremolite [Ca2Mg5Si8O22(OH)2 = TR]-pargasite [NaCa2Mg4Al3Si6O22(OH)2 = PG] in the ternary system tremolite-pargasite-cummingtonite [Mg7Si8O22(OH)2 = MC] that were synthesized previously by Sharma and Jenkins (1999). Representative samples across the join were examined in detail by HRTEM to document the presence and concentration of chain multiplicity defects (CMFs). There was relatively little change in the defect density with composition, with the tremolitic sample (TREM 23-13) having the highest defect concentration (6%) and the more PG-rich samples having slightly lower CMF concentrations (4-5%). CMFs with multiplicities of 1, 3, 4, 5, and 6 were observed, usually as isolated chains, with the most common being triple-chain slabs. Correction of the bulk composition of the tremolitic amphibole for the presence of these Mg-rich, wide-chain defects reduces the MC content from an apparent value of 8.5 to 4-7.5 mol% MC, depending on which composition is used for the triple-chain defect. The entire amphibole join was examined by FTIR spectroscopy in the OH-stretching region (3000-3800 cm-1) for the purpose of determining the presence of short-range order. A total of 10 component bands were fitted to the spectra across the join. These bands were assigned to specific cation configurations on the basis of earlier studies of the FTIR spectra of chemically simplified amphibole joins pertinent to this study. The extent of short-range order was qualitatively determined by comparing the observed intensities for groups of related bands, corrected for differences in their molar absorptivities, to their calculated intensities based on random-mixing probabilities. From this exercise, it is observed that the intensities of sodic amphibole configurations are consistently high, tremolite is lowest near the middle of the join, and aluminous amphibole configurations cross over from being higher (at low Al contents) to being lower (at high Al contents) than expected near the middle of the join. These differences between observed and predicted band intensities may reflect the presence of deviations in the thermodynamic activities of amphibole components from those predicted on the basis of random-mixing models.

Mid-IR bands of synthetic calcic amphiboles of tremolite-pargasite series and of natural calcic amphiboles

Abstract Mid-IR spectra (4000-400 cm-1) of synthetic calcic amphiboles in the tremolite-pargasite series and of various natural calcic amphiboles have been investigated. The pargasite substitution, a combination of the Tschermak (=[4]Al[6]Al[6]Mg-1[4]Si-1) and edenite (=[4]Al[A]Na[4]Si-1[A]□-1) substitutions, causes the following features in the region 1200-600 cm-1. (1) Weak [4]Al-O stretching bands appear at 895 and 815 cm-1 that are distinct from the 955 and 925 cm-1 Si-O stretching bands in tremolite. (2) There is a reduction in the intensity and frequency of the Si-O-Si symmetric bending band (=“chain breathing” mode) at 750 cm-1 in tremolite, and there is an appearance of medium-strong composite bands having a weak shoulder on the high-frequency side near 690 cm-1. These bands are assigned to Si-O-Al deformation bands. (3) Two OH-libration bands at 690 and 650 cm-1 become weak and broad composite bands from 720 to 610 cm-1. And (4) because the intensity and frequency of the band at 640 cm-1 in tremolite is affected neither by deuteration nor by the pargasite substitution, this band is ascribed to O-[T2]Si-O bending. Even in pargasite, most T2 sites are occupied by Si, so that the O2-[T2]Si-O4 bending mode will be dominant in this amphibole. The same behavior occurs for the synthetic fluoro-gallian tremolite-pargasite series but with larger downward frequency shifts-Ga-O stretching bands appear at 880 and 780 cm-1, and an Si-O-Ga bending band appears at 605 cm-1. The major T-O-T and O-T-O deformation bands in synthetic amphiboles are readily apparent in natural calcic amphiboles whose compositions are near the tremolite-pargasite join.

Shirozulite, KMn2+3 (Si3Al)O10(OH)2, a new manganese-dominant trioctahedral mica: Description and crystal structure

Abstract Shirozulite is a new Mn-dominant trioctahedral mica from the Taguchi mine, Aichi Prefecture, Japan. The mineral occurs in tephroite-rhodochrosite ores in contact with a Ba-bearing, K-feldspar vein. Shirozulite formed during regional low-P/T metamorphism and, thereafter, suffered thermal metamorphism from a local granodiorite. Grains of shirozulite are up to 0.5 mm across and have a typical micaceous habit. Color: dark reddish brown. Cleavage: (001), perfect. Optical properties: biaxial negative, 2Vx = very small. Strongly pleochroic: X = pale yellow, Y = Z = pale brown, absorption X < Y ≈ Z. Refractive indices: nα = 1.592(2), nβ ≈ nγ = 1.635(2). The structural formula is (K0.90Ba0.09) (Mn2+ 1.53Mg0.94Fe2+0.20Ti0.04Al0.29) (Si2.54 Al1.46) O10 [(OH)1.97F0.03], and the end-member composition is KMn2+3AlSi3O10(OH)2. Density: obs. = 3.20(3) g/cm3 by pycnometer, calc. = 3.14(2) g/cm3. Shirozulite is monoclinic, C2/m, 1M polytype, a = 5.3791(7), b = 9.319(1), c = 10.2918(9) Å, β = 100.186(9)°, V = 507.8(1) Å3. The six strongest lines in the powder X-ray diffraction pattern are as follows: d (Å), l (%), (hkl): 10.16, 100, (001); 2.654, 96, (1̅31); 3.386, 51, (003); 1.556, 48, (3̅̅13); 2.467, 46, (1̅32); 2.202, 36, (1̅33). The crystal structure has been refined to an R value of 4.1% based on 663 observed reflections collected with MoKα X-radiation from a single crystal. The mean bond lengths, tetrahedral rotation, and octahedral flattening angles are as follows: = 1.668, = 2.118, = 2.103, (inner) = 2.995, and (outer) = 3.376 Å, α = 8.36°, ψM1 = 58.5°, ψM2 = 58.2°. The apparent element distribution coefficient analyses among coexisting manganese or manganoan silicate minerals indicate that the trioctahedral mica structure cannot contain larger amounts of Mn2+ relative to Mg and Fe2+ than in olivine, pyroxenoid, and garnet.

Assignment of infrared OH-stretching bands in manganoan magnesio-arfvedsonite and richterite through heat-treatment

Abstract Infrared OH-stretching bands in some heat-treated A-site occupied and IVAl-free (or nearly free) manganoan sodic-calcic and sodic amphiboles, manganoan magnesio-arfvedsonites and richterites, have been assigned. Two OH-stretching bands, (MgMgMg)-OH-A-O2-/F-/Cl- and (MgMgMg)-OH- □(□= vacancy) configurations, persist to high temperature. With increasing temperature, the OHstretching band, A*, of the (MgMgMg)-OH-A-OH (A = A site cation) configuration shifts downward from 3730 to near 3700 cm-1 with formation of the (MgMgMg)-OH-A-O2- configuration; the repulsive interaction between the proton and the A cation is removed through dehydrogenation of OH at the O3 site, coupled with movement of the A cation toward the dehydrogenated side. In natural Fand Cl-bearing sodic-calcic and sodic amphiboles, two kinds of (MgMgMg)-OH-A stretching bands are observed at around 3730 and 3700 cm-1, in which A-site alkali ions move toward the F-(Cl-)- substituted O3 site. In this manner, (MgMgMg)-OH-K and (MgMgMg)-OH-Na bands shift downward 20 cm-1 and 26-29 cm-1 by heat-treatment, respectively, reflecting the different size of the A-site cations.

X-ray Rietveld refinement and FTIR spectra of synthetic (Si,Ge)-richterites

Abstract Richteritic amphiboles in which tetrahedral Si was substituted for Ge were synthesized using internally heated gas vessels at 795~905 °C and 720~756 MPa. There is complete solid-solution between IVSi and IVGe richterite. The materials were characterized by scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), analytical transmission electron microscopy (AN-TEM), electron microprobe analysis (EMPA), X-ray diffraction (XRD) Rietveld structure refinement, and Fourier-transform infrared spectroscopy (FTIR). X-ray diffraction data for the richterites indicate that, with increasing Ge replacement for Si, all cell parameters (= a, b, c, and β) increase linearly and the rotation angle of the double chains increases. Refinement of the Ge and Si contents at the tetrahedral sites indicate that the Ge content at the T2 site is greater than at the T1 site for amphiboles of intermediate composition. Deuteration experiments were also made for the purpose of FTIR analysis. Infrared OH/OD-stretching bands attributed to the configurations (MgMgMg)-OH/ OD-ANa(K) and (MgMgMg)-OH/OD-A⃞ (⃞ = vacancy) were observed. The frequency of the former bands decreases linearly with increasing Ge content, while the frequency and the intensity of the latter band decreases with increasing Ge content. Both sets of OH/OD-stretching bands show a continuous or one-mode change along the compositional join without any identifiable fine structure, indicating a lack of any short-range ordering within the tetrahedral double chain. The Si-O (at 1200~800 cm-1) and Ge-O (at 950~700 cm-1) stretching bands show similar continuous down-frequency shifts, and interactions of their modes are very small. The chain deformation bands of Si•Si-O, Si•Ge-O, and Ge•Ge-O are observed at 770~650, 660~590, and 590~510 cm-1, respectively, with the frequency range of their absorption bands becoming narrower with increasing Ge content. A weak and broad OH librational band appears at 600 cm-1 in Si-richterite. With increasing Ge for Si substitution this band shifts upward in frequency, becoming centered at 650 cm-1 in Ge-richterite, which is the opposite behavior to the downward frequency shift of the OH/OD-stretching vibrations. The most notable aspect of this study is the continuous changes that are observed in the structure (cell dimensions, bond distances) and infrared spectra of richterite with replacement of Si by Ge. The only long-range ordering effect that was clearly observed was the preference of Ge over Si at the tetrahedral T2 site for intermediate compositions. Evidence for short-range ordering that can be observed in the OH-stretching region of the Ge-analogue of talc was not observed in Ge-richterite.

The crystal chemistry of the gedrite-group amphiboles. I. Crystal structure and site populations

Abstract The crystal structures of twenty-five orthorhombic Fe-Mg-Mn amphiboles, a = 18.525-18.620, b = 17.806-18.034, c = 5.264-5.303 Å, V = 1737.6-1776.7, space group = Pnma, Z = 4, have been refined to R indices in the range 2.1-7.8% using 790-1804 unique observed reflections measured with Mo-Kα X-radiation on a Bruker P4 automated four-circle diffractometer equipped with a 1K CCD detector. The quality of the refinements is strongly a function of the [4]Al content of the crystals because of unmixing in the central part of the series due to the presence of a low-temperature solvus. The amphibole crystals were analysed by electron microprobe subsequent to collection of the X-ray intensity data and span the anthophyllite-gedrite series from 0.17-1.82 [4]Al a.p.f.u. Mössbauer spectroscopy shows that the amphiboles of this series commonly contain small but significant amounts of Fe3+. The amount of [4]Al is linearly related to the grand distance by the equation = 1.6214 + 0.171 [4]Al, R = 0.980; the slope of this relation is not significantly different from that characteristic of a hard-sphere model. The distances indicate the following site preference for [4]Al: T1B > T2B > T1A >> T2A. The distances are compatible with all [6]Al and Fe3+ ordered at the M2 site. The grand distance is related to the mean radius of the constituent cations, , by the equation <<M1,2,3-O>> = 1.4684 + 0.8553(7) .

Assignment of infrared OH-stretching bands in calcic amphiboles through deuteration and heat treatment

Abstract Infrared OH-stretching bands of calcic amphiboles in the magnesiohornblende-tschermakite/ferrotschermakite and edenite-pargasite/hastingsite series have been assigned by deuteration and heat treatment in air. Mössbauer spectra indicate that with increasing temperature for heat-treatment in air, Fe2+ at the M1 and M3 sites is first converted to Fe3+ through dehydrogenation, and then Fe2+ at the M2 site is oxidized at higher temperature. The quadrupole-splitting parameters of (oxidized) Fe3+ at the M1 and M3 sites and Fe3+ at the M2 site are much larger than in natural (= non-dehydrogenated) amphiboles, indicating that dehydrogenation of O3H causes large electric-field-gradients at the M1-3 sites. The intensity of absorption of Fe3+ at M2 decreases with heating temperature, which is consistent with the migration of Fe3+ at M2 to the M1 and/or M3 sites. The (MgMgAl)-OH band, designated K*T, occurs at ~3678 cm-1 in Fe2+-poor pargasitic amphiboles, and is assigned to the configuration (MgMgAl)-OH-A(Na,K): T1SiT1Al. Three (2.4) of the following four types of band systems occur with decreasing band frequency: (1) A*.D* bands at 3730.3675 cm-1, associated with (M1M1M3)-OH-A(Na,K):T1SiT1Si configurations; (2) A*T.D*T bands at 3725.3650 cm-1, associated with (M1M1M3)-OH-A(Na,K): T1SiT1Al configurations; (3) A.D bands at 3680.3620 cm-1 , associated with (M1M1M3)-OH-A□: T1SiT1Si (□ = vacancy) configurations; and (4) AT.DT bands at 3650.3580 cm-1 , associated with (M1M1M3)-OH-A□: T1SiT1Al configurations. In addition, A**T, E*T, and K**T bands ascribed to the configurations (MgMgMg)-OH-A(Na,K)-O3O2.:T1SiT1Al, (MgMgFe3+)-OHA( Na,K)-O3O2.:T1SiT1Al, and (MgMgAl)-OH-A(Na,K)-O3O2.:T1SiT1Al are important constituents of the spectra of oxidized magnesiohornblende and pargasite. The high frequency bands, A*.D*, are particularly weak, indicating short-range order involving local association of the T1SiT1Al configuration with a locally occupied A-site.

The crystal chemistry of the gedrite-group amphiboles. II. Stereochemistry and chemical relations

Abstract The general formula of the amphiboles of this series may be written as NaxMg2(Mg(5-y)Aly)(Si(8-z)Alz)O22(OH)2, where Mg = Mg + Fe2+ + Mn2+ and Al = Al + Fe3+ + Ti. The individual distances are linear functions of their [4]Al content, and the [4]Al content is strongly ordered in the following way: T1B > T1A >> T2B >> T2A. The , and distances are linear functions of the mean ionic radius of their constituent cations. End-member compositions may be written as follows: A⃞Mg2Mg5Si8O22(OH)2; ANaMg2Mg5(Si7Al)O22(OH)2; A⃞Mg2(Mg3Al2)(Si6Al2)O22(OH)2; ANaMg2(Mg3Al2)(Si5Al3)O22(OH)2. These compositions define a plane in xyz space across which the data of Schindler et al. (2008), measured on amphiboles from amphibolites, follow a tightly constrained trajectory. Anthophyllite-gedrite amphiboles equilibrated under significantly different P-T conditions (e.g. igneous rocks, contact-metamorphic rocks) follow trends that diverge from this trajectory, with greater Na and [4]Al contents and relatively smaller [6]Al contents. Detailed examination of the local bond topology involving the A and M2 sites indicates that the maximum degree of bond-valence compensation will occur for incorporation of ANa and M2Al in the ratio 4:10, and hence 2.5 ANa = M2Al in these amphiboles. This relation closely fits the data of Schindler et al. (2008), suggesting that the variation in chemical composition in anthophyllite-gedrite amphiboles is strongly constrained by the anion bond-valence requirements of the Pnma amphibole structure. We further suggest that different compositional trends for ortho-amphiboles equilibrated under different P-T conditions are the result of the valence-sum rule operating with (different) bond-lengths characteristic of these P-T conditions.

EXPERIMENTAL STUDY ALONG THE MAGNESIO-HORNBLENDE--GLAUCOPHANE JOIN

Abstract Amphiboles have played a leading role in metamorphic petrology, from helping to define several metamorphic facies to forming the basis of geothermobarometry, if their thermodynamic mixing properties can be calibrated to the temperature and pressure of formation. Compositional variations of sodium- and sodium-calcium-amphiboles may reveal important information about paleo-subduction zones but have not been studied as much as the more common calcium-amphiboles. In this study we investigate the mixing properties of amphibole solid solutions between magnesio-hornblende and glaucophane [BCa2C(Mg4Al)T(AlSi7)O22(OH)2-BNa2C(Mg3Al2)T(Si8)O22(OH)2] as a binary sub-join within the ternary amphibole system tremolite-glaucophane-tschermakite where the principal substitutions are Ca for Na at the B, Al for Mg at the C, and Al for Si at the T crystallographic sites. Amphiboles were made from mixtures of reagent oxides at 10 mol% increments between magnesio-hornblende and glaucophane, formed in a piston-cylinder press at 735-860 °C and 1.3-2.5 GPa for 72-216 h giving good yields (92-100 wt%). A positive deviation is present in the volume-composition plot, even after correcting volumes for non-binary components, supporting the presence of a positive deviation in the enthalpy of mixing (ΔHmix) along this join. Fourier transform infrared spectra (FTIR) were obtained in the range of 350-4000 cm-1 for the mid-infrared spectra (MIR) for the purpose of estimating the extent of short-range ordering and for autocorrelation analysis, and, in the 650-50 cm-1, for far-infrared spectra (FIR) for autocorrelation analysis. Autocorrelation analysis gave δΔCorr values, which further support a positive deviation in the ΔHmix along the magnsio-hornblende-glaucophane join, although the δΔCorr maximum did not occur at the calcium-poor (i.e., glaucophane-rich) portion of the join as expected. Synthetic end-member glaucophane and magnesio-hornblende were mixed in a molar ratio of 1:1 and allowed to equilibrate by homogenization for variable durations in the range of 600-800 °C at 2.0 GPa to determine the maximum-width of the miscibility gap. These compositional re-equilibration experiments suggested the presence of an asymmetric miscibility gap (steeper toward glaucophane) with a critical-point below 700 °C. Combining the results of this study with previously published results on the tremolite-glaucophane join allowed refinement of several asymmetric formalism mixing parameters (i.e., WGl,Ts = 20 kJ, αTs = 1.2) and modeling of the miscibility gap within the tremolite-glaucophane-tschermakite ternary system. The results showed that the composition of the critical point is very close to the maximum in the autocorrelation parameter δΔCorr, as one would predict. An important implication of this study is that low-temperature immiscibility between calciumand sodium-rich amphiboles may be more important than the role of pressure, as proposed by Brown (1977), in accounting for the change in B-site Na contents of metamorphic amphiboles.

Far-infrared spectra of synthetic [4][(Al2−xGax)(Si2−yGey)](OH,OD,F)2-kinoshitalite: Characterization and assignment of interlayer Ba-Oinner and Ba-Oouter stretching bands

Abstract Far-infrared spectroscopy and X-ray diffraction Rietveld structure-refinement of synthetic kinoshitalite (Kn) solid solutions, BaMg3[(Al2-xGax)(Si2-yGey)]O10(OH,OD,F)2: (x = 0.0-2.0, y = 0.0-2.0), show that there is complete solid solution for all compositions in each (OH/OD)- and F-series: [4][Al2(Si2-yGey)]-, [4][(Al2-xGax)Si2]-, [4][Ga2(Si2-yGey)]-, [4][(Al2-xGax)Ge2]-Kn, and in OH/OD-for-F substituted [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions. In the far-infrared region, 170-40 cm-1, three kind of bands are observed; an in-plane tetrahedral torsional mode, an interlayer Ba-Oinner stretching vibration and a Ba-Oouter stretching vibration. With increasing tetrahedral [4]Al-for-[4]Ga and Si-for-Ge substitution, the frequencies and intensities of the tetrahedral in-plane torsional bands decrease in both the (OH/OD)- and F-bearing phases, but in the [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions, the frequencies are unaffected by (OH/OD)-for-F substitution. The frequencies of both the Ba-Oinner and Ba-Oouter stretching bands increase with increasing [4]Al-for-[4]Ga and Si-for-Ge substitution, but the frequencies of the Ba-Oinner stretching bands decrease with increasing (OH/OD)-for-F substitution in the [4](Al2Si2)-, [4](Ga2Si2)-, [4](Al2Ge2)-, [4](Ga2Ge2)-Kn end-member compositions. The frequency difference between the Ba-Oinner and Ba-Oouter stretching bands is linearly related to the tetrahedral rotation angles (α), and these differences are about 10 cm-1 larger in the (OH/OD)-bearing phases than in the corresponding F-bearing phases. The ranges of absorption frequencies and their corresponding deformation modes are as follows: (1) inplane tetrahedral torsional mode, 105-150 cm-1; (2) Ba-Oinner stretching vibration, 105-140 cm-1; and (3) Ba-Oouter stretching vibration, 75-90 cm-1.