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This paper describes a high-pressure synchrotron infrared (IR) spectroscopy study of the synthetic amphibole Na(NaMg)[Mg.sub.5] [Si.sub.8][O.sub.22][(OH).sub.2]. This compound has P[2.sub.1]/m symmetry at room conditions; its IR OH-stretching spectrum consists of two main bands at 3743 and 3715 [cm.sup.-1], which are assigned to the two symmetrically independent O-H groups in the P structure (sample 403, Iezzi et al. 2004a). For increasing pressure, both bands shift toward higher frequency, suggesting a shortening of the O-H bond. In addition, the two bands progressively merge to give a single, symmetric and broad absorption band at 20-22 GPa. This behavior suggests that at 20-22 GPa there is a unique O-H group in the structure, indicative of a C-lattice type. The IR data thus show that the examined sample undergoes a P[2.sub.1]/m [left and right arrow], C2/m phase-transition at that pressure. Upon release of pressure, the initial two-band pattern is immediately recovered indicating that the pressure-induced phase-transition is reversible, as already observed for the same kind of transition induced by temperature. By analogy with structurally related pyroxenes, and taking into account the possible crystal structural topologies of amphiboles, we suggest that the C2/m polymorph stable at high pressure is characterized by fully kinked double-chains. Keywords: Amphibole, synthesis, IR spectroscopy, high-pressure, phase-transition

We describe here the synthesis of amphiboles along the nominal Na(NaMg) [Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2]-Na(LiMg)[Mg.sub.5][Si.sub.8] [O.sub.22][(OH).sub.2] join, at 800 [degrees]C, 0.4 GPa. High amphibole yields (>90%) plus minor quartz and enstatite have been obtained at all compositions; amphibole crystals are acicular and their size rarely exceeds 20-30 x 0.5-3 [micro]m. TEM analysis shows the presence of h+k odd reflections in all samples, indicative of a P-lattice. By similarity with closely related amphiboles from the literature (e.g., Oberti et al. 2000; Camara et al. 2003) a P[2.sub.1]/m space group was assigned to the amphiboles synthesized here. Refined cell-parameters from X-ray powder-patterns show a linear decrease as a function of increasing Li at M4, a and [beta] being the most affected parameters. The four infrared OH-stretching spectra all show two main bands at 3741-3748 and 3712-3716 [cm.sup.-1], respectively. They are assigned to two independent O-H groups in the P[2.sub.1]/m structure, interacting with a strongly delocalized [sup.A]Na. The spectra show in addition two minor absorptions at about 3688 and 3667 [cm.sup.-1], respectively; these bands are assigned to vacant A-sites in the structure and indicate slight departure of the nominal composition toward cummingtonite. The present work shows that one apfu of [sup.B]Na can also be completely replaced by one apfu of [sup.B]Li ([M.sup.+]), in synthetic Na([M.sup.+]Mg)[Mg.sub.5][Si.sub.8][O.sub.22] [(OH).sub.2], and that all compositions have P[2.sub.1]/m symmetry at ambient conditions. Keywords: amphibole synthesis, FTIR spectroscopy, TEM, XRPD, amphibole symmetry

This paper reports the results of hydrothermal synthesis in the system [Na.sub.2]O-MgO-FeO-[Fe.sub.2][O.sub.3]-Si[O.sub.2][H.sub.2]O. Four samples of stoichiometric magnesioriebeckite composition, ideally [[][Na.sub.2][Mg.sub.3][Fe.sup.3+.sup.2][Si.sub.8] [O.sub.22](OH).sub.2], were run at 700-800 [degrees]C, 0.4 GPa, and redox conditions varying from NNO (Nickel-Nickel Oxide) to NNO + 2.3 log f[o.sub.2]. Powder XRD and SEM-EDX show a high (>85%) amphibole yield for all samples; however, in no case was the end-member composition attained. EMP analyses show that the amphiboles obtained deviate strongly from nominal stoichiometry toward magnesio-arfvedsonite [[Na[Na.sub.2][Mg.sub.4][Fe.sup.3+][Si.sub.8][O.sub.22](OH).sub.2]]. Powder XRD patterns were indexed in the space group C2/m; refined cell-parameters reflect variations in the amphibole composition, and the cell volume is correlated linearly with the A-site occupancy. Mossbauer spectra show that in all samples, [Fe.sup.3+] is completely ordered at M2, whereas [Fe.sup.2+] occurs at the M1, M3, and M4 sites. The [Fe.sup.3+]/[Fe.sup.2+] ratio is a function of f[o.sub.2]: for increasing oxidation conditions, there is significant increase in [sup.M2][Fe.sup.3+] and decrease in [Fe.sup.2+], notably in [sup.M4][Fe.sup.2+]. Mossbauer spectra also show significant variation in [sup.M1][Fe.sup.2+] and [sup.M3][Fe.sup.2+] quadrupole splitting as a function of the [Fe.sup.3+] content in the amphibole. IR spectra in the OH-stretching region show a well-resolved quadruplet at frequencies

Synthetic Na[Na.sub.2][Mg.sub.5][Si.sub.8][O.sub.21][(OH).sub.3] is the first triclinic member of the amphibole group, and has a tripling of the unit cell in the b direction. The space group is C[bar.1] and the triple-b repeat gives Z = 6. The unit-cell parameters are: a = 9,883(2), b = 54.082(9), and c = 5.277(1) [Angstrom], [alpha] = 90.045(4)[degrees], [beta] = 103.068(3)[degrees] [gamma] = 89.960(4)[degrees], and V = 2748(1) [[Angstrom].sup.3]. The crystal structure has been refined to [R.sub.1] = 7.6 and w[R.sub.2] = 16.7% for the 1835 reflections with [F.sub.o] > 4[[sigma].sub.F] and for 4832 supercell reflections in the 2[theta] range 2-25[degrees], respectively. The structure is pseudo-monoclinic, but both the intensity distribution and refined model indicate space group C[bar.1]. Compared with the common C2/m amphibole structure, the two halves of an I-beam unit are no longer mirror-related, and the overall structure can be rationalized in terms of two different types of I-beam occurring in the unit cell. The first (with multiplicity 2) is centrosymmetric, and the second (with multiplicity 4) is non-centrosymmetric. There are also significant displacements of the cations, especially at the M4 sites, from their corresponding locations in the C2/m structure. The correlated displacements of Na atoms at the M4 sites permit incorporation of excess protons in pseudo-tetrahedral cavities between two adjacent chains of tetrahedra belonging to different I-beams. Bond-valence calculations and crystal-chemical analysis suggest that excess protons are bonded to O atoms at the O4 sites, and are hydrogen bonded to O atoms at adjacent O2 sites. The infrared spectrum of the amphibole in the principal OH-stretching region has a triplet of sharp bands at 3740, 3727, and 3711 [cm.sup.-1], which are assigned to the three independent 'normal' O3-H groups in the triclinic structure. There is an additional intense and very broad absorption at 3430 [cm.sup.-1] that is resolved only when adsorbed moisture is removed. This band is assigned to the extra H in the structure. All the details provided by structure refinement and the proposed location of the excess H atoms is in accord with previous HRTEM and [sup.29]Si- and [sup.1]H MAS NMR studies of this amphibole.

In the present work, we characterize the amphibole Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] synthesized at 0.4 GPa and 750, 800, and 850 [degrees]C, and 0.5 GPa, 900 [degrees]C. Experiments at 800 and 900 [degrees]C yielded crystals suitable for single-crystal data collection. Structure refinement shows that synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has P[2.sub.1]/m symmetry at room T. The two non-equivalent tetrahedral double-chains differ in their degree of stretching and kinking. The infrared spectrum of synthetic Na(NaMg)[Mg.sub.5][Si.sub.8][O.sub.22][(OH).sub.2] has two well-defined absorption bands at 3742 and 3715 [cm.sup.-1] which can be assigned to O-H bands associated with the two independent anion sites (O3A and O3B) in the structure. The higher frequency band is assigned to the shorter O3B-H2 bond, and the lower tYequency band is assigned to the longer O3A-H1 bond. The broader shape of the 3743 [cm.sup.-1] band is consistent with a stronger interaction of the H2 atom with [sup.A]Na, which is confirmed by structure refinement. Increasing Tof synthesis causes a progressive departure from the ideal stoichiometry via the A[[cube root of].sub.1][sup.B][Mg.sub.1][sup.A][Na.sub.-1][sup.B][Na.sub.-1] substitution, as confirmed by EMPA, structure refinement, and FFIR spectroscopy.

Amphiboles were hydrothermally synthesized at 500[degrees]C and 4 kbar in the system [Li.sub.2]O-[Na.sub.2]O-FeO-[Fe.sub.2] [O.sub.3]-Si[O.sub.2]-[H.sub.2]O, with nominal compositions along the riebeckite [[][Na.sub.2][Fe.sup.2+.sub.3][Fe.sup.3+.sub.2]+[Si.sub.8] [O.sub.22] ([OH).sub.2]]-ferri-clinoferroholmquistite [[][Li.sub.2] [Fe.sup.2+.sub.3][Fe.sup.3.sub.2]+[Si.sub.8][O.sub.22]([OH).sub.2]] join, where the exchange vector is Na[Li.sub.-1] at the B-site. Experimental products were characterized by powder XRD and SEM-EDAX, confirming very high amphibole yield along the join (>95%, plus minor quartz). The XRD patterns can be indexed in C2/m, and the refined cell parameters show linear variation as a function of composition. For the [sup.B]Li end-member, the IR spectrum shows a single sharp main band centered at 3614 [cm.sup.-1], which is assigned to the FeFeFe-O[H-.sup.A][] configuration. With increasing [sup.B]Na in the mineral, this band broadens and shifts 4 [cm.sup.-1] to higher frequencies. This effect can be attributed to the change in M4 site occupancy. Minor [sup.A]Na (partial solid-solution toward arfvedsonite) is also observed with increasing sodium in the system. Mossbauer spectroscopy confirms the cation distribution provided by IR data, and shows that a small, but significant amount of [Fe.sup.2+] occurs at M4 along the join. Infrared spectroscopy shows that the Li end-member has a very ordered structure, whereas intermediate compositions show local heterogeneities associated with the presence of two different B sites occupied by Na or Li.

Detailed X-ray single-crystal structure refinements and complete (SIMS + EMPA) microprobe chemical analyses of a series of non-metamict samples of hellandite recently found in Latium (Italy), together with a critical re-evaluation of the existing data on hellandite, allow a better understanding of the crystal-chemistry of the hellandite group. Relative to the crystal structure determined by Mellini and Merlino (1977) for a Y-rich sample from Predazzo (Italy), a new tetrahedrally coordinated site has been detected; this may be fully, or in part, occupied by Li and Be. These cations occur at the center of the tetrahedral cavity where the H atom, which is bonded to the O5 O atom, protrudes; thus the H content in hellandite is constrained to values [less than or equal to] 2 - (Li + Be + F). A new general formula for hellandite-group minerals is proposed: [X.sub.4][Y.sub.2]Z[T.sub.2][[B.sub.4][Si.sub.4][O.sub.22]][W.sub.2], where X = Na, Ca, Y, LRE[E.sup.3+] at the eightfold-coordinated M3 and M4 sites; Y = Ca, Y, HRE[E.sup.3+], [Th.sup.4+ at the eightfold-coordinated M2 site; Z = Al, [Mn.sup.3+], [Fe.sup.3+], [Ti.sup.4+] at the octahedral M1 site; T = [square] (vacancy), Li, Be at the new tetrahedrally coordinated site; and W = OH, F, [O.sup.2-] at the 05 site. Eight root end-member compositions were identified; four of these correspond to known compositions: hellandite-(REE), tadzhikite-(REE), and two new minerals, mottanaite-(Ce) and ciprianiite, which are described in a companion paper (Della Ventura et al. 2002). The root-name tadzhikite must be used for samples with [Ti.sup.4+] > 0.5 apfu, with no reference to the OH content. Hellandite is an REE mineral. Rare-earth elements occur at two distinct sites (M2 and M4), with a strong preference for M2 (particularly HREE). Thus, one or two Levinson modifiers can be added to the root name to correctly describe the species. A sequence of incorporation based on crystalchemical arguments is provided and allows evaluation of the site populations for nomenclature purposes. This redefinition of the hellandite group and the new nomenclature rules have been approved by the IMA Commission on New Minerals and Mineral names (code 00-F).

Two new minerals of the hellandite group were found within alkali-syenitic ejecta enclosed in pyroclastic formations of the Roman Comagmatic Province (Latium, Italy). Mottanaite-(Ce) [ideally [sup.X][Ca.sup.Y.sub.4][(CeCa).sup.Z][Al.sup.T][Be.sub.2] [([Si.sub.4][B.sub.4][O.sub.22]).sup.W][O.sub.2]] and ciprianiite [ideally [sup.X][Ca.sup.Y.sub.4][[(Th,U)(REE)].sup.Z][Al.sup.T][[square].sub.2] ([Si.sub.4][B.sub.4][O.sub.22]) [sup.W][(OH, F).sub.2]] occur as transparent, brown-colored, tabular euhedral crystals in miarolitic cavities and voids of the ejecta, which consist mainly of sanidine and plagioclase (An ranging from 20 to 80%), with minor amounts of feldspathoid, clinopyroxene and/or clinoamphibole, magnetite, titanite, and zircon. Locally, accessory minerals include britholite-(Ce), baddeleyite, phosphate to silico-phosphate phases close in composition to the brabantite-cheralite series, thorite, fluorite, danburite, and vonsenite. The genesis of the new hellandite end-members can be related to late-stage post magmatic hydrothermal fluids enriched in Zr, Ti, REEs, and actinide elements. Both mottanaite-(Ce) and ciprianiite have a vitreous luster and are non-fluorescent. Cleavage is absent in mottanaite-(Ce), fair to good in ciprianiite, {100}. Twinning is frequently observed in ciprianiite. Due to the strong intra-crystalline chemical zoning and twinning, physical properties could be measured only for mottanaite-(Ce). [D.sub.meas] is 3.61(4) g/[cm.sup.3], [D.sub.calc] is 3.88 g/[cm.sup.3]. Mottanaite-(Ce) is biaxial negative, with [alpha] = 1.680(5), [beta] = 1.694(2), [gamma] = 1.708(5); 2[V.sub.meas] ~90 [degrees]. Both minerals are monoclinic, space group P2/a, Z = 2. Unit-cell parameters for the crystals studied are: a = 19.032(9) [Angstrom], b = 4.746(3) [Angstrom], c = 10.248(5) [Angstrom], [beta] = 110.97(5) [degrees], V = 864.3(8) [[Angstrom].sup.3] for mottanaite-(Ce), and a = 19.059(5) [Angstrom], b = 4.729(1) [Angstrom], c = 10.291(4) [Angstrom], [beta] = 111.33(2) [degrees], V = 864.0(5) [[Angstrom].sup.3] for ciprianiite. Single-crystal structure refinement confirmed the presence of a further distorted tetrahedral site which was first detected in a hellandite-(Ce) sample from Latium (Oberti et al. 1999). This site is occupied by Be ([+ or -] Li) in stoichiometric mottanaite-(REE), whereas it mainly hosts hydrogen (bonded to the 05 oxygen atom) in ciprianiite and hellandite-(REE); solid solution between the end-members is possible, as shown by the studied samples. The chemical composition of the refined crystals was obtained by combining EMPA (for medium-Z elements) and SIMS analyses (for low- and high-Z elements); their results are in excellent agreement with the chemical information obtained from the structure refinements. The crystal-chemical formulae of the crystals studied, recalculated on the basis of 24 anions, are [sup.M3,4][Ca.sup.M2.sub.4] [[RE[E.sub.1.45][Ca.sub.0.37][(Th,U).sup.4+.sub.0.17] [Y.sub.0.01]].sub.[SIGMA]=2][sup.M1][([Al.sub.0.50] [Fe.sup.3+.sub.038][Mg.sub.0.03] [Ti.sup.4+.sub.0.07]).sub.[SIGMA]=0.99][sup.T] ([Be.sub.1.18][Li.sub.0.02][[square].sub.0.37]) [B.sub.3.99][Si.sub.3.98][O.sub.22][sup.O5][([O.sup.2-.sub.1.04] [F.sub.0.53]O[H.sub.0.43]).sub.[SIGMA]=2] for mottanaite-(Ce); and [sup.M3,4][Ca.sub.4][sup.M2][[RE[E.sup.3+.sub.0.72] [(Th,U).sup.4+.sub.0.66][Ca.sub.0.60][Y.sub.0.02]].sub.[SIGMA]=2] [sup.M1][([Al.sub.0.48][Fe.sup.3+.sub.0.38][Ti.sup.4+.sub.0.10][Mg.sub.0.05] [Mn.sup.3+.sub.0.02]).sub.[SIGMA]=1.03][sup.T] ([Be.sub.0.82][[square].sub.0.60][Li.sub.0.04]) [B.sub.4.00][Si.sub.4.00][O.sub.22][sup.O5][([O.sup.2-.sub.0.97]O [H.sub.0.54][F.sub.0.49]).sub.[SIGMA]=2] for ciprianiite.

The crystal structure of a Th-U-rich, Y-poor hellandite-(Ce) occurring in a volcanic ejectum from Capranica (Vico volcanic complex, Latium, Italy) was refined to an R index of 1.5% for 2226 observed reflections [I > 3 [Sigma](I)]. Hellandite from Capranica is monoclinic P2/a with a = 19.068 (8), b = 4.745 (2), c = 10.289 (3) [Angstrom], [Beta] = 111.18 (3)[degrees]. The extra-framework (distorted) tetrahedral cavity, usually occupied by H in hellandite, is partially (35-40%) occupied by Be and/or Li. When this is the case, the [[B.sub.4][Si.sub.4][O.sub.22]] tetrahedral chains of hellandite are locally cross-linked to form a sheet, which is built up by the same types of rings as in semenovite but in a different arrangement. The structure refinement and the electron- and ion-microprobe analyses provide constraints on hellandite-group crystal chemistry. The presence of (4)(Be, Li) at the Be site is coupled with the substitution of F and/or O for OH at the O5 site; the amount of extra-framework cations thus constrains the OH content in the general formula. X-ray data exclude the presence of other OH sites in the structure, in agreement with direct H determination by ion probe (1.02 H apfu + 0.98 F apfu = 2.0 apfu of monovalent anions at 05 vs. 4.5-8 apfu proposed previously). The complete chemical analysis also eliminates the need for a vacancy at the REE sites. The general formula of hellandite-group minerals is thus[(Ca,REE).sub.8] (Th,U,Y,REE).sub.4][ Ti,[Fe.sup.3+],Al).s b.2][(O,F,O[H.sub.x]).sub.4][(Be,Li).sub.4-x][Si.sub.8][B.sub.8][O.sub.44], where electroneutrality is assured by the appropriate mixing of heterovalent substituents at the M sites.

In this work, we address the kinetics of dehydrogenation occurring at high temperatures (HT) in riebeckite, a sodic amphibole with the ideal composition Na 2 Fe 2 3 + Fe 3 2 + Si 8 O 22 (OH) 2. ${\rm{N}}{{\rm{a}}_2}{\rm{Fe}}_2^{3 + }{\rm{Fe}}_3^{2 + }{\rm{S}}{{\rm{i}}_8}{{\rm{O}}_{22}}{({\rm{OH}})_2}.$ We performed isothermal experiments on both powders and single-crystals up to 560 °C and monitored the O-H stretching signal by Fourier transform infrared (FTIR) spectroscopy. Single-crystals show an initial increase in IR absorption intensity due to increasing vibrational amplitudes of the O-H bond stretching, not observed for powders. The OH-intensities vs. time were fitted using the formalism for first-order reactions. The calculated activation energies for H+ diffusion in riebeckite are 159 ± 15 kJ/mol for powders and 216 ± 20 kJ/mol for single crystals, respectively. The exponential factor m in the Avrami-Erofeev equation obtained for crystals ranges between 1.02 and 1.31, suggesting that, unlike powders, the dehydration process in crystals is not a purely first-order reaction. This implies that a second energy barrier must be considered, i.e., diffusion of H+ through the crystal. FTIR imaging showed that H+ diffusion occurs mainly perpendicular to the silicate double-chain. Our results confirm that the release of H+ from riebeckite occurs after the irreversible Fe2+-to-Fe3+ exchange, thus at temperatures >550 °C. To be effective, the process needs the presence of external oxygen that, by interacting with H+ at the crystal surface, triggers the release of H2O molecules. This implies that oxidizing conditions are required for the amphibole to be an efficient water source at depth. [ABSTRACT FROM AUTHOR]

Amphiboles were synthesized along the joins tremolite-fluorotremolite, richterite-fluororichterite and potassic-richterite-potassic-fluororichterite at 750 [degrees] C and 1 kbar P([H.sub.2]O). Infrared spectra of the amphiboles were recorded in the principal OH-stretching region. Amphiboles of the tremolite-fluorotremolite series show one-mode behavior, a single band due to a local [Mathematical Expression Omitted] ([Mathematical Expression Omitted] = vacancy) arrangement; this behavior is consistent with no coupling between NNN (next-nearest-neighbor) 03 anions either through the O3-O3 edge or across the vacant A-site cavity. The amphiboles of the richterite-fluororichterite and potassic-richterite-potassic-fluororichterite series show two-mode behavior, two bands due to the local arrangements MgMgMg-OH-[.sup.A]Na-OH and MgMgMg-OH-[.sup.A]Na-F (and their K equivalents); this behavior is consistent with coupling between NNN O3 anions across the filled A-site cavity through Na or K that occupies the A-site. A mathematical model is developed to describe local (OH,F) ordering in amphiboles as a function of F content. The variation in infrared band intensities is consistent with complete short-range disorder of OH and F in the synthetic amphiboles of the richterite-fluororichterite and potassicrichterite-potassic-fluororichterite series.

Amphiboles were synthesized at 750 [degrees] C, 1 kbar ([H.sub.2]O) for compositions at 20% intervals along the join potassium-richterite-tremolite. Structural variations, site occupancies, and modal analyses of the experimental products (amphibole + minor diopside, quartz, and enstatite) were characterized by Rietveld structure refinement, with final [R.sub.Bragg] indices in the range 4-6%, and by infrared spectroscopy in the principal OH-stretching region. Amphibole compositions were determined by (1) site-scattering refinement for the A and M4 sites that are occupied by (K, [square]) ([square] = vacancy) and (Na,Ca), respectively; and (2) mass-balance calculations involving the modal analysis and the nominal experimental product composition. These measurements agree within 1% absolute and show close agreement with electron-microprobe compositions for the two samples that we could analyze. Deviations from nominal amphibole composition are up to 19% absolute. The resulting relations between cell dimension and composition are linear. The major change in cell dimensions is a decrease of 0.25 [Angstrom] in a with increasing tremolite component. The infrared spectra show two principal peaks at 3735 and 3675 [cm.sup.-1], corresponding to the local arrangements MgMgMg-OH-AK (the Kr band) and MgMgMg-OH-A[square] (the Tr band), respectively. The relative variation in peak intensity as a function of amphibole composition shows that the molar absorptivities of the two bands are significantly different. The ratio of the molar absorptivities for the two bands is 2.2.

We used the JUMBO monochromator at SSRL to measure the Al K-edge X-ray absorption spectra of synthetic micas having variable Al content and occupancy, from 0 to 2/3 in the octahedral M positions, and 0 to 2/4 in the tetrahedral T positions. The measured Al K edges differ markedly, but the differences may have a common explanation: (1) Micas containing 1/3 Al in M or 1/4 Al in T have K edges that differ in the energy and intensity of the first two features, which are related to interaction of Al with its first-shell nearest neighbors (O and OH or F). They are nearly identical to the K edges of reference minerals such as albite (tetrahedral Al only) or grossular (octahedral Al only). (2) Micas containing Al in both M and T have K edges that can be interpreted as a weighed combination of the simple edges.

Amphiboles were synthesized at 750 [degrees] C, 1 kbar ([H.sub.2]O) on the binary joins (nickel, magnesium)-richterite and (magnesium, cobalt)-richterite. Structural variations and site occupancies were characterized by Rietveld structure refinement, with final [R.sub.Bragg] indices in the range 4-9%, and by powder infrared spectroscopy in the principal OH-stretching region. Site-occupancy refinement of Ni-Mg and Mg-Co distributions give the partition coefficients over M1,3 and M2 where [K.sup.M2+] = [([M.sup.2+]/Mg).sub.M1,3]/[([M.sup.2+]/Mg).sub.M2], and [M.sup.2+] = [Ni.sup.2+] or [Co.sup.2+], [Mathematical Expression Omitted] and [Mathematical Expression Omitted]. Both [K.sub.d] values are greater than 1.0, whereas [6]r([Ni.sup.2+]) < [6]r(Mg) < [6]r([Co.sup.2+]); this indicates that cation size is not the primary factor affecting the ordering of Ni-Mg and Mg-Co over the octahedral sites. The infrared spectra of intermediate binary compositions show fine structure caused by ordering of Ni-Mg or Mg-Co over the M1,3 sites and by ordering of Na and [square] (vacancy) at the A site; thus intermediate compositions show an eight-band spectrum in the principal OH-stretching region. Precise band intensities were derived by nonlinear least-squares fitting of Gaussian band shapes to the observed spectra. The relative observed intensities of the combinations of bands [Mathematical Expression Omitted] and [Mathematical Expression Omitted] are in accord with the equations of Burns and Strens (1966), indicating that there is no significant variation in molar absorptivity with frequency (energy) for individual bands within a single sample (spectrum). Combined with the results of Skogby and Rossman (1991) on polarized single-crystal infrared spectra of amphiboles, this result suggests that different local configurations of M1,3 cations in amphiboles couple such that the transition probabilities of the associated OH groups are equal.

By stacking the grain-size data interpreted from many logs of borings (down to 200 m deep) in a sector of the Po River valley, we have obtained a curve of the clay + silt fraction vs. depth that matches the oxygen isotope curve of paleotemperatures for the late Quaternary. Contour maps and plots for selected subzones show that the northern half of the study area is characterized by this correlation, whereas the southern part adds only a random low-amplitude noise to the curve. The suggested dependence of grain size of fluvial deposits on climate in a part of the study area would provide an indirect way to date these not otherwise datable sediments, allowing an evaluation of the sedimentation rates and of the total subsidence since the last interglacial.

The Rietveld structure refinement of amphiboles which are prepared along with the K(NaCa)Mg5(SiTi)O22(OH,F)2 from the (OH)2 to F2 atoms per formula unit (ap fu) indicates that the positively ionized Ti is fully ordered at the T2 site. The single-phase amphibole with 0.6 ap fu of titanium is determined for the OH end-member beginning composition. The complex amphibole-priderite-titanite-rutile, produced along the OH-F, joins with a reduction in the end-member fluor-richterite to 0.16 ap fu. A primary band for the titanium-rich OH-end member located at 3728 lines per centimeter is found in the infrared spectra of the principal OH-stretching region.

The synthesis and characterization of potassium richterite containing variable amounts of Nickel, Magnesium and Cobalt in octahedral strips is a result of the hydrothermal syntheses. For all compositions in the system, the monophase amplibole products were obtained at 750 degree C, 1kbar (H2O). The variations in structure and site occupancies were marked by Rietveld structure refinement, with final R (Bragg) indices ranging from 2-4%. The individual cell dimensions along the Ni-Mg-Co revealed a break at the Mg composition, suggesting that constituent cation sizes determine the cell volume and grand mean (M-O) bend length, and not the substituent cations.