Characterisation of tobelite (NH4)Al2[AlSi3O10](OH)2 and ND4-tobelite (ND4)Al2[AlSi3O10](OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

Tobelite (NH₄) Al₂ [AlSi₃O₁₀] (OH)₂, the ammonium analogue of muscovite, and its deuterated form ND₄-tobelite (ND₄) Al₂ [AlSi₃O₁₀] (OD)₂ have been synthesised at 600?°C and 200 and 500 Mpa using a well homogenised, stoichiometric SiO₂-Al₂O₃ oxide mix with Al₂O₃ in excess of 5 mol% and a 25% NH₃ solution whose relative abundance was such that the amount of NH₄ ⁺ stoichiometrically available was in excess of 50%. Characterisation of both tobelite and ND₄-tobelite using IR-spectroscopy, Rietveld refinement of X-ray powder diffraction data, and electron microprobe analysis indicate that, similar to K⁺ in muscovite, the NH₄ ⁺ or ND₄ ⁺ molecule occupies the interlayer site. IR absorption bands caused by NH₄ ⁺ and ND₄ ⁺ can be explained, to a very good approximation, on the basis of T_d symmetry. Nevertheless, substantial line broadening and the occurrence of shoulders indicate a deviation from ideal T_d symmetry. However, even at 77?K, no discrete splitting of the degenerate states could be confirmed. The OH stretching frequencies observed for synthetic tobelite are quite similar to those for muscovite, indicating that the replacement of K⁺ by NH₄ ⁺ has no effect. The low FWHH of the OH bands indicate that the hydroxyl groups are well ordered within the structure. Rietveld refinement of tobelite and ND₄-tobelite indicates that all samples synthesised consist of the 3 different mica polytypes which are typical of muscovite – namely 1M (C2/m), 2M ₁ (C2/c) and 2M ₂ (C2/c). Tobelite and ND₄-tobelite synthesised at 500 Mpa principally contain the 1M polytype, whereas the principle polytype for ND₄-tobelite synthesised at 200 Mpa, is 2M ₂. Rietveld refinement of X-ray diffraction spectra for tobelite synthesised at 200 Mpa was problematic due to the very broad FWHH of the X-ray peaks indicating poor crystallinity. In comparision to synthetic muscovite, the cell dimensions observed for tobelite and its deuterated analogue are quite similar except for the lattice constant c. Due to the larger radius of NH₄ ⁺ or ND₄ ⁺ compared to K⁺ cation, the c-direction is expanded form 10.275 Å in muscovite to approximately 10.540 Å in tobelite and ND₄-tobelite.     

Characterization of NH4-phlogopite (NH4) (Mg3) [AlSi3O10] (OH)2 and ND4-phlogopite (ND4) (Mg3) [AlSi3O10] (OD)2 using IR spectroscopy and Rietveld refinement of XRD spectra

 A synthesis technique is described which results in >99% pure NH₄-phlogopite (NH₄) (Mg₃) [AlSi₃O₁₀] (OH)₂ and its deuterium analogue ND₄-phlogopite (ND₄) (Mg₃) [AlSi₃O₁₀] (OD)₂. Both phases are characterised using both IR spectroscopy at 298 and 77 K as well as Rietveld refinement of their X-ray powder diffraction pattern. Both NH₄ ⁺ and ND₄ ⁺ are found to occupy the interlayer site in the phlogopite structure. Absorption bands in the IR caused by either NH₄ ⁺ or ND₄ ⁺ can be explained to a good approximation using T_d symmetry as a basis. Rietveld refinement indicates that either phlogopite synthesis contains several mica polytypes. The principle polytype is the one-layer monoclinic polytype (1M), which possesses the space group symmetry C2/m. The next most common polytype is the two-layer polytype (2M ₁ ) with space group symmetry C2/c. Minor amounts of the trigonal polytype 3T with the space group symmetry P3₁12 were found only in the synthesis run for the ND₄-phlogopite. Electron microprobe analyses indicate that NH₄-phlogopite deviates from the ideal phlogopite composition with respect to variable Si/Al and Mg/Al on both the tetrahedral and octahedral sites, respectively, due to the Tschermaks substitution ^VIMg²⁺+^IVSi⁴⁺↔^VIAl³⁺+^IVAl³⁺ and with respect to vacancies on the interlayer site due to the exchange vector ^XII(NH₄)⁺+^IVAl³⁺↔^XII□+^IVSi⁴⁺. Received: 30 August 1999 / Accepted: 2 October 2000     

Hydrothermally grown buddingtonite,an anhydrous ammonium feldspar (NH4AlSi3O8)

Ammonium feldspar was grown hydrothermally from a gel, having a stöchiometric Al₂O₃·6SiO₂ composition. As a source for NH ₄ ⁺ , a 25 percent NH₃ solution was used. Internal Cr/CrN and graphite/methane buffers fixed the fugacity of NH₃ during the experiments. Unit cell parameters of the synthetic ammonium feldspar are a: 0.8824 (5) nm, b: 1.3077 (8) nm, c: 0.7186 (4) nm, β: 116.068 (12)°, V: 0.7448 (34) nm₃. the X-ray power diffraction pattern is measured and indexed in accordance to the space group C2/m. Infrared and thermal gravimetric analyses provide no evidence for the presence of structurally bound water molecules in the crystal structure of synthetic ammonium feldspar. Hydrothermally grown anhydrous ammonium feldspar is shown to be identical to the mineral buddingtonite by the similarity of the data between the synthetic and natural materials. There may be justification for considering natural buddingtonite as an anhydrous feldspar with the ideal formula NH₄Si₃O₈. Reexamination of natural specimens is desirable.     

Synthesis, structure, thermodynamic properties, and stability relations of K-cymrite, K[AlSi3O8]·H2O

Single-phase K-cymrite, K[AlSi₃O₈]·H₂O, has been synthesized in the P-T range 3≤P(GPa)≤4 and 350≤T(°C)≤650, and characterized by a variety of techniques like SEM, FTIR, and ²⁹Si MAS-NMR. Its thermal expansivity and compressibility have been measured up to 375?°C and 6.0?GPa, respectively. Within the uncertainty of the microchemical determination of H₂O by Karl-Fischer titration, it invariably contains 1?mol of H₂O per mol of KAlSi₃O₈. Under the SEM, it appears a small idiomorphic prisms. It is optically negative, with n _o=1.553(1) and n _e=1.521(1). FTIR spectrum identifies the water in its structure as molecular H₂O. Its lattice constants are a=5.3348(1)?Å, c=7.7057(1) Å, V= 189.924 ų, the space group being P6/mmm. The ²⁹Si MAS-NMR suggests a weak short-range order of Al and Si in the symmetrically equivalent tetrahedral sites. A Rietveld structure refinement demonstrates that it is isostructural with cymrite (BaAl₂Si₂O₈·H₂O), the structure comprising double tetrahedral sheets with H₂O molecules residing in their cavities, K serving as an interlayer cation. Whereas cymrite, with its ordered tetrahedral Al/Si distribution, shows a Pm symmetry, the weak short-range Al/Si order in K-cymrite (abbreviated below as KCym) makes it crystallize in the space group P6/mmm. Three reversal experiments on the reaction K[AlSi₃O₈]·H₂O (KCym)=K[AlSi₃O₈] (Kfs)+H₂O, executed in this study, confirm the earlier results of Thompson (1994) and supplement her data. A simultaneous treatment of those reversals, together with the thermodynamic data for Kfs and H₂O available in the literature, helps derive the standard enthalpy of formation (?4233±9.4?kJ/mol) and standard entropy (276.3±10.2 J/K·mol) for K-cymrite. The computed phase relations of KCym in the KAlSi₃O₈-H₂O binary are shown in Figure 4 for three different values of a_{H 2}O. Given a 5?°C/km isotherm in a subducting slab of metasediments in a ultra-high-pressure metamorphic environment, KCym will be expected to grow by hydration of Kfs, unless the a_{H 2}O had been substantially less than 0.5. Nevertheless, how far it can survive exhumation of the subducted terrain will depend critically on the rate of uplifting and on the a_{H 2}O prevailing during that process.     

Low-temperature behaviour of ammonium ion in buddingtonite [N(D/H)<Subscript>4</Subscript>AlSi<Subscript>3</Subscript>O<Subscript>8</Subscript>] from neutron powder diffraction

The structural response of buddingtonite [N(D/H)₄AlSi₃O₈] on cooling has been studied by neutron diffraction. Data have been collected from 280 K down to 11 K, and the crystal structure refined using the Rietveld method. Rigid-body constraints were applied to the ammonium ion to explore the structural properties of ammonium in the M-site cavities at low-temperature. Low-temperature saturation is observed for almost all the lattice parameters. From the present in situ low-temperature neutron diffraction studies, there is no strong evidence of orientational order–disorder of the ammonium ions in buddingtonite.     

Ammonium ion behaviour in feldspar: variable-temperature infrared and 2H NMR studies of synthetic buddingtonite, N(D,H)4AlSi3O8

The behaviour of the ammonium ion in synthetic buddingtonite, N(D,H)₄AlSi₃O₈, has been studied by infrared (IR) spectroscopy from 20 K to 298 K and by ²H NMR spectroscopy from 120 K to 298 K. IR spectra were collected from 500 to 3500 cm^–1. Static ²H NMR spectra collected at 298 K and 120 K are very similar, consisting of a single sharp isotropic resonance, indicating complete averaging of quadrupolar interactions and implying that at these temperatures the ammonium ion is in rapid (<1 s) randomised motion within the M-site cavity of the feldspar framework. NMR spectroscopy indicates that the splitting of the internal modes of the ammonium ion observed by IR spectroscopy is not due to freezing in of the ammonium ion. This observation rules out the formation of a preferred N–H...O hydrogen bond, with precession of the ion about it, as proposed by Kimball and Megaw (1978), because any N–H...O hydrogen bond must be very weak and transient in nature. Contraction of the cavity site upon cooling imposes a distortion upon the ammonium ion that affects vibrational modes. This distortion does not affect the motion of the ammonium ion as observed on the NMR time-scale.     

IR spectroscopic characterization of NH4-analcime

 The IR spectrum of ammonium-exchanged natural analcime (basalt, Nidym River, Siberian platform) exhibits several features that suggest a lowered symmetry for the NH₄ ⁺ ion and that this is influenced by hydrogen bonding within the framework. These features are: the pronounced splitting into three components and high-frequency shift of the ν₄-bending mode; appearance of the ν₁-stretching mode which is predicted to become IR-active when the ideal T _d symmetry of NH₄ ⁺ ion is violated, and the low-frequency shift of the ν₁- and ν₃-stretching modes. The absence of absorption lines in the 1800–2400-cm⁻¹ region indicates that hydrogen bonding between the framework and the NH⁺ ₄ ion is very weak. The three-component splitting of the ν₄-bending mode indicates that the symmetry of NH⁺ ₄ ion is lower than C_3v. This implies that at least two N–H bonds of the NH⁺ ₄ ion are disturbed by hydrogen bonding. Computer analysis of the normal vibrations of the NH⁺ ₄ molecule for different symmetry types (using harmonic approximation) indicates that the best fit to the observed ν₄ triplet frequencies for C₁ symmetry implies a deviation of the valent angle ∠H–N–H from ideal T _d symmetry of around ±2.5°. The factors governing the behaviour of the NH⁺ ₄ ion in the analcime structure are discussed. The geometry of the nearest environment of the NH⁺ ₄ ion in the analcime structure is analyzed with respect to the present IR data. Received: 2 January 2002 / Accepted: 26 June 2002 Acknowledgements We thank Dr. D. Harlov and an anonymous reviewer for their helpful comments, as well as Dr. I.A. Belitsky and Dr. S.V. Goryainov for discussion of the material. This work is supported by RFBR grants 01-05-65414, 00-05-65305 and 02-05-65313.     

Ammonium halos in lithogeochemical exploration for gold at the Horse Canyon carbonate-hosted deposit,Nevada, U.S.A.: use and limitations

Rock samples taken from the oxidized, open pit (to depths of 55 m) of the carbonate-hosted Horse Canyon gold deposit contain no NH₄⁺. Samples from the unoxidized zone at the bottom of the pit in the “protore”, however, contain as much as 1.44% NH₄⁺ (1.12% N); the presence of NH₄⁺ was confirmed by mass spectrometry. From these results it is concluded that samples from any oxidized environment (e.g. outcrops, soils) likely will have lost all, or most, of any NH₄⁺ originaly present in minerals such as buddingtonite of NH₄⁺-containing clays. This limits the use of NH₄⁺ halos in lithogeochemical exploration programs. The most reliable application of lithogeochemical studies based on NH₄⁺ halos will likely be based on samples from drill cores and other situations where the samples have always been in a reducing environment.Analysis for NH₄⁺ by IR spectroscopy, the method preferred by most previous workers, is subject to several inherent problems, the main one being the presence of the CO₃²⁻ ion whose absorbance at about 1430 cm⁻¹ coincides with that used to determine NH₄⁺. The removal of CO₃²⁻ and other phases (e.g. organic matter) which may cause analytical difficulties by chemical treatment is not recommended because it is not possible to predict the effect of such treatment on any one of several possible minerals which may contain NH₄⁺. It is suggested that the most efficient method of analysis of rock samples for NH₄⁺ (or N which can be assumed to represent NH₄⁺) will be to analyze the samples after heating to 550°C, at which temperature all organic NH₄⁺ (and N) is removed by oxidation. Any remaining NH₄⁺ (N) determined on this residue can be assumed to originate in NH₄⁺-containing mineral structures.     

Crystal chemistry of selenates with mineral-like structures: V. Crystal structures of (H3O)2[(UO2)(SeO4)2(H2O)](H2O)2 and (H3O)2[(UO2)(SeO4)2(H2O)](H2O), new compounds with rhomboclase and goldichite topology

The crystal structures of two new compounds (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O)₂ (1, orthorhombic, Pnma, a = 14.0328(18), b = 11.6412(13), c = 8.2146(13) Å, V = 134.9(3) ų) and (H₃O)₂[(UO₂)(SeO₄)₂(H₂O)](H₂O) (2, monoclinic, P2₁/c, a = 7.8670(12), b = 7.5357(7), c = 21.386(3) Å, β = 101.484(12)°, V = 1242.5(3) ų) have been solved by direct methods and refined to R ₁ = 0.076 and 0.080, respectively. The structures of both compounds contain sheet complexes [(UO₂)(SeO₄)₂]^2? formed by cornershared [(UO₂)O₄(H₂O)] bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (100) plane in structure 1 and to (?102) in structure 2. The [(UO₂)(SeO₄)₂(H₂O)]^2? layers are linked by hydrogen bonds via interlayer groups H₂O and H₃O⁺. The sheet topologies in structures 1 and 2 are different and correspond to the topologies of octahedral and tetrahedral complexes in rhomboclase (H₂O₂)⁺[Fe(SO₄)₂(H₂O)₂] and goldichite K[Fe(SO₄)₂(H₂O)₂](H₂O)₂, respectively.     

Mg4Ga8Ge2O20: A new synthetic analog of the mineral sapphirine

A new sapphirine-analog, Mg₄Ga₈Ge₂O₂₀, has been identified and characterized in the system MgO-Ga₂O₃-GeO₂ in samples quenched from 1400° C, 1 atm. It crystallizes with a triclinic unit-cell and is isostructural with the mineral sapphirine-1Tc, Mg₄Al₈Si₂O₂₀. It is stable over a broad range of stoichiometry associated with the exchange reaction 2 Ga³⁺ = Mg²⁺ }+Ge⁴⁺. TEM examination shows the common occurrence of polysynthetic twinning and suggests the existence of a high-temperature triclinic→monoclinic transition. The observation of sapphirine/spinel intergrowths is consistent with the structural relationships described previously.     

Crystal chemistry of selenates with mineral-like structures. VI. Hydrogen bonds in the crystal structure of [(H5O2)(H3O)(H2O)][(UO2)(SeO4)2]

The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) Å, β = 103.880(13)°, V = 1182.4(3) ų), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]^2? sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the ( $ \bar 1 The crystal structure of a new compound, [(H₅O₂)(H₃O)(H₂O)][(UO₂)(SeO₄)₂] (monoclinic, P2₁/n a = 8.3105(15), b = 11.0799(14), c = 13.227(2) ?, β = 103.880(13)°, V = 1182.4(3) ?³), has been solved by direct methods and refined to R ₁ = 0.036. The structure is based on [(UO₂)(SeO₄)₂]²⁻ sheet complexes formed by corner-shared UO₇ pentagonal bipyramids and SeO₄ tetrahedrons. The sheets are parallel to the (01) plane. Oxonium ions and water molecules forming [(H₃O)·(H₂O)·(H₅O₂)]²⁺ complexes are interlayer. Among minerals, the existence of (H₅O₂)⁺ has been unambiguously confirmed only in rhomboclase, (H₅O₂)⁺[Fe₂(SO₄)₂(H₂O)₂]. Original Russian Text ? S.V. Krivovichev, 2008, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2008, No. 2, pp. 123–130.     

Channel constituents in synthetic beryl: ammonium

 The infrared spectra and electron paramagnetic resonance (EPR) of channel constituents in beryls synthesized hydrothermally in the presence of NH₄Cl were investigated. Two forms of ammonium ion were observed to be incorporated into the c -channel. IR-spectra show the double band at 3295 and 3232 cm⁻¹ and two broad bands between 2600 and 3000 cm⁻¹ which were assigned to the NH₃ molecule and NH₄ ⁺ ion, respectively. Similar N–H stretching vibrations are also observed in Regency hydrothermal synthetic beryls and can be used to separate these synthetic beryls from their natural counterparts. After γ-irradiation of hydrothermally grown samples at 77 K, the EPR of the NH₃ ⁺(I) radical was observed. The NH₃ ⁺(II) radical replaces the NH₃ ⁺(I) radical when the sample is heated to room temperature. Both the NH₃ molecule and the NH₃ ⁺ radical have their C₃ symmetry axes perpendicular to the crystal c-axis. The spin Hamiltonian parameters of the NH₃ ⁺(I) are axial-symmetric due to the rapid rotation of the radical about the c-axis. The NH₃ ⁺(II) radical has a low symmetry and shows a hindered rotation because of its shift from the c-axis position and an interaction with the proton in the near neighbourhood. Possible models for the paramagnetic centres are discussed. Received: 16 May 2000 / Accepted: 5 July 2001     

Distribution and isotope composition of nitrogen in Bay of Quinte (Lake Ontario) sediments

The concentrations and isotopic compositions of the various forms of nitrogen in silty clay sediments from the Bay of Quinte (Lake Ontario) have been determined. The total organic-N content is high throughout the sediment profiles and generally decreases with depth. On the contrary, exchangeable NH⁺₄-N concentration is quite low and tends to increase with depth in two out of three sediment cores examined. The concentration of non-exchangeable NH⁺₄-N and the 6 N HCl hydrolyzable NH⁺₄-N are relatively constant with depth. Among the N fractions analyzed, the exchangeable NH⁺₄-is most enriched in ¹⁵N. In most cases, the δ ¹⁵ N values of the N fractions remain relatively constant with sediment depth. There is no apparent correlation of δ ¹⁵ N values with the N concentration for any of the individual N fractions. The observed ranges in the δ ¹⁵ N values are: exchangeable NH⁺₄, + 5–+10‰; 6 N HCl hydrolyzable total N and 6 N HCl hydrolyzable NH⁺₄-N, + 3.5–+5.5‰.     

Cymrite as mineral clathrate: An overlooked redox insensitive transporter of nitrogen in the mantle

We study the phase relations and mineral chemistry in the systems muscovite–NH₃–N₂-H₂O and eclogite + muscovite–NH₃–N₂-H₂O at 6.3–7.8 GPa, 1000–1200 °C, and oxygen fugacity (fO₂) from ∼IW (Fe–FeO) to ∼ NNO (Ni–NiO) equilibria. The quenched H₂O-bearing fluids differ in nitrogen speciation from NH₃-rich to N₂–rich, and the respective N₂/(NH₃+N₂) ratio varies from <0.1 to ∼ 1. N-bearing K-cymrite is obtained in association with a kyanite-garnet-jadeite ± muscovite ± corundum assemblage in the muscovite–NH₃–N₂-H₂O system and coexists with pyrope-almandine garnet and omphacite in the eclogite + muscovite–NH₃–N₂-H₂O system. The presence of an N-bearing fluid in the studied systems stabilizes the K-cymrite structure. Muscovite does not convert to K-cymrite in the absence of NH₃–N₂-bearing fluid up to 7.8 GPa and 1070–1120 °C. According to FTIR and Raman spectroscopy, K-cymrite in equilibrium with an N-rich fluid can capture a huge amount of nitrogen in cages of its framework, mainly as N₂ molecules at fO₂ ∼NNO and predominantly as NH₃ molecules at fO₂ ∼IW. The storage capacity of K-cymrite with respect to nitrogen increases from 2.9 to 6.3 wt% with increase of fO₂. FTIR spectroscopy of muscovite equilibrated with K-cymrite shows that the clathrate mechanism of nitrogen entrapment by aluminosilicates (as neutral N₂ and NH₃ molecules) is much more efficient than the K⁺ → (NH₄)⁺ substitution. The structure of N-bearing K-cymrite (K,(NH₄⁺))[AlSi₃O₈]·(N₂,NH₃,H₂O) determined using X-ray single-crystal diffraction is very similar to that of H₂O-bearing K- and Ba-cymrites. It includes aluminosilicate layers consisting of double six-membered tetrahedral rings and cation sites statistically occupied with K⁺, Ba²⁺ and (NH₄)⁺ on the six-fold symmetry axis in interlayer space. The N₂ and NH₃ molecules are located near the cage centers and, unlike H₂O molecules, are included in the coordination environment of the cations. Our study confirms that NH₃- and N₂-rich K-cymrite may be stable in metapelites and can act as a redox insensitive carrier of nitrogen to >250 km mantle depths in downgoing slabs. The stability field of N-rich K-cymrite in the presence of an N₂–H₂O–NH₃-bearing fluid is inferred to be P ≥ 4 GPa in metasediments rich in K-feldspar and P ≥ 6 GPa in those containing phengite. As the slab material sinks deeper than 250–300 km where N-bearing K-cymrite may lose stability, the releasing nitrogen may migrate to metal-saturated mantle and become stored there in γ−Fe, Fe₃C, metal melt, or even iron nitride phases.     

Kanonaite, (Mn 0.76 3+ Al0.23Fe 0.02 3+ )[6]Al[5][O¦SiO4], a new mineral isotypic with andalusite

Kanonaite forms rare porphyroblasts up to 12mm long in a gahnite— Mg-chlorite — coronadite — quartz schist occurring near Kanona, Zambia. The composition is (microprobe analysis): SiO₂ 32.2, Al₂O₃ 33.9, Mn as Mn₂O₃ 32.2, Fe₂O₃ 0.66, ZnO 0.13, MgO 0.04, BaO 0.04, TiO₂ 0.01, CaO 0.01, PbO 0.01, CuO 0.01, total 99.21, corresponding to $$\left( {{\text{Mn}}_{{\text{0}}{\text{.76}}}^{{\text{3 + }}} {\text{Al}}_{{\text{0}}{\text{.23}}} {\text{Fe}}_{{\text{0}}{\text{.015}}}^{{\text{3 + }}} } \right)_{1.005}^{\left[ 6 \right]} {\text{AL}}_{1.00}^{\left[ 5 \right]} \left[ {{\text{O}}_{{\text{1}}{\text{.00}}} |{\text{Si}}_{{\text{0}}{\text{.99}}} {\text{O}}_{{\text{4}}{\text{.00}}} } \right]$$ The mineral is greenish black, strongly pleochroic with X(∥a) yellow green, Y(∥b) bluish green, Z(∥c) deep golden yellow, biaxial positive, with 2V = 53°(3°), α = 1.702, β = 1.730, γ = 1.823. Vickers microhardness (100 gram load) ranges between 906 and 1017kp/mm². The structure is orthorhombic, isotypic with andalusite, space group Pnnm, a = 0.7953(2), b = 0.8038(2), c = 0.5619(2) nm, V = 0.3592(1) nm³, a/b = 0.9895(3), c/b = 0.6990(3), S.G.(_x) = 3.395 g/cm³, Z = 4. The strongest X-ray powder lines are (d in nm, I, hkl):0.5669, 100, 110; 0.4590, 75, 011 and 101; 0.3577, 90, 120 and 210; 0.2827, 94, 220; 0.2517, 90, 310 and 112; 0.2212, 83, 320, 122 and 212. Comparison of the intensities of 373 observed X-ray reflections with those calculated for several models of Mn³⁺-distribution indicates octahedral coordination of all or most of the manganese present. Interpretation of magnetic measurements (μ_eff = 3.15B.M. per Mn atom at 25 ° C) indirectly supports octahedral coordination of Mn³⁺. The name of the mineral is for Kanona, a town near the type locality. The name is proposed for the end member Mn^{3+ [6]}Al^[5][O¦SiO₄] and for members of the solid-solution series towards andalusite with octahedral Mn³⁺>Al. The presently described mineral may be referred to as aluminian kanonaite.     

The crystal and magnetic structure of Li-aegirine LiFe3+Si2O6: a temperature-dependent study

Single crystals of Li-aegirine LiFe³⁺Si₂O₆ were synthesized at 1573?K and 3?GPa, and a polycrystalline sample suitable for neutron diffraction was produced by ceramic sintering at 1223?K. LiFe³⁺Si₂O₆ is monoclinic, space group C2/c, a=9.6641(2)?Å, b= 8.6612(3)?Å, c=5.2924(2)?Å, β=110.12(1)^° at 300?K as refined from powder neutron data. At 229?K Li-aegirine undergoes a phase transition from C2/c to P2₁ /c. This is indicated by strong discontinuities in the temperature variation of the lattice parameters, especially for the monoclinic angle β and by the appearance of Bragg reflections (hkl) with h+k≠2n. In the low-temperature form two non-equivalent Si-sites with 〈SiA–O〉=1.622?Å and 〈SiB–O〉=1.624?Å at 100?K are present. The bridging angles of the SiO₄ tetrahedra O3–O3–O3 are 192.55(8)° and 160.02(9)° at 100?K in the two independent tetrahedral chains in space group P2₁ /c, whereas it is 180.83(9)° at 300?K in the high-temperature C2/c phase, i.e. the chains are nearly fully expanded. Upon the phase transition the Li-coordination changes from six to five. At 100?K four Li–O bond lengths lie within 2.072(4)–2.172(3)?Å, the fifth Li–O bond length is 2.356(4)?Å, whereas the Li–O3?A bond lengths amount to 2.796(4)?Å. From ⁵⁷Fe Mössbauer spectroscopic measurements between 80 and 500?K the structural phase transition is characterized by a small discontinuity of the quadrupole splitting. Temperature-dependent neutron powder diffraction experiments show first occurrence of magnetic reflections at 16.5?K in good agreement with the point of inflection in the temperature-dependent magnetization of LiFe³⁺Si₂O₆. Distinct preordering phenomena can be observed up to 35?K. At the magnetic phase transition the unit cell parameters exhibit a pronounced magneto-striction of the lattice. Below T _N Li-aegirine shows a collinear antiferromagnetic structure. From our neutron powder diffraction experiments we extract a collinear antiferromagnetic spin arrangement within the a–c plane.     

Evaluation of Jordanian zeolite tuff as a controlled slow-release fertilizer for NH4 +

 The exchange and release properties of the natural phillipsite tuff from the Aritain area in Jordan were evaluated by studying the exchange properties of this natural zeolite in the NH₄ ⁺–Na⁺ system. Exchange isotherms at 18, 35, and 50  °C showed that phillipsite exchanged NH₄ ⁺ preferably over Na⁺ at all temperatures. However, the selectivity coefficient for NH₄ ⁺ decreased with decreasing temperature. The release of NH₄ ⁺ from phillipsite saturated with ammonium sulfate took place in two stages characterized by different SO₄ ^2– : NH₄ ⁺ ratios. Aritain phillipsite from NE Jordan could be processed and used as NH₄ ⁺ slow-release fertilizers. The use of NH₄ ⁺-phillipsite tuff offers an option to the widely used soluble NH₄-fertilizers in agciculture to avoid environmental problems associated with nitrogen contamination of surface water and groundwater. Received: 19 December 1996 · Accepted: 13 May 1997     

Ammonium excretion by benthic invertebrates and sediment-water nitrogen flux in the Gulf of Mexico near the Mississippi River outflow

Benthic macroinvertebrate biomass and ammonium excretion rates were measured at four stations in the Gulf of Mexico near the Mississippi River mouth. Calculated areal excretion rates were then compared to sediment-water nitrogen fluxes measured in benthic bottom lander chambers at similar stations to estimate the potential importance of macroinvertebrate excretion to sediment nitrogen mineralization. Excretion rates for individual crustaceans (amphipods and decapods) was 2–21 nmoles NH₄ ⁺ (mg dry weight)^?1 h^?1. The mean excretion rates for the polychaetes, Paraprionaspio pinnata [6–12 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1] and Magelona sp. [27–53 nmoles NH₄ ⁺ (mg dry weight)^?1h^?1], were comparable or higher than previous measurements for similar size benthic or pelagic invertebrates incubated at the same temperature (22±1°C). Although the relatively high rates of excretion by these selective feeders may have been partially caused by experimental handling effects (e.g., removal from sediment substrates), they probably reflected the availability of nitrogen-rich food supplies in the Mississippi River plume. When the measured weight-specific rates were extrapolated to total areal biomass, areal macroinvertebrate excretion estimates ranged from 7 μmole NH₄ ⁺ m^?2h^?1 at a 40-m deep station near the river mouth to 18 μmole NH₄ ⁺ m^?2h^?1 at a shallower (28-m deep) station further from the river mouth. The net flux of ammonium and nitrate from the sediments to the water measured in bottom lander chambers in the same region were 15–53 μmole NH₄ ⁺ m^?2h^?1 and ?25–21 μmole NO₃ ^? m^?2h^?1. These results suggest that excretion of NH₄ ⁺ by macroinvertebrates could be a potentially important component of benthic nitrogen regeneration in the Mississippi River plume-Gulf shelf region.     

Thermal decomposition of NH4-analcime

The thermal decomposition of ammonium-exchanged natural analcime is characterized by gas chromatography, IR spectroscopy and X-ray diffraction. The de-ammoniation and dehydroxylation proceed in parallel throughout the decomposition, which evidences the instability of the protonated analcime framework. The mechanism of degassing of NH₄-analcime changes throughout its decomposition. At the initial step, the mechanism of de-ammoniation consists in thermal dissociation of NH₄⁺ molecule onto NH₃ and proton (framework OH group) and diffusion of NH₃ out of the structure. Subsequent decomposition and removal of the OH groups lead to a progressive loss of crystallinity. At this step, an apparent activation energy for NH₃ desorption is estimated to be 145(±13) kJ mol^–1. This value is within the upper limit of the activation energy characteristic for the NH₃ desorption from proton centres in large-pore zeolites. At the final step, the adsorption of NH₃ and protons onto the defect centres in the amorphosed aluminosilicate framework results in a significant increase of an apparent activation energy for the de-ammoniation and dehydroxylation up to 270(±20) kJ mol^–1.     

Phase transitions in langbeinites I: Crystal chemistry and structures of K-double sulfates of the langbeinite type M 2 + + K2(SO4)3, M + +=Mg,Ni, Co,Zn, Ca

The crystal structures of the langbeinite type M ₂ ^{+ +} K₂(SO₄)₃ with M ^{+ +}=Mg, Ni, Co, Zn, Ca in their cubic phase (P 2 ₁ 3) and Ca₂K₂(SO₄)₃ in its orthorhombic phase (P 2 ₁ 2 ₁ 2 ₁) are determined. Whereas the SO₄-tetrahedra in these compounds are almost undistorted, the two symmetry-independent coordination polyhedra of M ^{+ +} are highly distorted octahedra with trigonal site symmetry in P 2 ₁ 3. The deformation of the oxygen octahedra and the off-centering of M ^{+ +} along the trigonal axis show systematic dependences on the ionic radii and the electronegativities of the M ^{+ +}-ions. The correlations are remarkably different for the two symmetry-independent M ^{+ +}-ions indicating different M ^{+ +} — O bonding. The octahedral deformations show also linear correlations with the phase transition temperatures (P 2 ₁ 3 — P 2 ₁ 2 ₁ 2 ₁) of the different compounds. This observation leads to a new model for the phase transition mechanism which is based on thermal instabilities of the M ^{+ +} — O and K — O polyhedral distortions. The cubic high temperature phase is characterized by high symmetric oxygen coordinations around M ^{+ +} which distort with decreasing temperature. At T _c the trigonal site symmetry is broken in such a way that the K — O coordination becomes denser at the expense of a wider and less symmetric M ^{+ +} — O coordination.