Ion exchange between tectosilicates with the nepheline-kalsilite framework and molten MNO3 or MO (M = Li,Na, K,Ag)

Natural nepheline, a synthetic Na-rich nepheline, and synthetic kalsilite were ion exchanged in molten MNO₃ or MCl (M = Li, Na, K, Ag) at 220–800° C. Crystalline products were characterized by wet chemical and electron microprobe analysis, single crystal and powder X-ray diffraction, and transmission electron microscopy and diffraction. Two new compounds were obtained: Li-exchanged nepheline with a formula near (Li,K_0.3,□)Li₃[Al₃(Al,Si)Si₄O₁₆] and a monoclinic unit cell with a = 951.0(6) b = 976.1(6) c = 822.9(5)pm γ = 119.15°, and Ag-exchanged nepheline with a formula near (K,Na,□)Ag₃[Al₃(Al,Si)Si₄O₁₆] and a hexagonal unit cell with a = 1007.4(8) c = 838.2(1.0) pm. Both compounds apparently retain the framework topology of the starting material. Ion exchange isotherms and structural data show that immiscibility between the end members is a general feature in the systems Na-Li, Na-Ag, and Na-K. For the system Na-K, a stepwise exchange is observed with (K,D)Na₃[Al₃(Al,Si)Si₄O₁₆] as an intermediate composition which has the nepheline structure and is miscible with the sodian end member (Na,□)Na₃[Al₃(Al,Si)Si₄O₁₆], but not with the potassian end member (K,□)₄[Al₃(Al,Si)Si₄O₁₆] which shows the kalsilite structure; there was no indication for the formation of trior tetrakalsilite (K/(K + Na)≈0.7) at the temperatures studied (350 and 800° C). The exact amount of vacancies □ on the alkali site depends upon the starting material and was found to be conserved during exchange, with ca 0–0.2 and 0.3–0.4 vacancies per 16 oxygen atoms for the synthetic and natural precursors, respectively. Thermodynamic interpretation of the Na-K exchange isotherms shows, as one important result, that the sodian end member is unstable with respect to the intermediate at K/(K+Na)≈0.25 by an amount of ca 45 kJ/mol Na in the large cavity at 800° C (52 kJ/mol at 350° C).     

Phase relations in the system NaAlSiO4-NaGaSiO4

Phase relations in the system NaAlSiO₄-NaGaSiO₄ to 945° C at 1 kbar P(H₂O) are dominated by stability of Na(Al,Ga)SiO₄ with the beryllonite-type structure. The nepheline structure is restricted to NaAlSiO₄-rich compositions at moderate and high temperature. Structure-composition relationships are controlled by space-fitting requirements of both framework and cavity cations, as in related systems. The two-phase (nepheline-type+beryllonite-type) field has been delineated from the end-member NaAlSiO₄ composition up to the peritectic point at about 945° C (and 60 mol% NaGaSiO₄), using a volume-composition relationship for the beryllonite-type phase, phase appearance, and electron microprobe analysis. At end-member NaGaSiO₄ composition, the beryllonite-type phase is stable to the melting point (902±5° C). At end-member NaAlSiO₄ composition, the beryllonite-type?nepheline-type transformation occurs at 348±2° C, and is associated with an increase in molar volume of 2.4% and enthalpy of 5170±40J·mol^?1. Thus, end-member NaAlSiO₄ nepheline, and probably all sub-potassic nephelines as well, are metastable at very-low geological temperatures.     

Crystal chemistry of nephelines from ijolites and nepheline-rich pegmatites: influence of composition and genesis on the crystal structure investigated by X-ray diffraction

Ten nepheline single crystals from five different localities representing rocks from nepheline-syenite pegmatites to urtite, ijolite and cancrinite-ijolite were investigated chemically and structurally. The chemical compositions were determined by electron microprobe, whereas the crystal structures were refined against X-ray diffraction data to R values of 0.022?C0.031. In all cases the crystal structures conformed to space group P6₃, but at the same time all crystals produced some satellite reflections testifying to the modulation effects in their structure. This work confirms the presence of satellite reflections as a universal property of nepheline and resolves some controversy about this issue present in older works. The samples show a range of compositions with variable excess of Si over Al combined with vacancies in the alkaline sites. Vacancies are present exclusively in the symmetric ditrigonal channels occupied preferentially by K atoms in natural nepheline. The influence of the variable amounts of K, Na and vacancies to the increase of volume at this site is shown to be in the order ??>K>Na. It has been confirmed that natural nepheline can partly depart from Loewenstein??s rule and show some degree of disorder in the Si/Al distribution. Most of the samples revealed a practically full Si-Al-ordering, but the crystals from one of the nepheline-syenite pegmatites in Langesundsfjord, Norway and from the type locality of ijolite, Iivaara, Finland, showed a disorder of 10?C20% suggesting a different cooling history for these rocks.     

Heat capacity and entropy of rutile (TiO<Subscript>2</Subscript>) and nepheline (NaAlSiO<Subscript>4</Subscript>)

 From heat capacities measured adiabatically at low temperatures, the standard entropies at 298.15 K of synthetic rutile (TiO₂) and nepheline (NaAlSiO₄) have been determined to be 50.0 ± 0.1 and 122.8 ± 0.3 J mol⁻¹ K, respectively. These values agree with previous measurements and in particular confirm the higher entropy of nepheline with respect to that of the less dense NaAlSiO₄ polymorph carnegieite. Received: 23 July 2001 / Accepted: 12 October 2001     

Crystallization paths of leucite-bearing lavas: Examples from Italy

The salic phases found in leucite-basanites, -trachytes, and -phonolites may be used to portray crystallization in the system NaAlSiO₄-KAlSiO₄-CaAl₂Si₂O₈-SiO₂, the phonolite pentahedron. Only two lavas have been found that contain the assemblage leucite-nepheline-plagioclase-sanidine and liquid, a natural pseudo-invariant assemblage (at 900° C±100) equivalent to the isobaric invariant point of the four component system. The diversity of phases in this group of lavas illustrates the role of halogens in controlling their crystallization paths. Thus the presence of F in the leucite-basanites has stabilized magnesian biotite and suppressed sanidine, as has been found in other basanitic lavas (Brown and Carmichael 1969). The presence of Cl in these same lavas has induced the crystallization of sodalite, which takes the place of nepheline in the groundmass. However in the leucite-trachytes, biotite has suppressed olivine and coexists with sanidine and leucite. The presence of S may produce haüyne at the expense of nepheline, and in general sulphate minerals, which include apatite, have the role in lavas of low silica activity that pyrrhotite plays in liquids of high silica activity. Both pyroxenes and titaniferous magnetites in this suite of lavas are very aluminous. Groundmass crystals of pyroxene may have one-fifth of Si replaced by Al. Other phases which occur occasionally are melanite garnet and a potassium-rich hastingsite, but neither ilmenite nor a sulphide mineral has been found. Phenocryst equilibration temperatures, derived from olivine and Sr-rich plagioclase, are generally in the range from 1,050° C to 1,150° C. The high content of incompatible elements (e.g., K, Ba, Rb, F, Sr, P) in these lavas suggests that they represent a small liquid fraction from a mantle source which possibly contains phlogopite.     

Spinelloid phases in the nickel gallosilicate system

The phase diagram of the NiO-Ga₂O₃- SiO₂ system has been investigated at high temperatures (up to 1550° C) and 1 atm. pressure. The only ternary phases observed in this system are the spinelloid phases I, II, and V, forming on the NiGa₂O₄-Ni₂SiO₄ pseudo-binary join at temperatures above 1400° C. Our results show that phase V is stable at higher temperature than phase I, while phase II may be metastable. The stability of phases I and V has been confirmed by the successful growth of single crystals from a silica-rich flux. Phases I, II, and V have been characterized by powder X-ray diffraction and transmission electron microscopy. The dimensions of their orthorhombic unit-cells are: (I) a= 5.7741(5), b=11.712(1), c=8.2387(9) Å; (II) a= 5.765(1), b= 17.619(3), c= 8.238(2) Å; (V) a= 5.7914(4), b=8.7809(7), c=8.2346(6) Å. High resolution electron microscopy has also revealed the formation of microscopic intergrowths similar to those previously observed for the spinelloid phases of the NiO-Al₂O₃-SiO₂ system.     

Defects and short-range order in nepheline group minerals: a silicon-29 nuclear magnetic resonance study

²⁹Si magic-angle spinning nuclear magnetic resonance (NMR) spectra are presented for seven crystalline phases of the nepheline group: natural nephelines from a plutonic environment (Bancroft, Ontario) and a volcanic deposit (Mt. Somma, Italy), kalsilite, synthetic pure Na nepheline, carnegieite, and two samples of orthorhombic KAlSiO₄. In all phases, nearly all of the Si sites have four Al neighbors, indicating nearly complete Al-Si ordering. Excess Si over the 1:1 stoichiometric Si/Al ratio appears to substitute randomly for Al on an ordered lattice, adding Si sites with 3 and 0 Al neighbors in a 3:1 ratio. Various types of structural disorder, including Al-Si disorder, that are reported from some x-ray diffraction studies are probably long range in nature and are due to the presence of ordered domains. In naturally occurring nepheline, the relative abundance of T sites with three-fold local symmetry is maintained at the ideal stoichiometric value of 1/4, even when the K/(K+Na) ratio is substantially lower. This is in agreement with conclusions reached about the average structure from x-ray data. The distinction between the two sites, at least in terms of the local structure that is reflected in ²⁹Si NMR chemical shifts, is lost in a pure Na nepheline sample.     

Inversions in sub-potassic nephelines

Sub-potassic nephelines in the system NaAlSiO₄(Ne)-KAlSiO₄(Ks) were synthesized under a variety of conditions and studied at room temperature and up to 1000 °C using an X-ray powder diffractometer. At low temperatures they do not have the hexagonal structure determined by Hahn and Buerger (1955) for natural nepheline. Samples with 0.7 to 2.5 mole % Ks have an orthorhombic supercell with parameters equivalent to a, 3a, 3c where a and c are Hahn and Buerger structure cell parameters. Nephelines with 0 to 0.7% Ks consist of two phases with different c axes; one of these phases has the orthorhombic supercell.Pure-Na nephelines (NaAlSiO₄) invert to a hexagonal phase with the Hahn and Buerger structure at 190 °±10 °C; this inversion temperature decreases with increasing Ks and a sample with 0.5% Ks inverts at 170 °±5 °C. The inversion is reversible and is displacive. Another reversible inversion begins at 875 °±10 °C in pure-Na nepheline; this inversion increases in temperature with increasing Ks and a sample with 1.8% Ks begins to invert at 960 °±10 °C.Superstructures with anomalous low-temperature cell parameters in sub-potassic nephelines are attributed to reversible collapse of the framework about the larger cation sites which must be occupied by small Na in subpotassic nephelines. Superstructures in natural nephelines are also related to framework collapse at a displacive inversion.     

Etude des solutions solides des néphélines (Na,K)AlSiO4 et (Na,Rb)AlSiO4

Ion exchange equilibrium of nepheline solid solutions (Na, K)AlSiO₄ and (Na, Rb)AlSiO₄ with hydrothermal solutions has been studied at 600°C and 2000 bars. The behaviour of dilute solid solutions was specially investigated.Na-Rb ion exchange data can be represented satisfactorily by a model taking into account the existence of two different sites in the structure of nepheline. At 600°C Rb atoms substitute almost exclusively for Na atoms situated in the larger sites. On the other hand, this model only partially applies to Na-K ion exchange equilibrium.Finally, the importance of the ion exchange data concerning extremely dilute solutions to calculate activity-composition diagrams is emphasized with special reference to the nepheline solid solutions.     

Ephesite,Na(LiAl2) [Al2Si2O10] (OH)2: II. Thermodynamic analysis of its stability and compatibility relations,and its geological occurrences

The trioctahedral mica ephesite, Na(LiAl₂) [Al₂Si₂O₁₀] (OH)₂, has a large -T stability field in the quaternary system NaAlSiO₄-LiAlSiO₄-Al₂O₃-H₂O. At temperatures below 400–500° C it coexists with diaspore, while at higher temperatures it occurs with corundum, until it decomposes to nepheline +eucryptite+corundum+H₂O at 600–800° C (Fig. 1). Nature faithfully reflects these phase relations; ephesite is found to coexist with diaspore or corundum in silicadeficient metamorphosed rocks or in hydrothermally altered nepheline-syenite pegmatite.Thermodynamic analysis of phase relations of ephesite in the silica saturated portion of the quinary system NaAlSiO₄-LiAlSiO₄-Al₂O₃-SiO₂-H₂O shows that the assemblage quartz+ephesite is always metastable with respect to paragonite+spodumene or paragonite+petalite at temperatures down to approximately 300° C (Fig. 3). At lower temperatures, a number of other phases like bikitaite, cookeite, Na-montmorillonite, and analcime are stabilized. Stability and compatibility relations involving these phases are presently not amenable to thermodynamic treatment due to lack of suitable data. Nevertheless, the absence of the assemblage quartz+ephesite in nature seems to vindicate our conclusion that it is metastable down to at least 300° C.The frequently encountered assemblage quartzspodumene (or petalite)-microcline-albite of some lithium pegmatites contains muscovite (±lepidolite), rather than paragonite. The absence of paragonite in such rocks is best explained by the inherent metastability of the phase-pair paragonite+microcline with respect to muscovite+albite. The pegmatite bulk compositions plot in the four-phase field spodumene (petalite)-microcline-muscovite-albite, cutting out paragonite from the observed assemblage Thus, absence of paragonite-spodumene or paragonitepetalite in nature reflects lack of suitable bulk compositions in rocks.     

Iron titanium oxides in equilibrium with metallic iron

The iron titanium oxide phases ulvite, ilmenite and ferropseudobrookite were synthesized in equilibrium with metallic iron at 1000 ° C, 1130 ° C and 1300 ° C in CO₂/H₂ gas mixtures.The composition of the phases were determined by wet chemical and electron microprobe analyses and by direct oxygen determination.The chemical composition of the Fe-Ti oxide phases in equilibrium with metallic iron is sensitively influenced by temperature and by bulk composition.Ulvite in equilibrium with wüstite does not contrivalent titanium in the whole temperature range up to the eutectic temperature at 1312 °C.Ulvite+ilmenite phase assemblages contain trivalent titanium only at temperatures above 1200 ° C.A ferropseudobrookite phase is stable under the given conditions at temperatures above 1068 °C (Ender and Woermann, 1977).Ferropseudobrookite in equilibrium with metallic iron always contains trivalent titanium.Deviations from stoichiometric compositions of the solid solution phases are generally small. Thus recalculation of microprobe data to stoichiometric solid solutions does not involve a major error.From: Bruno Simons, Diplomarbeit Aachen, October 1974     

Thermodynamics of feldspathoid solutions

We have developed models for the thermody-namic properties of nephelines, kalsilites, and leucites in the simple system NaAlSiO₄?KAlSiO₄?Ca_0.5AlSiO₄?SiO₂?H₂O that are consistent with all known constraints on subsolidus equilibria and thermodynamic properties, and have integrated them into the existing MELTS software package. The model for nepheline is formulated for the simplifying assumptions that (1) a molecular mixing-type approximation describes changes in the configurational entropy associated with the coupled exchange substitutions □Si?NaAl and □Ca? Na₂ and that (2) Na⁺ and K⁺ display long–range non-convergent ordering between a large cation and the three small cation sites in the Na₄Al₄Si₄O₁₆ formula unit. Notable features of the model include the prediction that the mineral tetrakalsilite (“panunzite”, sensu stricto) results from anti-ordering of Na and K between the large cation and the three small cation sites in the nepheline structure at high temperatures, an average dT/dP slope of about 55^°/kbar for the reaction over the temperature and pressure ranges 800–1050 ^°C and 500–5000 bars, roughly symmetric (i.e. quadratic) solution behavior of the K–Na substitution along joins between fully ordered components in nepheline, and large positive Gibbs energies for the nepheline reciprocal reactions and and for the leucite reciprocal reaction      

Behoite and mimetite from the Saharjok alkaline intrusion,Kola Peninsula

The detailed study of the mineral composition of the nepheline syenite pegmatite from the Saharjok Intrusion has resulted in the finding of behoite and mimetite, a mineral species identified in the Kola region for the first time. The pegmatite body at the contact between nepheline syenite and essexite is unusual in textural and structural features and combination of mineral assemblages including unique beryllium mineralization. Behoite Be(OH)₂ is an extremely rare beryllium mineral. It occurs as powderlike aggregates in the leaching cavities between euhedral pyroxene crystals. Behoite was identified by comparison of X-ray powder diffraction data of the studied mineral phase and behoite from the Be-bearing tuff in the type locality of this mineral (Utah, United States). Mimetite was found in the same pegmatite of the Saharjok intrusion. It forms unusual parallel-fibrous aggregates with individual fibers as long as ∼1 mm and only ∼1 μm across. X-ray powder diffraction data and the chemical composition characterize the mineral as hexagonal phase Pb₅[AsO₄]₃Cl. Both behoite and mimetite are the products of late hydrothermal alteration of primary minerals (meliphanite, galena, arsenopyrite, and loellingite). The secondary phases freely crystallized in the cavities remaining after the leached nepheline.     

Thermochemistry of yavapaiite KFe(SO4)2: Formation and decomposition

Yavapaiite, KFe(SO₄)₂, is a rare mineral in nature, but its structure is considered as a reference for many synthetic compounds in the alum supergroup. Several authors mention the formation of yavapaiite by heating potassium jarosite above ca. 400°C. To understand the thermal decomposition of jarosite, thermodynamic data for phases in the K-Fe-S-O-(H) system, including yavapaiite, are needed. A synthetic sample of yavapaiite was characterized in this work by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and thermal analysis. Based on X-ray diffraction pattern refinement, the unit cell dimensions for this sample were found to be a = 8.152 ± 0.001 Å, b = 5.151 ± 0.001 Å, c = 7.875 ± 0.001 Å, and β = 94.80°. Thermal decomposition indicates that the final breakdown of the yavapaiite structure takes place at 700°C (first major endothermic peak), but the decomposition starts earlier, around 500°C. The enthalpy of formation from the elements of yavapaiite, KFe(SO₄)₂, ΔH°_f = −2042.8 ± 6.2 kJ/mol, was determined by high-temperature oxide melt solution calorimetry. Using literature data for hematite, corundum, and Fe/Al sulfates, the standard entropy and Gibbs free energy of formation of yavapaiite at 25°C (298 K) were calculated as S°(yavapaiite) = 224.7 ± 2.0 J.mol⁻¹.K⁻¹ and ΔG°_f = −1818.8 ± 6.4 kJ/mol. The equilibrium decomposition curve for the reaction jarosite = yavapaiite + Fe₂O₃ + H₂O has been calculated, at pH₂O = 1 atm, the phase boundary lies at 219 ± 2°C.     

Feldspar and oxide thermometry of granulites in the Adirondack Highlands

Thermometry of regionally metamorphosed granulites of the Adirondack Highlands has been undertaken using feldspar and iron-titanium-oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An_18–30) and microcline perthite (Or_57–87) pairs from charnockites and granitic gneisses give K_D[Na/(Na+Ca+K]_plag/[Na/(Na+Ca+K)]_or = 2–3 yielding temperatures of 650 ° to 750 ° C in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pressures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magnetite (ulvöspinel_ss 16–45) and ilmenite (hematite_ss 4.7–6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ° to 800 ° C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10^–20 and 10^–16 and always lie within 10⁺¹ of the f buffered by fayalite-magnetite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulvöspinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically reasonable results. The inferred 750 ° and 700 ° C isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790 ± 50 ° C) occur between Saranac Lake and Tupper Lake, New York.Contribution No. 336 from the Mineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan, 48109. U.S.A.     

Potential of microbial methane formation in a high-temperature hydrocarbon seep

Hydrocarbon seepage is a surface expression where fluids mixed with sediments and hydrocarbons are expelled through fracture systems that potentially tap into gas–petroleum reservoirs. Hydrocarbons released from most seeps appear to be thermogenic on the basis of their relative abundance and isotopic composition. The potential for subsurface microbial processes modifying these geochemical fingerprints remains poorly constrained. In this study, microcosm incubations were conducted on mud slurries supplied with/without various methanogenic precursors at temperatures ranging from ambient conditions to 90 °C, in order to assess microbial CH₄ formation in the subsurface beneath hydrocarbon seeps. The analyses indicated that CH₄ production was positive at ?80 °C, regardless of whether or not or which precursors were added. However, the pattern of CH₄ production rates varied with the precursor and temperature. In general, the optimum CH₄ production from H₂/CO₂ and formate occurred over a wide range of temperatures (?40 °C), whereas that from acetate, methanol and methylamine was restricted to relatively lower temperatures (40–50 °C). The CH₄ recoveries, together with the C isotopic compositions of CH₄, further indicated that the quantities of CH₄ produced could not completely account for the quantities of precursor consumed, suggesting that a complex metabolic network was involved in the transformation of the added precursor and organic C inherited from inoculated sediments. Microbial CH₄ was estimated to constitute 7–61% of the CH₄ observed using experimentally-derived apparent isotope fractionations as the end member compositions. This illustrates the possibility that microbial CH₄ produced at shallower depths could quantitatively and isotopically alter deeply-sourced thermogenic CH₄ in hydrocarbon seep environments.     

The evolution of a grospydite from the Roberts Victor mine,South Africa

A grospydite from Roberts Victor contains the most Ca-rich garnets yet found in South African kimberlite xenoliths and also sub-micron sized sodic nepheline in melted and quenched clinopyroxene. Three stages can be recognised in the textural evolution of the grospydite. The first is the development of a layering of large kyanite laths. Kyanite together with complex aluminous clinopyroxene precipitated and accumulated from an evolved residual eclogitic liquid which has penetrated across the garnet join so that garnet no longer precipitated. Solidus conditions for the Roberts Victor grospydite are estimated as T = 1350–1550 ° C, P = 27–39 kbars. Adjacent layers in the grospydite have slightly different mineral compositions suggesting that the small-scale layering (1–5 cm) in this, and associated rocks, may be related to varying activities of R₂O₃ components and possibly to f _{o 2}.The second stage is represented by a necklace texture in which all the garnet and some kyanite developed along grain boundaries of clinopyroxenes with triple-point textures. This is interpreted as an example of incoherent, grain-boundary exsolution resulting from large subsolidus volume changes. The conditions for subsolidus equilibration are estimated to be T= 1120–1320 ° C, P = 42–56 kbars.Moderate Ca-contents in garnet and excess Al^[6] in clinopyroxene may be subsolidus indicators of eclogite samples evolving towards grospydite at the solidus.The third stage is represented by the melting of jadeite-rich clinopyroxenes and quenching to glass, nepheline and plagioclase. Most of the glass has a composition similar to clinopyroxene, except for K₂O, though local areas of different glass, possibly the result of phase separation, also occur. The melting process seems to be a low-pressure feature involving limited addition of H₂O at temperatures between 900–1000 ° C. Water-absent melting could indicate temperatures up to 1500 ° C.The temperatures and pressures assigned to the three-stage evolution of this grospydite imply formation at moderate pressures and subsolidus equilibration at higher pressures. This is equated with downgoing mantle/asthenosphere tectonic processes. After entrainment in a kimberlite magma the grospydite fragment apparently ascended rapidly, thus allowing low-pressure melting and quenching.     

Petrogenetic relationships between melilitite and lamproite

A detailed petrological study of the S. Venanzo olivine melilitite and Cupaello kalsilitite, located at the NE margin of the Roman Comagmatic Region of Italy has provided new data to evaluate their genetic relationships and related tectonic/magmatic conditions. Early crystallized olivine (Fo₉₂) from S. Venanzo is compatible with crystallization from near primary mantle melts, while late-crystallized olivine from both rocks (Fo_88–79) reflects primarily the high Ca of the host liquids, attained under the volcanic crystallization regime. Magnesiochromite inclusions in the early-crystallized olivine are consistent with near-primary melts close to lamproite in composition. Nepheline and kalsilite from both lavas contain high alkalis (+Ca), relative to Al, and thus correspond more closely with nepheline compositions from carbonatite-related assemblages, than with those from a wide compositional range of Alban Hills lavas. Coexisting melilite has high (Na+K)/Al, reflecting the Peralkalinity Index of the bulk rocks. Diopside and phlogopite from both lavas are characteristic of lamproites and groundmass kimberlites in their high Mg/(Mg+Fe²⁺) ratio (0.86–0.95; 0.80–0.90, respectively) and T-site (Si+Al) deficiencies. Götzenite, Ca₂Na[Zr, Ti]Si₂O₇(O, OH, F)₂, identified in both lavas, is typical of nephelinite-ijolite assemblages. On the other hand, khibinskite, K₄Zr₂Si₄O₁₄, found in the Cupaello lava, may be regarded as a Si-poor variety of wadeite, a mineral characteristic of lamproites. Clinopyroxene and monticellite, coexisting as late-crystallized phases in both lavas, suggest a common P-T liquid path of thermal descent in the system CaMgSi₂O₇–CO₂ in the presence of excess CO₂, but with different intersections with the åkermanite stability field. Substantial differences in SiO₂ saturation combined with high Mg number and liquidus temperatures experimentally determined at atmospheric pressure in both lavas (1276° C and 1260° C, respectively) indicate that a parent-daughter relationship is unlikely under the volcanic P-T regime. In La Roche's Rm-Ri-Rs diagram, the S. Venanzo composition fall close to the Oldoinyo Lengai alkalic pyroxenite trend of Donaldson and Dawson, while the Cupaello compositions follow a lamproitic trend, consistent with that indicated by the quartz-normative glass of the Gaussberg lava. Mantle compositions corresponding to wehrlite-clinopyroxenite and enriched in H₂O, CO₂, F, and LILE, are favoured as potential sources for the lavas. Their origin is probably related to subcrustal fluid transfer promoted by the Tyrrhenian mantle doming.     

Fluid inclusion and sulphur isotope evidence for syntectonic mineralisation at the Elura mine,southeastern Australia

Fluid inclusion and sulphur isotope data for the discordant, metasediment-hosted massive sulphide deposit at Elura are consistent with a syntectonic origin of the orebodies. Thermometric and laser Raman microprobe analyses indicate that two-phase, primary fluid inclusions are low salinity and H₂O-CO₂-CH₄ types. Inclusion fluids from quartz in ore yield homogenisation temperatures (T_h) ranging from 298 ° to 354 °C (mean 320 °C). They are likely to have been trapped close to the solvus of the H₂O-CO₂-(CH₄-NaCl) system and thus should give temperatures of the mineralising fluid. An additional, low T_h population of later fluid inclusions is recognised in quartz from ore and syntectonic extension veins in the adjacent wallrock. T_h's for these low CO₂bearing inclusions range from 150 to 231 °C (mean 190 °C), and should be considerably lower than true trapping temperatures. Sulphur isotopic composition (³⁴S) of pyrite, sphalerite, pyrrhotite and galena ranges from 4.7 to 12.6% and indicates a sulphur source from underlying Cobar Supergroup metasediments. An average temperature of 275 °C from the sphalerite-galena sulphur isotopic thermometer suggests isotopic re-equilibration below peak metamorphic temperatures.     

Shonkin Sag laccolith,Montana

A variety of rock types is developed in the Shonkin Sag laccolith, with extreme compositions represented by the porphyritic pseudoleucite-bearing chilled margin, shonkinitic in composition, and by the final differentiate, chemically a nepheline syenite. During differentiation the pyroxenes changed in composition from Ca-rich varieties through aegirine-augite to acmite; there is no evidence of an immiscibility gap between Ca-rich and Na-rich pyroxenes. Olivine compositions changed from approximately Fa₂₀ to Fa₄₀, but in marked contrast the coexisting biotites exhibit a more extensive compositional range, from annite₂₄ to annite₁₀₀; the crystal margins of annites in the most evolved rocks are manganophyllite-rich. Titanomagnetites are TiO₂-poor varieties. Arfvedsonite and melanite occur in the most evolved syenites.From mineralogical and thermodynamic data initial and final temperatures of crystallization of the various rock types have been calculated. At an estimated total vapor pressure of 310 bars, the temperature of intrusion was 985° C. Final crystallization of the laccolith took place below 700° C, and crystallization intervals for most rock types are of the order of 170° C. Over the total magmatic temperature range the activity of silica in the melt decreased from 0.13 to 0.09. Oxygen fugacity falls with temperature approximately parallel to the synthetic fayalite-magnetite-quartz oxygen buffer until olivine disappears. The crystallization of Na-rich pyroxenes does not demand an increase in the fugacity of oxygen, but rather requires that the oxygen fugacity fall less rapidly with temperature than would be the case if olivine and magnetite were present.