Structural thermal behavior of paragonite and its dehydroxylate: a high-temperature single-crystal study

 The lattice constants of paragonite-2M₁, NaAl₂(AlSi₃)O₁₀(OH)₂, were determined to 800 °C by the single-crystal diffraction method. Mean thermal expansion coefficients, in the range 25–600 °C, were: α_a = 1.51(8) × 10⁻⁵, α_b = 1.94(6) × 10⁻⁵, α_c = 2.15(7) ×  10⁻⁵ °C⁻¹, and α_V = 5.9(2) × 10⁻⁵ °C⁻¹. At T higher than 600 °C, cell parameters showed a change in expansion rate due to a dehydroxylation process. The structural refinements of natural paragonite, carried out at 25, 210, 450 and 600 °C, before dehydroxylation, showed that the larger thermal expansion along the c parameter was mainly due to interlayer thickness dilatation. In the 25–600 °C range, Si,Al tetrahedra remained quite unchanged, whereas the other polyhedra expanded linearly with expansion rate proportional to their volume. The polyhedron around the interlayer cation Na became more regular with temperature. Tetrahedral rotation angle α changed from 16.2 to 12.9°. The structure of the new phase, nominally NaAl₂ (AlSi₃)O₁₁, obtained as a consequence of dehydroxylation, had a cell volume 4.2% larger than that of paragonite. It was refined at room temperature and its expansion coefficients determined in the range 25–800 °C. The most significant structural difference from paragonite was the presence of Al in fivefold coordination, according to a distorted trigonal bipyramid. Results confirm the structural effects of the dehydration mechanism of micas and dioctahedral 2:1 layer silicates. By combining thermal expansion and compressibility data, the following approximate equation of state in the PTV space was obtained for paragonite: V/V ₀ = 1 + 5.9(2) × 10⁻⁵ T(°C) − 0.00153(4) P(kbar). Received: 12 July 1999 / Revised, accepted: 7 December 1999     

High-temperature study and thermal expansion of phlogopite

 Unit-cell dimensions of a natural phlogopite from Pargas, Finland, have been determined in the temperature interval of 27–1050 °C by X-ray powder diffraction technique. Expansion rates vary discontinuously with temperature with a break at 412 °C. Below this temperature, the linear expansions (α) for a, b and c axis lengths are 3.74 × 10⁻⁵ K⁻¹, 1.09 × 10⁻⁵ K⁻¹, and 1.19 × 10⁻⁵ K⁻¹, respectively, and above that they are 0.86 × 10⁻⁵ K⁻¹, 0.80 × 10⁻⁵ K⁻¹, and 1.93 × 10⁻⁵ K⁻¹. The volume thermal expansion coefficients are 6.26 × 10⁻⁵ K⁻¹ and 3.71 × 10⁻⁵ K⁻¹ for low-temperature and high-temperature intervals, respectively. The observed kink in the rate of thermal expansions with temperature could be due to the different mode of structural changes. Thermogravimetric analysis of the sample indicates the oxidation of iron in the temperature range of 500–600 °C and dehydroxylation as well as decomposition of phlogopite in the temperature range of 900–1200 °C. Received: 8 September 1998 / Accepted: 28 February 2000     

Thermal equations of state of dioctahedral micas on the join muscovite–paragonite

 Powder diffraction measurements at simultaneous high pressure and temperature on samples of 2M1 polytype of muscovite (Ms) and paragonite (Pg) were performed at the beamline ID30 of ESRF (Grenoble), using the Paris-Edinburgh cell. The bulk moduli of Ms, calculated from the least-squares fitting of V–P data on each isotherm using a second-order Birch–Murnaghan EoS, were: 57.0(6), 55.1(7), 51.1(7) and 48.9(5) GPa on the isotherms at 298, 573, 723 and 873 K, respectively. The value of (∂K _T /∂T) was −0.0146(2) GPa K⁻¹. The thermal expansion coefficient α varied from 35.7(3) × 10⁻⁶ K⁻¹ at P ambient to 20.1(3) × 10⁻⁶ K⁻¹ at P = 4 GPa [(∂α/∂P) _T = −3.9(1) × 10⁻⁶ GPa⁻¹ K⁻¹]. The corresponding values for Pg on the isotherms at 298, 723 and 823 K were: bulk moduli 59.9(5), 55.7(6) and 53.8(7) GPa, (∂K _T /∂T) −0.0109(1) GPa K⁻¹. The thermal expansion coefficient α varied from 44.1(2) × 10⁻⁶ K⁻¹ at P ambient to 32.5(2) × 10⁻⁶ K⁻¹ at P = 4 GPa [(∂α/∂P) _T = −2.9(1) × 10⁻⁶ GPa⁻¹ K⁻¹]. Thermoelastic coefficients showed that Pg is stiffer than Ms; Ms softens more rapidly than Pg upon heating; thermal expansion is greater and its variation with pressure is smaller in Pg than in Ms. Received: 28 January 2002 / Accepted: 5 April 2002     

Thermal expansion of ZnSiO<Subscript>3</Subscript> high-pressure phases

Thermal expansion of ZnSiO₃ high-pressure clinopyroxene and ilmenite phases was measured in the temperature range 100–620 K by the X-ray powder diffraction method. Interpolation and extrapolation of experimental data were performed by the procedure based on the Debye-Mie-Gruneisen theory in the range from 50 to 1,500 K. Temperature dependencies of molar volumes and coefficients of bulk thermal expansion of ZnSiO₃ phases were determined.     

The study of the kinetics and the mechanism of dehydroxylation in muscovite by ESR on Fe3+

 Dehydroxylation of muscovite in the form of small lamellae at 923 <T <1173 K was studied by Electron Spin Resonance (ESR) on Fe³⁺. The kinetics of the process has been established to be described by the model of continuous nucleation on the large surface planes of the small plates. Determined by experimental data the rate constant of the process k is shown to be that of dehydroxylation itself. The activation energy obtained by data at T<1100 K is 97.5 KJ·mol⁻¹. The nonlinear dependence of ln(k) on 1/T is explained by the theory of transitions induced by the fluctuative preparation of a potential barrier as a result of thermal oscillations of ions in the lattice. At high temperatures the potential curve of the hydroxyl's proton is transformed so that it can overcome the barrier from one potential well to the other (from one hydroxyl site to the adjacent one). Such transformations of the curve can be caused by the oscillations of large structural clusters (∼1·10⁻²² kg) with the frequency ∼4.5·10¹² s⁻¹. Received: 3 August 1995 / Accepted: 13 April 1997     

Thermal expansion and crystal structure of FeSi between 4 and 1173 K determined by time-of-flight neutron powder diffraction

The thermal expansion and crystal structure of FeSi has been determined by neutron powder diffraction between 4 and 1173?K. No evidence was seen of any structural or magnetic transitions at low temperatures. The average volumetric thermal expansion coefficient above room temperature was found to be 4.85(5)?×?10^?5?K^?1. The cell volume was fitted over the complete temperature range using Grüneisen approximations to the zero pressure equation of state, with the internal energy calculated via a Debye model; a Grüneisen second-order approximation gave the following parameters: θ_D=445(11)?K, V ₀=89.596(8)?ų, K ₀′=4.4(4) and γ′=2.33(3), where θ_D is the Debye temperature, V ₀ is V at T=0?K, K ₀′ is the first derivative with respect to pressure of the incompressibility and γ′ is a Grüneisen parameter. The thermodynamic Grüneisen parameter, γ_th, has been calculated from experimental data in the range 4–400?K. The crystal structure was found to be almost invariant with temperature. The thermal vibrations of the Fe atoms are almost isotropic at all temperatures; those of the Si atoms become more anisotropic as the temperature increases.     

Garnet–omphacite–phengite thermobarometry of eclogites from the coesite-bearing unit of the southern Dora-Maira Massif, Western Alps

Using relevant geothermobarometric methods, P–T-data were collected for the reconstruction of the metamorphic evolution of 34 eclogite samples taken from small lenses and boudins within the ultrahigh-pressure (UHP) metamorphic coesite-bearing Brossasco-Isasca Unit (BIU) of the Dora-Maira Massif. The mineral phases used (clinopyroxene, garnet, phengite), or growth zones thereof, were identified as being coexistent for different stages of metamorphism on the basis of careful petrographic studies. Of several published geothermobarometers, the garnet–clinopyroxene thermometer of Powell [Powell, R., 1985. Regression diagnostics and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. J. Metamorph. Geol., 3, pp. 231–243.] combined with the garnet–clinopyroxene–phengite barometer after Waters and Martin [Waters, D., Martin, H.N., 1993. The garnet–clinopyroxene–phengite barometer. Terra Abstr., 5, pp. 410–411.] was chosen here, because it provided the most reliable results. Nevertheless, the scatter of P–T-data points for the prograde (stage I), peak metamorphic (stage II), and retrograde (stage III) development of the eclogites is still considerable. Among the many possible reasons for this inconsistency discussed, a partial lack of equilibration of some of the eclogites during their metamorphic history should be taken into account. Despite the data scatter, an average P–T-path could be estimated, which includes the following coordinates: for stage I: 15 kbar/500°C; 25 kbar/570°C; 32 kbar/650°C; for stage II: 36 kbar/720°C; and for stage III: 24 kbar/680°C and 14 kbar/620°C. This is in fair agreement with P–T-paths derived earlier for other rock types of the BIU on the basis of other geothermobarometers.     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln = Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1100 K) and SmCaAl[Al₂O₇] (up to T=1024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data were obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶×K⁻¹+3.6(7)×10⁻⁹ΔT×K⁻² and α₃=15.0(1)×10⁻⁶×K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶×K⁻¹+2.0(2)×10⁻⁹ΔT×K⁻² and α₃=8.5(2)×10⁻⁶×K⁻¹+2.0(3)×10⁻⁹ΔT×K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)×10⁻⁶×K⁻¹+1.7(2)×10⁻⁹ΔT×K⁻² and α₃=9.344(5)×10⁻⁶×K⁻¹. The expansion mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ‘proper’ layer structure, the aluminates show increased framework structure behavior. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)–(Ln ³⁺+Al³⁺) in the melilite-type structure. This article has been mistakenly published twice. The first and original version of it is available at .     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln=Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1,100 K) and SmCaAl[Al₂O₇] (up to T=1,024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data was obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶ K⁻¹+3.6(7)×10⁻⁹ΔT K⁻² and α₃=15.0(1)×10⁻⁶ K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶ K⁻¹+2.0(2)×10⁻⁹ΔT K⁻² and α₃=8.5(2)×10⁻⁶ K⁻¹+2.0(3)×10⁻⁹ΔT K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)× 10⁻⁶ K⁻¹+1.7(2)×10⁻⁹ΔT K⁻² and α₃=9.344(5)×10⁻⁶ K⁻¹. The expansion-mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ’proper’ layer structure, the aluminates show increased framework structure behaviour. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)-(Ln ³⁺+Al³⁺) in the melilite-type structure. Electronic Supplementary Material Supplementary material is available for this article at     

Experimental synthesis and phase relations of phengitic muscovite from 6.5 to 11 GPa in a calcareous metapelite from the Dabie Mountains, China

The stability and phase relations of phengitic muscovite in a metapelitic bulk composition containing a mixed H₂O+CO₂ fluid were investigated at 6.5–11 GPa, 750–1050°C in synthesis experiments performed in a multianvil apparatus. Starting material consisted of a natural calcareous metapelite from the coesite zone of the Dabie Mountains, China, ultrahigh-pressure metamorphic complex that had experienced peak metamorphic pressures greater than 3 GPa. The sample contains a total of 2.1 wt.% H₂O and 6.3 wt.% CO₂ bound in hydrous and carbonate minerals. No additional fluid was added to the starting material. Phengite is stable in this bulk composition from 6.5 to 9 GPa at 900°C and coexists with an eclogitic phase assemblage consisting of garnet, omphacite, coesite, rutile, and fluid. Phengite dehydrates to produce K-hollandite between 8 and 11 GPa, 750–900°C. Phengite melting/dissolution occurs between 900°C and 975°C at 6.5–8 GPa and is associated with the appearance of kyanite in the phase assemblage. The formation of K-hollandite is accompanied by the appearance of magnesite and topaz-OH in the phase assemblage as well as by significant increases in the grossular content of garnet (average X_grs=0.52, X_py=0.19) and the jadeite content of omphacite (X_jd=0.92). Mass balance indicates that the volatile content of the fluid phase changes markedly at the phengite/K-hollandite phase boundary. At P≤8 GPa, fluid coexisting with phengite appears to be relatively CO₂-rich (XCO₂/XH₂O=2.2), whereas fluid coexisting with K-hollandite and magnesite at 11 GPa is rich in H₂O (XCO₂/XH₂O=0.2). Analysis of quench material and mass balance calculations indicate that fluids at all pressures and temperatures examined contain an abundance of dissolved solutes (approximately 40 mol% at 8 GPa, 60 mol% at 11 GPa) that act to dilute the volatile content of the fluid phase. The average phengite content of muscovite is positively correlated with pressure and ranges from 3.62 Si per formula unit (pfu) at 6.5 GPa to 3.80 Si pfu at 9 GPa. The extent of the phengite substitution in muscovite in this bulk composition appears to be limited to a maximum of 3.80–3.85 Si pfu at P=9 GPa. These experiments show that phengite should be stable in metasediments in mature subduction zones to depths of up to 300 km even under conditions in which aH₂O1. Other high-pressure hydrous phases such as lawsonite, MgMgAl-pumpellyite, and topaz-OH that may form in subducted sediments do not occur within the phengite stability field in this system, and may require more H₂O-rich fluid compositions in order to form. The wide range of conditions under which phengite occurs and its participation in mixed volatile reactions that may buffer the composition of the fluid phase suggest that phengite may significantly influence the nature of metasomatic fluids released from deeply subducted sediments at depths of up to 300 km at convergent plate boundaries.     

Computational study of tetrahedral Al–Si and octahedral Al–Mg ordering in phengite

 As part of a wider study of the nature and origins of cation order–disorder in micas, a variety of computational techniques have been used to investigate the nature of tetrahedral and octahedral ordering in phengite, K₂ ^[6](Al₃Mg)^[4](Si₇Al)O₂₀(OH)₄. Values of the atomic exchange interaction parameters J _n used to model the energies of order–disorder were calculated. Both tetrahedral Al–Si and octahedral Al–Mg ordering were studied and hence three types of interaction parameter were necessary: for T–T, O–O and T–O interactions (where T denotes tetrahedral sites and O denotes octahedral sites). Values for the T–T and O–O interactions were taken from results on other systems, whilst we calculated new values for the T–O interactions. We have demonstrated that modelling the octahedral and tetrahedral sheets alone and independently produces different results from modelling a whole T–O–T layer, hence justifying the inclusion of the T–O interactions. Simulations of a whole T–O–T layer of phengite indicated the presence of short-range order, but no long-range order was observed. Received: 8 August 2002 / Accepted: 14 February 2003 Acknowledgements The authors are grateful to EPSRC (EJP) and the Royal Society (CIS) for financial support. Monte Carlo simulations were performed on the Mineral Physics Group's Beowulf cluster and the University of Cambridge's High Performance Computing Facility.     

Thermal expansion of Mg(OH)<Subscript>2</Subscript> brucite under high pressure and pressure dependence of entropy

An equation of state for Mg(OH)₂ brucite under high-pressure and high-temperature conditions has been obtained by measuring temperature dependence of volume up to 600 K at ambient pressure and pressure dependence of volume up to 16 GPa at 300, 473, 673, and 873 K with in situ X-ray diffraction. Pressure dependence of entropy of brucite has been calculated with thermal expansion coefficient and volume which are derived from the present EoS. This dependence indicates that generation of secondary OH dipoles affects entropy. The OH dipoles probably appear around 2 GPa and the number seems not to change over 8 GPa at 300 K.     

Molar volume and thermal expansion of glaucophane

The molar volume of glaucophane [Na₂Mg₃Al₂Si₈O₂₂(OH)₂] has been determined in this study by correcting synthetic glaucophane-rich amphiboles made in the system Na₂O–MgO–Al₂O₃–SiO₂–H₂O for very small deviations from ideal glaucophane composition using recent volume data on key amphibole components. The derived unit-cell volume for end-member glaucophane is 862.7±1.6 Å³, which gives a molar volume of 259.8±0.5 cm³/mol and a calculated density of 3.016±0.006 g/cm³. This value has been corroborated through an essentially independent method by correcting the volumes of natural sodic amphiboles reported in the literature for non-glaucophane components, particularly including calcium-rich components, to yield a value of 861.2±1.9 Å³. The unit-cell volume derived from the synthetic amphiboles, which is considered here to be more reliable, is somewhat smaller than that reported previously in the literature. A thermal expansion (α_V) at 298 K of 1.88±0.06×10^?5/K was derived from unit-cell volumes measured in the range of 25–500°C for a synthetic glaucophane sample, which is noticeably smaller than previously reported.     

High-temperature thermal expansion of lime, periclase, corundum and spinel

The high-temperature cell parameters of lime (CaO), periclase (MgO), corundum (Al₂O₃), and spinel (MgAl₂O₄) have been determined from 300 up to 3000 K through X-ray diffraction experiments with synchrotron radiation. The good agreement found with dilatometric results suggests that vacancy-type defects do not make a large contribution to thermal expansion for these oxides, even near the melting point, justifying the use of X-ray diffraction for determining volume properties up to very high temperatures. Thermal expansion coefficients were determined from the measured cell volumes with equations of the form α=α₀ + α₁ T + α₂/T ². Along with available isobaric heat capacity and compressibility data, these derived coefficients clearly show that anharmonic effects contribute little to the isochoric heat capacities (C _v ) of CaO, MgO, and Al₂O₃, which do not depart appreciably from the 3nR Dulong and Petit limit. Received: 31 March 1999 / Revised, accepted: 23 June 1999     

High temperature dehydroxylation of muscovite-2M1: a kinetic study by in situ XRPD

Muscovite-2M₁ shows a major phase transition at about 800°C, which is generally attributed in the literature to the structural dehydroxylation process, although a number of structural models have been proposed for the dehydroxylated phase, and different transformation mechanisms have also been put forward. The observed first order transformation involves an increase in the cell volume, and it is not clear to date how the cell expansion is related to the loss of hydroxyl groups. The phase change has been re-investigated here by in situ high temperature powder diffraction, both in non-isothermal and isothermal modes, to combine for the first time the structural and the kinetic interpretation of the transformation. The results unequivocally confirm that the reaction taking place in the temperature range 700–1000°C is truly a dehydroxylation process, involving the nucleation and growth of the high temperature dehydroxylated phase, having Al in 5-fold coordination. Structural simulations of the basal peaks of the powder diffraction patterns indicate that the model originally proposed by Udagawa et al. (1974) for the dehydroxylated phase correctly describes the high temperature phase. The kinetic analysis of the isothermal data using an Avrami-type model yields values for the reaction order compatible with a reaction mechanism limited by a monodimensional diffusion step. Apparent activation energy of the process in vacuum is about 251 kJ/mol. Experiments carried out at temperatures much higher than the onset temperature of the reaction show that the dehydroxylation reaction overlaps with the reaction of formation of mullite, the final product in the reaction pathway. Received: 24 April 1998 / Revised, accepted: 12 October 1998     

The combined effect of moisture and temperature on the anomalous expansion behaviour of marble

Three calcitic marble types often used as cladding material and different in lattice preferred orientation (texture), grain shape preferred orientation, grain size distribution and grain interlocking are investigated to study the combined effect of thermal cycles (day-night) and moisture on the decay of marble, particularly on the bowing phenomenon. Repetitive heating-cooling under dry conditions leads to considerable inelastic residual strain only in the first thermal cycle. The residual strain continuously increases again if water is present, whereby the moisture content after a thermal cycle has a certain impact on the decay rate. The water-enhanced thermal dilatation strongly correlates with the deterioration rate obtained from the laboratory bow test. All applied approaches reveal that the texture in combination with the grain shape preferred orientation control the intensity and anisotropy of marble deterioration, thus, the cutting direction of facade panels has to be considered in terms of durability. On-site analysis from building facades confirm the laboratory results such as the bowing tendency of different marbles, directional dependence, relevance of moisture content during cyclical heating and the loss of strength due to environmental impact.Special Issue: Stone decay hazards     

Effect of pressure on the thermal expansion of MgO up to 8.2 GPa

Isobaric volume measurements for MgO were carried out at 2.6, 5.4, and 8.2 GPa in the temperature range 300–1073 K using a DIA-type, large-volume apparatus in conjunction with synchrotron X-ray powder diffraction. Linear fit of the thermal expansion data over the experimental pressure range yields the pressure derivative, (∂α/∂P) _T , of −1.04(8) × 10⁻⁶ GPa⁻¹ K⁻¹ and the mean zero-pressure thermal expansion α_{0, T}  = 4.09(6) × 10⁻⁵ K⁻¹. The α_{0, T} value is in good agreement with results of Suzuki (1975) and Utsumi et al. (1998) over the same temperature range, whereas (∂α/∂P) _T is determined for the first time on MgO by direct measurements. The cross-derivative (∂α²/∂P∂T) cannot be resolved because of large uncertainties associated with the temperature derivative of α at all pressures. The temperature derivative of the bulk modulus, (∂K _T/∂T) _P , of −0.025(3) GPa K⁻¹, obtained from the measured (∂α/∂P) _T value, is in accord with previous findings. Received: 2 April 1999 / Revised, accepted: 22 June 1999     

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.     

阿尔泰可可托海3号脉伟晶岩型稀有金属矿床云母和长石的矿物学研究及意义

可可托海3号脉伟晶岩型稀有金属矿床是阿尔泰造山带产出的规模最大的伟晶岩脉,其完美的同心环状结构分带举世闻名。云母和长石作为3号脉9个结构带的贯通性矿物,由外向内表现不同的结构和成分特征。其中,云母由白云母系列向锂云母系列演化,白云母呈黄-绿色中细粒→白色或绿色中粗粒-巨晶→白色或绿色书状集合体→白色或绿色中粗粒-巨晶,锂云母呈玫瑰紫中细粒鳞片状或楔状集合体,BSE图像下云母表现出成分分带及不平衡和交代结构;长石主要为钾长石和钠长石,及少量斜长石,钾长石主要呈块体产出,钠长石呈细粒→叶片状→薄片状产出。本次研究通过电子探针(EMPA)和激光剥蚀等离子质谱(LA-ICP-MS)获得3号脉各结构带云母和长石的主微量成分。3号脉云母具有高Li(249×10^-6~35932×10^-6)、Rb(1240×10^-6~22825×10^-6)、Cs(35.9×10^-6~13980×10^-6)、Ta(13.3×10^-6~447×10^-6)含量、低K/Rb值(4.23~59.4)和K/Cs值(6.53~2368),钾长石具有低K/Rb值(35.4~1865),且由外向内,随K/Rb值降低,云母的Li、Rb、Cs、F、Ta含量升高,表明3号脉是一个由外向内结晶的分异演化程度较高的伟晶岩脉。另外,连续相邻结构带中云母和长石的主微量成分呈振荡变化,该现象主要受熔体不混溶过程的控制,也受矿物结晶不平衡影响,而熔体不混溶过程也是控制3号脉结构分带的机制之一。外部带(I-IV带)和内部带(V-VIII带)的云母和碱性长石在成分(FeO、Li、Rb、Cs、F、Ta含量和K/Rb值及K/Cs值)和结构(不平衡和交代结构)上具有明显差异,内部带演化程度明显加大,流体组分比例升高,表明体系由以熔体为主的阶段(外部带)进入以熔流体为主相对不稳定的阶段(外部带)。结合野外观察的证据,促使体系在IV带和V带间发生突然转变而进入熔流体阶段的是一个泄压事件。