Characterization of beryl (aquamarine variety) from pegmatites of Minas Gerais,Brazil

Eight samples of the beryl variety aquamarine were selected from four pegmatites in the Governador Valadares and Araçuaí regions in northeastern Minas Gerais State, Brazil. These samples were fully characterized by chemical analysis, infrared and UV-visible spectroscopy, thermal analyses, and high-temperature X-ray diffraction (from room temperature up to 800 °C). Several physical and chemical properties of beryl were found to depend on the amount of water and ions residing in the structural channels. The thermal expansion coefficients from room temperature to about 800 °C are temperature-independent, with αa ? ?3.2 × 10^?6 ° C^?1 and αc ? ?8.7 × 10^?6 ° C^?1. The contraction of both a and c unit-cell parameters with increasing temperature and the shift of the infrared band centered at about 1200 cm^?1 were tentatively ascribed to interactions between channel water and the silicate rings.The color of beryl seems to be dictated by the relative proportions of Fe³⁺ in the octahedralsites and of fe²⁺ in the channels. Thus, deep-blue samples have little Fe³⁺, whereas greener samples have more Fe³⁺ or less channel Fe²⁺.     

Characterization of beryl (aquamarine variety) by Mössbauer spectroscopy

The Mössbauer spectra of several blue beryls have been obtained in the temperature range of 4.2–500 K. A common feature observed in all room-temperature spectra is the presence of an asymmetric Fe²⁺ doublet (ΔE _Q ?～?2.7?mm?s^?1, δ?～?1.1?mm?s^?1), with a very broad low-velocity peak. This asymmetry seems to be related to a relaxation process involving ferrous ions and water molecules in the structural channels, as suggested by Price et?al. (1976). Surprisingly, the spectrum at 500?K also shows a broad, but symmetrical, doublet, with a clear splitting of the lines indicating the presence of at least two Fe²⁺ components. The room-temperature spectrum obtained after the 500?K run shows the same features as prior to the heating. At 4.2?K the spectrum of a deep blue beryl was well fitted with four symmetrical doublets, one of which could be related to Fe²⁺ in the structural channels. Ferrous ion was also found to occupy the octahedral and tetrahedral sites, whereas ferric ion is most probably located in the octahedral site. A meaningful fit of the room-temperature spectrum, as well as an explanation for the temperature dependence of the Mössbauer spectra, are discussed. Finally, it is believed that the color in beryl will be dictated by the relative proportions of Fe³⁺ in the octahedral sites and of Fe²⁺ in the channels.     

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe³⁺ in blue beryl occupies a substitutional Al³⁺ site and in green beryl is localized in the structural channels between two O₆ planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe²⁺. Thermal treatments above 200° C in green beryl cause the reduction of Fe³⁺ into Fe²⁺ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe³⁺ into Fe²⁺ is composed of two first order processes; the first one has an activation energy ΔE ₁=0.30 eV and the second one has an activation energy ΔE ₂=0.46 eV.     

Blue, complexly zoned, (Na,Mg,Fe,Li)-rich beryl from quartz-calcite veins in low-grade metamorphosed Fe-deposit Sk��ly near Ryma?ov, Czech Republic

Syn-tectonic quartz-calcite veins containing blue beryl are enclosed in hematite > magnetite-rich portions of the low-grade metamorphosed Fe-deposit Skály near Ryma?ov, Czech Republic. Aggregates of pale to deep blue beryl, up to 2?cm in diameter, are associated with euclase, clinochlore, hematite, albite and dravite. Complexly zoned beryl crystals consist of skeletal aggregates of beryl I randomly distributed within volumetrically dominant beryl II with narrow rims of beryl III. All types of beryl have similar contents of Na (0.32?C0.49 apfu) and Mg (0.31?C0.41 apfu) but variable contents of Fe_tot (0.05?C0.34 apfu) and Al (1.20?C1.62 apfu). The LA-ICP-MS study yielded elevated contents of Li, up 1,314?ppm (0.28?wt.% Li₂O) in beryl I. The quartz-calcite veins represent an unusual type of low-T metamorphic-hydrothermal vein related to Fe-ore deposit characterized by single-stage fracturing and mobilization in a closed system at T~200?C300°C and CO ₃ ^2- as a major complexing agent for the mobility of Be.     

Thermal expansion,Debye temperature and Grüneisen parameter of synthetic (Fe,Mg)SiO3 orthopyroxenes

Thermal expansion properties of synthetic orthopyroxenes (Fe_0.20Mg_0.80)SiO₃, (Fe_0.40Mg_0.60)SiO₃, (Fe_0.50Mg_0.50)SiO₃, (Fe_0.75Mg_0.25)SiO₃ and (Fe_0.83Mg_0.17)SiO₃ were systematically studied by means of single-crystal x-ray diffraction in the temperature range from 296 to 1300 K. The measurements of unit cell dimensions as a function of temperature reveal that the a and c dimensions and the unit cell volume V increase nonlinearly with a positive curvature with rising temperature, whereas the b dimension behaves differently, depending on the total Fe content. For Mg-rich orthopyroxenes (Fe/(Fe+Mg)<30%), the b dimension expands similarly as the a and c dimensions, but it exhibits a nonlinear increase with a negative curvature for orthopyroxenes with Fe/(Fe+Mg)>30%. Together with the high temperature neutron diffraction data on enstatite (MgSiO₃) (McMullan, Haga and Ghose, unpublished) and x-ray diffraction data on ferrosilite (FeSiO₃) (Sueno et al. 1976), the measured unit cell dimensions were analyzed in terms of the Grüneisen theory of thermal expansion. The linear thermal expansion coefficients α _a and α _c both increase as temperature is elevated, with α _c increasing faster, while α _b changes gradually from increasing for Mg-rich orthopyroxenes to decreasing for Fe-rich orthopyroxenes. The relative magnitudes of linear thermal expansion coefficients are always in the order α _b >α _c >α _a between 300 and 500 K, but at higher temperatures, the order changes to α _c >α _b >α _a for Mg-rich orthopyroxenes and α _c >α _a >α _b for Fe-rich ones. The linear thermal expansion behavior is interpreted on the basis of the structural mechanical model of Weidner and Vaughan (1982). The anomalous behavior of α _b is mainly attributed to the changes in the Fe₂₊ population at the M2 site and the relative stiffness of the M2(Fe₂₊)-O bonds compared to the M2(Mg₂₊)-O bonds. The volume thermal expansion coefficients are nonlinear functions of temperature and lie between 23 and 49×10^?6/K. The previously reported results of mean volume thermal expansion coefficients appear to represent the α _V values characteristic of higher temperatures compared to our results. The thermal Debye temperatures are composition-dependent, decreasing linearly from 812 (MgSiO₃) to 561 K (FeSiO₃), and are systematically higher than the corresponding acoustic Debye temperatures. The Grüneisen parameters range from 0.85 to 0.89 and do not seem to vary with composition. The linear compressibilities derived from thermal expansion and elastic moduli data agree very well. The pressure derivatives of the isothermal bulk modulus (dK₀/dP) are also composition-dependent and decrease from 11.2 (MgSiO₃) to 8.77 (FeSiO₃). Such large values indicate possible anomalous elastic behavior of orthopyroxenes at high pressures in the Earth's upper mantle.     

Thermal expansion of single-crystal forsterite to 1023 K by Fizeau interferometry

Using a Fizeau interferometry technique, we have measured the coefficients of linear thermal expansion of single-crystal forsterite (Mg₂SiO₄) along three axial directions to 1023 K during heating and cooling cycles. Overall, the present data are consistent in magnitude (within 1 to 2%) with those previously reported but have less scatter. We used the Grüneisen statistical mechanical approach in analzying the data. The least-squares method was applied to evaluate thermal parameters (?, Q ₀, k and a) in two cases. The expansion coefficients in wider temperature ranges were extrapolated by using the parameters of solution 2 (i.e., solution by fixing ? and k). In contrast to earlier findings, our results show that for forsterite the Grüneisen parameter decreases with temperature, implying that it does not behave too differently from fayalite (Fe₂SiO₄) and periclase (MgO).     

Positron annihilation lifetime spectral changes on dehydration of natural beryls and cordierites

The effect on positron annihilation lifetime spectra, measured at room temperature, of the dehydration of single crystals of beryl and cordierite was studied. In each case, the spectra were satisfactorily fitted to three lifetime components. For the beryls, the dehydration considerably enhanced the intensity of the intermediate-lifetime component (I ₂) and reduced the intensity of the longest-lived component (I ₃). I ₂ is attributed to positron annihilation in the empty cages in the channels of the beryl structure and I ₃ to annihilation by a pick-off process via unknown foreign molecules. However, for the cordierites, the main effect of the dehydration was a slight (～10%) increase in the lifetime of the intermediate component, τ₂. Here I ₂ was relatively high both before and after dehydration and the increase in τ₂ is attributed to Si-Al ordering. No changes in the lifetime spectra were produced by γ irradiation.     

EPR investigation of NO2 and CO2 − and other radicals in beryl

A number of different impurities are located in the open channels of natural beryl crystals. The rare Maxixe beryl contains an unusual amount of NO₂. The isoelectronic CO₂ ⁻ radical is found in the irradiated Maxixe-type beryl. The NO₂ radicals are distributed in the Be–Al plane of the crystal, with the nitrogen atom close to the oxygens of the beryl cavity wall. These oxygens repel the negative CO₂ ⁻ radical, which is located at the center of the beryl cavity and rotates around its O–O axis, which is parallel to the crystal c-axis. When there is a nearby alkali ion at the center of the beryl channel, it reorients the CO₂ ⁻ radical so that its bisector is parallel to the c-axis and points toward the positive ion. Different signals are analyzed for Li⁺, Na⁺, and another counter-ion, which probably is Cs⁺. The related NO₃ and CO₃ ⁻ radicals are the color centers in the investigated deep blue beryls. The slow decay of the color, which makes these beryls useless as gem stones, is related to the decay of the hydrogen atoms which are present in these crystals. Evidence is given that NO₃ is created in Maxixe beryl by a natural process, while CO₃ ⁻ in Maxixe-type beryl has been created by irradiation. The temperature dependence of the EPR signals of these two radicals was investigated, but a definitive proof that they rotate at the center of the beryl cavity could not be given. EPR signals from some other radicals in beryl have been observed and described.     

四川雪宝顶钨锡铍矿床流体包裹体研究及其意义

四川雪宝顶钨锡铍矿床产于花岗岩体与三叠系地层大理岩的接触带,赋矿石英脉受大理岩中的劈理破碎带控制。绿柱石与白钨矿中的包裹体可分为熔融包裹体、流体熔融包裹体和流体包裹体3类。流体包裹体又可分为H2O包裹体、CO2包裹体和CO2-H2O包裹体,其中,绿柱石中以富含CO2-H2O包裹体为显著特征。加热时,富H2O相CO2-H2O包裹体完全均一至H2O相,富CO2相CO2-H2O包裹体完全均一至CO2相,而二者的完全均一温度和均一压力一致,表明它们是同期捕获的CO2-低盐水不混溶包裹体组合。与绿柱石相比,白钨矿中CO2-H2O包裹体数量明显减少,H2O包裹体数量增多,成矿压力与成矿温度均有所降低。含CO2流体在花岗岩体与大理岩接触带附近发生流体不混溶和相分离,CO2的出溶使成矿流体中pH值升高,f(O2)降低,导致钨的溶解度降低而沉淀,这是形成白钨矿的主要原因。     

Raman spectra and X-ray diffraction of tuite at various temperatures

High-temperature Raman spectra and thermal expansion of tuite, γ-Ca₃(PO₄)₂, have been investigated. The effect of temperature on the Raman spectra of synthetic tuite was studied in the range from 80 to 973 K at atmospheric pressure. The Raman frequencies of all observed bands for tuite continuously decrease with increasing temperature. The quantitative analysis of temperature dependence of Raman bands indicates that the changes in Raman frequencies for stretching modes (ν₃ and ν₁) are faster than those for bending modes (ν₄ and ν₂) of PO₄ in the present temperature range, which may be attributed to the structural evolution of PO₄ tetrahedron in tuite at high temperature. The thermal expansion of tuite was examined by means of in situ X-ray diffraction measurements in the temperature range from 298 to 923 K. Unit cell parameters and volume were analyzed, and the thermal expansion coefficients were obtained as 3.67 (3), 1.18 (1), and 1.32 (3) × 10^?5 K^?1 for V, a, and c, respectively. Thermal expansion of tuite shows an axial anisotropy with a larger expansion coefficient along the c-axis. The isothermal and isobaric mode Grüneisen parameters and intrinsic anharmonicity of tuite have been calculated by using present high-temperature Raman spectra and thermal expansion coefficient combined with previous results of the isothermal bulk modulus and high-pressure Raman spectra.     

The partial molar volume, thermal expansivity, and compressibility of H2O in NaAlSi3O8 liquid: new measurements and an internally consistent model

The molar volumes of 19 hydrous albitic liquids (1.9 to 6.1 wt% H₂O^total) were determined at one bar and 505–765 K. These volume data were derived from density measurements on hydrous glasses at 298 K, followed by measurements of the thermal expansion of each glass from 298 K to its respective glass transition temperature. The technique exploits the fact that the volume of a glass is equal to that of the corresponding liquid at the limiting fictive temperature (T _f′), and that T _f′ can be approximated as the temperature near the onset of the rapid increase in thermal expansion that occurs in the glass transition interval. The volume data of this study were combined with available volume data for anhydrous, Na₂O-Al₂O₃-SiO₂ liquids to derive the partial molar volume (±1) of the H₂O component in an albitic melt at ∼565 K and one bar. To extend the determination of to higher temperatures and pressures, the molar volumes of the hydrous albitic liquids determined in this study were combined with those measured by previous authors at 1023–1223 K and 480–840 MPa, leading to the following fitted values (±1) at 1673 K and one bar: (±0.46)×10⁻³ cm⁻³/mol-K, and dVˉ ^H ₂ ^{O total} /dP=−3.82 (±0.36)×10⁻⁴ cm³/mol-bar. The measured molar volumes of this study and those of previous authors can be recovered with a standard deviation of 0.5%, which is within the respective experimental errors. There is a significant difference between the values for derived in this study as a function of temperature and pressure and those obtained from an existing polynomial, primarily caused by the previous absence of accurate density measurements on anhydrous silicate liquids. The coefficients of thermal expansion (=4.72×10⁻⁴/K) and isothermal compressibility ( _T =1.66×10⁻⁵/bar) for the H₂O component at 1273 K and 100 MPa, indicate that H₂O^total is the single most expansive and compressible component in silicate liquids. For example, at 1473 K and 70 MPa (conditions of a mid-ocean ridge crustal magma chamber), the presence of just 0.4 wt% H₂O will decrease the density of a basaltic liquid by more than one percent. An equivalent decrease in melt density could be achieved by increasing the temperature by 175 degrees or the decreasing pressure by 230 MPa. Therefore, even minor quantities of dissolved water will have a marked effect on the dynamic properties of silicate liquids in the crustal environment. Received: 20 August 1996 / Accepted: 15 March 1997     

The thermal expansion of anhydrite to 1000° C

The thermal expansion of anhydrite, CaSO₄, has been measured from 22° to 1,000° C by X-ray diffraction, using the Guinier-Lenné heating powder camera. The heating patterns were calibrated with Guinier-Hägg patterns at 25° C, using quartz as internal standard. Heating experiments were run on natural anhydrite (Bancroft, Ontario), which at room temperature has lattice constants in close agreement with those of synthetic material. The orthorhombic unit cell at 22° C (space group Amma) has a=7.003 (1) Å, b=6.996 (2) Å and c=6.242 (1) Å, V=305.9 (2) ų. At room temperature, the thermal expansion coefficients α and β (α in °C^?1×10⁴, β in °C^?2×10⁸) are for a, 0.10, ?0.69; for b, 0.08, 0.19; for c, 0.18, 1.60; for V, 0.37, 1.14. Second-order coefficients provide an excellent fit over the whole range to 1,000° C.     

Partition of Sr,Ba, Ca,Y, Eu2+, Eu3+, and other REE between plagioclase feldspar and magmatic liquid: an experimental study

Plagioclase feldspar/magmatic liquid partition coefficients for Sr, Ba, Ca, Y, Eu²⁺, Eu³⁺ and other REE have been determined experimentally at 1 atm total pressure in the temperature range 1150–1400°C. Natural and synthetic melts representative of basaltic and andesitic bulk compositions were used, crystallizing plagioclase feldspar in the composition range An₃₅–An₈₅. Partition coefficients for Sr are greater than unity at all geologically reasonable temperatures, and for Ba are less than unity above approximately 1060°C. Both are strongly dependent upon temperature. Partition coefficients for the trivalent REE are relatively insensitive to temperature. At fixed temperature they decrease monotonically from La to Lu. The partition of Eu is a strong function of oxygen fugacity. Under extreme reducing conditions D_Eu approaches the value of D_Sr.     

A high P-T single-crystal X-ray diffraction study of thermoelasticity of MgSiO3 orthoenstatite

P-V-T data of MgSiO₃ orthoenstatite have been measured by single-crystal X-ray diffraction at simultaneous high pressures (in excess of 4.5 GPa) and temperatures (up to 1000 K). The new P-V-T data of the orthoenstatite, together with previous compression data and thermal expansion data, are described by a modified Birch-Murnaghan equation of state for diverse temperatures. The fitted thermoelastic parameters for MgSiO₃ orthoenstatite are: thermal expansion ?α/?P with values of a=2.86(29)×10^-5 K^-1 and b=0.72(16)×10^-8 K^-2; isothermal bulk modulus K _{T o} =102.8(2) GPa; pressure derivative of bulk modulus K′=?K/?P=10.2(1.2); and temperature derivative of bulk modulus K=?K/?T=-0.037(5) GPa/K. The derived thermal Grüneisen parameter is γ _th=1.05 for ambient conditions; Anderson-Grüneisen parameter is δ _{T o} =11.6, and the pressure derivative of thermal expansion is ?α/?P=-3.5×10^-6K^-1 GPa^-1. From the P-V-T data and the thermoelastic equation of state, thermal expansions at two constant pressures of 1.5 GPa and 4.0 GPa are calculated. The resulting pressure dependence of thermal expansion is Δα/ΔP=-3.2(1)× 10^-6 K^-1 GPa^-1. The significantly large values of K′, K, δ _T and ?α/?P indicate that compression/expansion of MgSiO₃ orthoenstatite is very sensitive to changes of pressure and temperature.     

The thermal expansion of synthetic aluminosilicate-sodalites,M 8(Al6Si6O24)X 2

Thermal expansion data, determined by powder X-ray diffraction methods are presented for 11 members of the (Li,Na,K,Rb)₈(Al₆Si₆O₂₄)Cl₂ solid solution series, 3 members of the (Na,K)₈(Al₆Si₆O₂₄)Br₂ solid solution series and Na₈(Al₆Si₆O₂₄)I₂. Only the latter showed a discontinuity in its expansion curve at 810° C wigh a mean linear expansion coefficient of 22.0×10^?6 °C^?1 below and 7.7×10^?6 °C^?1 above the discontinuity. The mean expansion coefficients from 0° to 500° C decrease gradually over the range of room temperature cell edges from 8.4 to 8.89 Å, then increase up to a cell edge of 9.01 Å above which they decrease sharply and extrapolate to a zero coefficient at 9.4 Å. These variations may be related to the expansion characteristics of the bonds between the cavity cations and cavity anions in different sodalites. The aluminosilicate-sodalites which show a discontinuity in their thermal expansion curves are those with large cavity anions, I^? or SO ₄ ^2? ; the discontinuity is believed to occur at the point when the x-coordinate of the cavity cation becomes 0.25.     

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.     

The positions of H+, Li+ and Na+ impurities in beryl

Electron Paramagnetic Resonance (EPR) measurements show that Li⁺ impurities are located at two different positions in beryl, one in the crystal lattice and the other in the crystal channel. The position of the Li⁺ impurity in the lattice is generally assumed to be at the site of a missing Be²⁺ ion. It is shown that this is not the case, but that the Li⁺ ion is located in a tetrahedron formed by the oxygens of one side of the Be tetrahedron and the nearest oxygen in the channel ring. This Li site has the coordinates (0.423, 0.344, 0.167) and can only be occupied when the neighbouring Be site is empty. There are four such sites around every Be tetrahedron at the distance of 1.46 Å from the Be site. The distance from the Li site to the oxygens of the Li tetrahedron is 1.84 Å. This compares favourably with the much smaller distance of 1.65 Å in the Be tetrahedron. Protons in beryl are trapped at or near these Li sites. Na⁺ is known to be located at the 2b position at the center of the silicate rings, where it is stabilized by one water molecule located at each of the two surrounding 2a sites. This is also the position of Li⁺ in the beryl channel. It is found that the presence of Na⁺ in the ring of six oxygens reduces the radius of this ring. The Na⁺ impurity has also been supposed to be located at position 2a alone and at 2b stabilized by only one water molecule. It is now proposed that Na⁺ and H₂O are located together in the Al–Be plane when only one water molecule is associated with Na⁺. The water oxygen is located at or near 2a and Na is closer to the Be site in tetrahedral beryl and closer to the Al site in octahedral beryl. It is proposed that the water protons are also located in the Al–Be plane, which would mean that there exists a third type of water in beryl. The origin of protons and OH^? ions in beryl is discussed and it is suggested that the plugs in the beryl channels are CO ₃ ^2? ions. Diffusion of OH^? ions and natural radiation may have led to the creation of NO₃ and the blue colour of Maxixe beryl.     

Partitioning of strontium between calcite,dolomite and liquids: An experimental study under higher temperature diagenetic conditions,and a model for the prediction of mineral pairs for geothermometry

The partitioning of Sr between calcite, dolomite and liquids is essentially independent of temperature between 150° and 350° C. The partition coefficients corrected for number of cation sites are b _calc=0.096 and b _dol= 0.048 for 1 mol cations/6 mol H₂O liquid. Upon dilution the partition coefficients increase, but their ratio stays constant at about 2∶1. This ratio is due to the fact that calcite has twice as many Ca-sites for Sr-substitution as dolomite. The 2∶1 relationship is also observed in natural calcite and dolomite which have undergone diagenesis. The temperature independence of partitioning is caused by the relatively small thermal expansion of calcite and dolomite. Thermal expansion between 25° and 400° C was found to follow the equations V _calc=7.0·10⁻⁴ T(°C)+36.95 and V _dol=6.9·10⁻⁴ T(°C)+32.24, V: cm³/mol. Therefore calcite and dolomite cannot serve as a temperature indicator. To have an ideal geothermometer a mineral pair with high and low thermal expansion is required. Literature date demonstrate that wurtzite, sphalerite, and galena are such minerals.     

A room-temperature phase transition in maximum microcline

A type of hysteresis, similar to that observed in the heat capacity measurements (Openshaw et al., 1979), has been found in the room temperature unit cell parameters of a microcline sample 71104 and likewise indicates the existence of two forms of this microcline. The A-form (obtained on cooling the sample to approximately 80 K) has significantly different values of b, β and V to the B-form which is the more stable form above 300±10 K. The transition from the A- to the B-form occurs over a period of months and has an associated ΔV of ?0.0011 nm³. The cell parameters of the B-form have been measured up to 1278 K and show significant changes: a and V increase, b constracts, and c is unchanged on increasing temperature. The calculated thermal expansion ellipsoid is nearly uniaxial and similar in shape to that for sanidine. Below room temperature the isobaric thermal expansion coefficent α_p, for a natural microcline is nearly four times as large as that for sanidine. Above room temperature α_p for 71104 microcline decreases markedly with increasing temperature. This implies a rapid change in the thermal expansion behavior of microcline which has been correlated with the proposed phase transition.     

A single-crystal neutron and X-ray diffraction study of pezzottaite, Cs(Be2Li)Al2Si6O18

The chemical composition and the crystal structure of pezzottaite [ideal composition Cs(Be₂Li)Al₂Si₆O₁₈; space group: ${\it{R}} \overline{\text{3}} $ c, a?=?15.9615(6) ?, c?=?27.8568(9) ?] from the type locality in Ambatovita (central Madagascar) were investigated by electron microprobe analysis in wavelength dispersive mode, thermo-gravimetric analysis, Fourier-transform infrared spectroscopy, single-crystal X-ray (at 298?K) and neutron (at 2.3?K) diffraction. The average chemical formula of the sample of pezzottaite resulted ^Cs1,Cs2(Cs_0.565Rb_0.027K_0.017)_Σ0.600 ^Na1,Na2(Na_0.101Ca_0.024)_Σ0.125Be_2.078Li_0.922 ^Al1,Al2(Mg_0.002Mn_0.002Fe_0.003Al_1.978)_Σ1.985 ^Si1,Si2,Si3(Al_0.056Si_5.944)_Σ6O₁₈·0.27H₂O. The (unpolarized) IR spectrum over the region 3,800–600?cm^?1 was collected and a comparison with the absorption bands found in beryl carried out. In particular, two-weak absorption bands ascribable to the fundamental H₂O stretching vibrations (i.e. 3,591 and 3,545?cm^?1) were observed, despite the mineral being nominally anhydrous. The X-ray and neutron structure refinements showed: (a) a non-significant presence of aluminium, beryllium or lithium at the Si1, Si2 and Si3 sites, (b) the absence (at a significant level) of lithium at the octahedral Al1, Al2 and Al3 sites and (c) a partial lithium/beryllium disordering between tetrahedral Be and Li sites.