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.     

The effect of thermal treatment on the 57Fe Mössbauer spectrum of beryl

Mössbauer spectra (MS) of blue, green and yellow beryl (ideally Be₃Al₂Si₆O₁₈) containing approximately 1% of iron were obtained at 295 and 500 K. Room temperature (RT) spectra of both blue and green samples showed the presence of an asymmetric Fe²⁺ doublet (ΔE _Q~2.7 mm/s, δ~1.1 mm/s), with a very broad low-velocity peak. There is no clear evidence for the presence of a ferric component. The MS of the yellow sample at RT consists of an intense central absorption with parameters typical for Fe³⁺ (ΔE _Q~0.4 mm/s, δ~0.29 mm/s), plus an apparently symmetrical Fe²⁺ doublet. This sample acquires a light-blue shade upon heating in air at about 620 K. Thermal treatments at high temperatures caused no significant changes in the MS, but the green and yellow beryl acquire a blue colour. All these results are interpreted in relation to the existence of channel water and the distribution of iron among the available crystallographic sites.     

Natural iron-containing blue and colorless euclase studied by electron paramagnetic resonance

Natural blue and colorless rare-gem mineral specimens of euclase from Brazil are investigated by electron paramagnetic resonance (EPR). Angular dependences of Fe³⁺ EPR spectra in three mutually perpendicular crystal planes are analyzed revealing g and D tensors with significant low-symmetry effects, as for example, the high asymmetry parameter E/D = 0.28. Fourth-order degree Stevens parameters are also included in analysis. The anisotropy of both g and D tensors is consistent with Fe³⁺ substituting for Al³⁺ ions in strongly distorted AlO₅(OH) octahedra in which the oxygen distances range from 1.85 to 1.98 Å. Fe³⁺ is not responsible for the blue color because colorless and blue euclase show nearly the same Fe³⁺ concentration as measured by EPR. However, total iron content in blue sample is much higher than in the colorless one suggesting that the existing model that Fe²⁺–Fe³⁺ intervalence charge transfer transition may explain the blue color of euclase.     

Cordierite III: the site occupation and concentration of Fe3+

Cordierite has the ideal formula (Mg,Fe)₂Al₄Si₅O₁₈ ^.x(H₂O,CO₂), but it must contain some Fe³⁺ to account for its blue color and strong pleochroism. The site occupation and concentration of Fe³⁺ in two Mg-rich natural cordierites have been investigated by EPR and ⁵⁷Fe Mössbauer spectroscopy. In addition, powder IR spectroscopy, X-ray diffraction, and TEM examination were used to characterize the samples. Single-crystal and powder EPR spectra indicate that Fe³⁺ is located on T₁1 in natural cordierites and not in the channels. The amount in Mg-rich cordierites is very small with an upper limit set by Mössbauer spectroscopy giving less than 0.004 cations per formula unit (pfu). Fe³⁺ in cordierite can, therefore, be considered insignificant for most petrologic calculations. Heat-treating cordierite in air at 1,000?°C for 2?days causes an oxidation and/or loss of Fe²⁺ on T₁1, together with an expulsion of Na⁺ from the channels, whereas heating at the Fe–FeO buffer produces little Fe³⁺ in cordierite. Heating at 1,000?°C removes all class I H₂O, but small amounts of class II H₂O remain as shown by the IR measurements. No evidence for channel Fe²⁺ or Fe³⁺ in the heat-treated samples was found. The blue color in cordierite arises from a broad absorption band (E//b and weaker with E//a) around 18,000?cm^?1 originating from charge-transfer between Fe²⁺ in the octahedron and Fe³⁺ in the edge-shared T₁1 tetrahedron. It therefore appears that all natural cordierites contain some tetrahedral Fe³⁺. The brown color of samples heated in air may be due to the formation of very small amounts of submicroscopic magnetite and possibly hematite. These inclusions in cordierite can only be identified through TEM study.     

Mössbauer spectra of kyanite,aquamarine, and cordierite showing intervalence charge transfer

The blue colors of several minerals and gems, including aquamarine (beryl, Be₃Al₂Si₆O₁₈) and cordierite (Al₃(Mg, Fe)₂Si₅AlO₁₈), have been attributed to charge transfer (CT) between adjacent Fe²⁺ and Fe³⁺ cations, while Fe²⁺→Ti⁴⁺ CT has been proposed for blue kyanites (Al₂SiO₅). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mössbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe²⁺ and Fe³⁺ were detected with MS, indicating that Fe²⁺→Fe³⁺ CT, Fe²⁺→Ti⁴⁺ CT, and Fe²⁺ and Fe³⁺ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe²⁺ and Fe³⁺ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm^?1 in E∥c spectra to Fe²⁺→Fe³⁺ CT between Fe cations replacing Al³⁺ ions 4.6Å apart along c. A band at 17,500 cm^?1 in E⊥c spectra of cordierite is generally assigned to Fe²⁺ (oct)→Fe³⁺ (tet) CT between cations only 2.74 Å apart. However, no Fe³⁺ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm^?1 band and reported to contain Fe³⁺. A quadrupole doublet with parameters consistent with tetrahedral Fe³⁺ appears in 77K MS, but the Fe³⁺/Fe²⁺ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mössbauer and chemically determined Fe³⁺/Fe²⁺ ratios for minerals exhibiting Fe²⁺→Fe³⁺ CT.     

Electron paramagnetic resonance spectroscopy of Fe<Superscript>3+</Superscript> ions in amethyst: thermodynamic potentials and magnetic susceptibility

Single-crystal and powder electron paramagnetic resonance (EPR) spectroscopic studies of natural amethyst quartz, before and after isochronal annealing between 573 and 1,173 K, have been made from 90 to 294 K. Single-crystal EPR spectra confirm the presence of two substitutional Fe³⁺ centers. Powder EPR spectra are characterized by two broad resonance signals at g = ~10.8 and 4.0 and a sharp signal at g = 2.002. The sharp signal is readily attributed to the well-established oxygen vacancy electron center E ₁′. However, the two broad signals do not correspond to any known Fe³⁺ centers in the quartz lattice, but are most likely attributable to Fe³⁺ clusters on surfaces. The absolute numbers of spins of the Fe³⁺ species at g = ~10.8 have been calculated from powder EPR spectra measured at temperatures from 90 to 294 K. These results have been used to extract thermodynamic potentials, including Gibbs energy of activation ΔG, activation energy E _a, entropy of activation ΔS and enthalpy of activation ΔH for the Fe³⁺ species in amethyst. In addition, magnetic susceptibilities (χ) have been calculated from EPR data at different temperatures. A linear relationship between magnetic susceptibility and temperature is consistent with the Curie–Weiss law. Knowledge about the stability and properties of Fe³⁺ species on the surfaces of quartz is important to better understanding of the reactivity, bioavailability and heath effects of iron in silica particles.     

The investigation of defects in shock-metamorphosed quartz

The shock-metamorphosed quartz exhibits thermal luminescence (TL) with maxima at 365 nm, 470 nm and 610–680 nm. By electron paramagnetic resonance (EPR) analysis E₁ type electron centers and hole centers have been found which originate from vacancies including those from the substitution of Al³⁺ and/or Fe³⁺, for Si⁴⁺. The EPR and TL spectra may be interpreted mainly in terms of vacancy type defects associated with dislocations in the crystal structure of quartz.     

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.     

绿柱石中铁离子的晶格占位及致色作用

绿柱石族宝石颜色丰富,一直是市场上重要的宝石品种。几十年来,国内外学者对不同颜色绿柱石的致色机理及改色工艺开展了广泛的研究。铁离子作为重要的致色过渡金属离子之一,其价态调控是绿柱石改色工艺的关键,所以铁离子的价态、晶格占位和对应的致色作用也一直是绿柱石研究的焦点。铁离子被认为可能存在于绿柱石中的铝氧八面体Al³⁺格位、硅氧四面体Si⁴⁺格位、铍氧四面体Be²⁺格位、结构通道2a或2b位和晶格间隙6g位等位置,是蓝、绿或黄等颜色绿柱石的主要致色元素。本文通过对绿柱石的晶体结构、铁离子的核外电子排布和晶格占位等方面文献资料的梳理及综合分析,认为Fe²⁺和Fe³⁺分别对蓝色和黄色绿柱石的呈色起着主导作用。当Fe²⁺和Fe³⁺处于晶格中不同位置时致色作用的具体差异,仍有待于进一步研究。     

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⁺.     

A reassessment of the role of iron in the 5,000–30,000 cm−1 region of the electronic absorption spectra of tourmaline

The E∥c and E ⊥ c polarized optical absorption spectra of a variety of blue/green tourmalines and a schorl were measured from room temperature down to helium temperatures. Heat treatments at 750–800° C in air and hydrogen were carried out on several green tourmalines. From the results obtained, absorptions at 7,900 and 13,800 cm^?1 in the E∥c spectra of tourmalines are assigned to Fe²⁺ in the b-site. In the same polarization, bands detected at 9,000 and 13,400 cm^?1 are attributed to Fe²⁺ in the smaller c position. In contrast to previous interpretations, the E ⊥ c polarized bands at 9,000 and 13,800 cm^?1 are not assigned to single ion transitions, but are largely associated with nearest neighbour Fe²⁺-Fe³⁺ pairs. Correlations between near-infrared band absorption coefficients and FeO concentration reinforce these assignments. The temperature dependence and the reaction to heat treatment of the strongly polarized (E⊥c?E∥c) band near 18,000 cm^?1 in blue and green tourmaline spectra are shown to be consistent with previous assignments of the band to Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer. Similar results are discussed for broad absorptions (also E⊥c?E∥c) found in the 22,000–25,000 cm^?1 region of the spectra of certain green and brown tourmalines. It is concluded that these absorptions are due to Fe²⁺+Ti⁴⁺→Fe³⁺+Ti³⁺ charge transfer. The proposal is made that the initial effect of heating green tourmalines in air and hydrogen is to reduce Fe³⁺ cations located in both b- and c-sites. Further heat treatment in air and hydrogen results in the oxidation of Fe²⁺→Fe³⁺ and leads to the generation of bands near 19,100 and 21,600 cm^?1. The newly formed bands are assigned to Fe³⁺-Fe³⁺ pairs.     

Temperature dependence of the 57Fe Mössbauer parameters in riebeckite

Mössbauer spectra for two riebeckite minerals were collected at temperatures in the range 4.2 to 500 K. The magnetic-ordering temperatures were found to be 33±1 and 31±1 K respectively. Fitting the paramagnetic spectra with a discrete number of doublets (three or four) did not lead to consistent results. Instead, a superposition of an Fe³⁺ (M2) doublet and one distributed ferrous component was found to produce adequate fits with reasonable parameter values. For both samples, a minor fraction of ferrous ions was observed to be present at the M4 sites and for one of the samples at the M2 sites as well. The temperature variations of the center shifts were well reproduced using the Debye model of the lattice vibrational spectrum to evaluate the second-order Doppler shift. The characteristic Mössbauer temperatures were calculated to be in the range 340–390 K for Fe²⁺, and 520 K for Fe³⁺. The temperature dependences of the various ferrous quadrupole splittings could not be explained in terms of the point-charge model and assuming a temperature-independent energy-level scheme for the ⁵D term. It is suggested that a gradual change with temperature of the orbital-level splittings takes place. All calculations yielded a positive sign for the principal component of the electric field gradient (EFG). The spectrum recorded at 4.2 K for one of the riebeckites was fitted with a superposition of an Fe³⁺ and a Fe²⁺ hyperfine-field distribution, the latter one primarily characterizing the Fe²⁺ (M1) cations. The following relevant hyperfine data were calculated: H _hf=161 kOe, ΔE _Q=3.11 mm/s, and V _zz<0, all referring to the maximum-propability values. For the second riebeckite at 4.2 K, an additional distributed ferrous component could independently be resolved. The two maximum-probability hyperfine fields were found to be 189 and 98 kOe and the corresponding ΔE _Q values 3.10 and 2.67 mm/s. Both components exhibit a negative V _zz. The subspectra were attributed to M1 and M3 sites respectively. The Fe³⁺ hyperfine fields are 548+-2 kOe for both riebeckites. The different values found for the Fe³⁺ quadrupole shift 2?_Q for the two samples is explained by a different angle between the hyperfine field and the EFG's principal axis. The magnetic spectra recorded at 15 K and higher, could not be reproduced adequately with reasonable parameter values.     

Measurements of iron concentration in kaolinites considering disorder broadening of EPR lines

Only one part of the EPR lines of a kaolinite spectrum of structural Fe³⁺ is clearly observable because of the overlapping of other lines with other spectra. For this reason, to determine the structural Fe³⁺ concentration we used the line near g=9, although it is not intense. A standard is needed: powders of ZnS containing given concentrations of Mn²⁺ (isoelectronic to Fe³⁺) were used for this purpose. Using the simulations of the EPR spectra, the concentration (number of Fe³⁺ per Al³⁺) is determined; it is in the range 10^?5 to 10^?4 for our samples. Considering that the crystal-field disorder around Fe³⁺ is responsible for line broadening, we looked for a possible effect of the broadening on the intensity of the EPR spectra. This effect is taken as a distribution of the parameter λ=B₂²/B₂⁰. The influence of the parameter λ and its statistical distribution on the position, shape, width and intensity of the EPR line has been calculated using simulation procedures. The correction due to the disorder on the calculated concentration is of the same order of magnitude as the precision measurement. This method can be applied for other kaolinites by comparing the area of their g=9 lines with known ones.     

Chromium and vanadium impurities in natural green euclase and their relation to the color

Natural specimens of green gemological euclase (chemical formula BeAlSiO₄(OH)) from Brazil were investigated by electron paramagnetic resonance (EPR) and optical absorption. In addition to iron-related EPR spectra, analyzed recently in blue and colorless euclase, chromium and vanadium-related EPR spectra were also detected in green euclase. Their role as color causing centers is discussed. The results indicate that Cr³⁺ ions substitute for Al³⁺ ions in the euclase structure. The EPR rotation patterns of Cr³⁺ with electron spin S = 3/2 were analyzed with monoclinic spin Hamiltonian leading to the parameters of g _xx , g _yy and g _zz equal to 2.018, 2.001 and 1.956 and electronic fine structure parameters of D = −8.27 GHz and E = 1.11 GHz, respectively, with high asymmetry ratio E/D = 0.13. For the vanadium-related EPR spectra the situation is different. It is concluded that vanadium is incorporated as the vanadyl radical VO²⁺ with electron spin S = 1/2 with nearly axial spin Hamiltonian parameters g_zz = 1.9447, g _xx  = 1.9740 g _yy  = 1.9669 and axial hyperfine interactions due to the nuclear spin I = 7/2 of the ⁵¹V isotope leading to A _zz  = 502 MHz, A _xx  = 150 MHz and A _yy  = 163 MHz. The green color of euclase is caused by two strong broad absorption bands centered at 17,185 and 24,345 cm⁻¹ which are attributed to the ⁴A_2g → ⁴T_2g, ⁴T_1g transitions of Cr³⁺, respectively. Vanadyl radicals may introduce some absorption bands centered in the near infrared with tail extending into the visible spectral range.     

Spectroscopic studies of synthetic and natural ringwoodite, γ-(Mg,Fe)<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

Synthetic ringwoodite γ-(Mg_1?x Fe _x )₂SiO₄ of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mössbauer spectroscopy. The Mössbauer spectra consist of two doublets caused by ^VIFe²⁺ and ^VIFe³⁺, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe³⁺/Fe_total ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed ⁵ T _2g → ⁵ E _g transitions of ^VIFe²⁺ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm^?1 weakly depend on temperature, whilst the Fe²⁺/Fe³⁺ IVCT band at ~16,400 cm^?1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe³⁺ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of ^VIFe²⁺ (the bands at ~8,600 and ~12,700 cm^?1) and Fe²⁺/Fe³⁺ IVCT transition (~18,100 cm^?1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm^?1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)₂SiO₄-spinels and that the single band is caused by the split spin-allowed ⁵ E → ⁵ T ₂ transition of ^IVFe²⁺. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stöffler in Lunar Planet Sci 29(1308), 1998).     

Electron paramagnetic resonance study of iron(III) and manganese(II) in the glassy and crystalline environments of synthetic fayalite and tephroite

Crystals of the olivine minerals, tephroite (Mn₂SiO₄) and fayalite (Fe₂SiO₄) containing manganese(II) and iron (II and trace of III), respectively, were synthesized. Glasses were prepared from these crystalline materials by a splat-quench technique. Measurement of electron paramagnetic resonance (EPR) of all these powdered samples at room temperature show that the g-factors of Mn²⁺ in both glassy and crystalline environments (g_eff = 2.004) are the same, although the EPR linewidths (for glass, ΔH_pp = 200 G; for crystals ΔH_pp = 287 G) suggest less clustering of paramagnetic Mn²⁺ ions in the glass. Mn²⁺ probably occupies a distorted octahedral site in the tephroite crystal structure, although a four-fold coordination is suggested from other spectroscopic investigation on this glass. The EPR parameters of Fe³⁺ in synthetic fayalite glass (g_eff = 2.01 and 6.00; ΔH_pp=150 and 1375 G, respectively, for the high and low field resonances) and powdered crystals (g_eff = 3.31 and ΔH_pp = 900 G) indicated that Fe³⁺ ion in the crystals, is probably located in a distorted tetragonal site M2 and an axial environment has been proposed in the glassy system.     

Use of ultrasonically modulated electron resonance to study S-state ions in mineral crystals: Mn2+, Fe3+, in tremolite

The use of ultrasonically modulated electron resonance (UMER) to study S-state ions in substitutional sites of mineral single crystals is discussed. Mn²⁺ and Fe³⁺ in natural single crystals of tremolite are used as examples. Combined electron paramagnetic resonance (EPR) and UMER measurements establish almost certainly that Mn²⁺ enters predominantly into the distorted M4 sites occupied by Ca²⁺ in the ideal tremolite structure and only to a minor extent into the M1, M2 and M3 sites normally occupied by Mg²⁺. Fe³⁺ in tremolite gives rise to the well known high spin resonance with g _eff?4.3 but there is considerable uncertainty as to the site of the impurity ion.     

Local relaxations around Fe3+ and Cr3+ in Al sites in minerals

Second-order zero-field splitting (ZFS) parameters from the literature for Fe³⁺ in twelve and for Cr³⁺ in seven minerals substituting for Al were evaluated by application of the superposition model. For Fe³⁺ in monoclinic site symmetries a fair agreement of the observed splitting patterns with those calculated from the crystal structure data was observed in most cases, but the distortions for Fe³⁺ appear to be usually larger than those of the unrelaxed Al sites. In cases of not too large local relaxation the unknown sign of the axialZFS parameterb ₀ ² could be predicted, in two cases a different sign than that reported was postulated. In cordierite and scolecite the reportedEPR spectra could thus be assigned to the sites with larger average bond distances. For Fe³⁺ in beryl the relaxation of the axial site can be deduced within narrow limits. For Cr³⁺ significantly larger differences between observed and calculatedZFS patterns are found suggesting additional relaxations due to the non-spherical electron distribution in the ground state of this ion.     

Determination of Fe3+ and Fe2+ concentrations in feldspar by optical absorption and EPR spectroscopy

Ferrous and ferric iron concentrations in feldspars with low total iron content (<0.32 wt% total Fe) were determined from optical and electron paramagnetic resonance (EPR) spectra to better than ±15 percent of the amount present. Optical spectra indicate that Fe²⁺ occupies two distorted M-sites in plagioclases of intermediate structural state. The linear dependence of the Fe²⁺/Fe total ratio on An content demonstrates that Fe²⁺ substitutes for Ca (not Na) so that the number of Ca-sites is a principal factor in iron partitioning in plagioclase. EPR powder spectra show that the number of sites for Fe³⁺ depends on structural state rather than on plagioclase chemistry. The observed linear correspondence of EPR double-integrated intensities with optical peak areas shows that all Fe³⁺ is tetrahedrally coordinated in both plagioclase and disordered potassium feldspar. Microcline perthites show, in addition to tetrahedral Fe³⁺, a signal due to axially coordinated ferric iron, which we associate with formation of hematite inclusions.     

Magnetic properties,characteristic spectra and colors of turquoise

The unique bright bluish-green color of turquoise as a high-grade jade has long received wide attention. The relationship between the color of turquoise and its composition and structure is described in this paper on the basis of chemical data, EPR, magnetic susceptibility, absorption and Mössbauer spectra. The results show that the basic color of turquoise (bright blue) is related to the existence of octahedrally on the amount of iron. EPR, magnetic susceblue through green to earth-yellow is dependentcoordinated Cu²⁺ and the shade variation from ptibility, and crystal-field spectra of Cu²⁺ have been analyzed and compared with the theoretical calculations. A preliminary discussion is also made of the color change as a function of temperature.