Single crystals of (1-x)Na0.5Bi0.5TiO3-xBaTiO3 (NBT-xBT) with a composition below (x = 0.013) and above (x = 0.074) the morphotropic phase boundary (MPB) were studied by in situ high-pressure x-ray diffraction with synchrotron radiation and Raman spectroscopy. We show that both compounds undergo a reversible pressure-induced phase transition to structures with Pnma symmetry, regardless of their symmetry at ambient conditions. The high-pressure phase resembles that recently reported for x(MPB) = 0.048 [Sci. Rep. 14, 18799 (2024)], as the critical pressure pc gradually increases with x. However, the pathway of developing the highpressure phase varies with the composition, revealing distinct ways of Ba incorporation into the NBT matrix for the pseudorhombohedral (x = 0.013) and tetragonal (x = 0.074) phases. For x < x(MPB) the Ba2+ cations are preferentially accommodated in the tetragonal-type nanodomains existing in the dominant pseudorhombohedral NBT matrix, whereas for x > x(MPB), Ba2+ is dispersed in both the rhombohedral nanodomains and the dominant tetragonal matrix. Furthermore, for x = 0.074 there are incommensurate structural modulations at ambient pressure, which up to p(c) remain unaffected by pressure, but vanish above p(c).

Abstract Metaheimite (IMA 2023-020a), PbCu 2 (AsO 4 )(OH) 3 , has been identified as a new secondary mineral at the Grosses Chalttal deposit, Mürtschenalp district, Glarus, Switzerland. It forms radial aggregates of blade-like crystals of light blue to turquoise blue colour. Metaheimite has a light blue streak and vitreous to silky lustre. Its calculated density is 5.47 g cm –3 . The empirical chemical formula based on seven anions per formula unit is (Pb 0.96 Ca 0.03 )Cu 1.98 (As 1.01 O 4 )(OH) 3 . Metaheimite is pseudo-orthorhombic, with monoclinic symmetry, space group P 2 1 /n and unit cell parameters a = 5.8347(4), b = 7.7528(6), c = 13.8899(9) Å, β = 90.018(3)°, V = 628.31(8) Å 3 and Z = 4. The five strongest lines in the calculated powder diffraction pattern are ( d in Å(I) hkl ) as follows: 6.945(100)002, 3.870(75)112, 3.169(78)014, 3.145(99)121 and 2.615(81)015. The crystal structure, refined to R obs = 6.13% for 1393 reflections with I > 3σ( I ), consists of layers similar to those occurring in heimite, PbCu 2 (AsO 4 )(OH) 3 ·2H 2 O, but with 6+2 coordinated Pb 2+ . The lack of water molecules in metaheimite causes different interlayer hydrogen bonding and consequently, different layer stacking order as compared to that in heimite. In metaheimite, Cu 2+ therefore occurs in a square-pyramidal coordination by five oxygen atoms. Hydrous species in metaheimite have been examined by Raman and by infrared spectroscopy. Metaheimite is structurally related to duftite, PbCu(AsO 4 )(OH) and may be considered a transitional state between heimite and duftite.

Anorthoyttrialite-(Y) occurs as inclusions in yttrian fluorite from the Stetind pegmatite, Narvik, Nordland, Norway, associated with allanite-(Ce), alnaperb & oslash;eite-(Ce), bastn & auml;site-(Ce), fluorite, hematite, hundholmenite-(Y), perb & oslash;eite-(Ce), rowlandite-(Y), schl & uuml;terite-(Y), synchysite-(Y), thal & eacute;nite-(Y), t & ouml;rnebohmite-(Ce) and vyuntspakhkite-(Y). It forms translucent tabular or needle shaped crystals, which are colourless or white, yellow or brownish. It is optically biaxial (-) with alpha = 1.705(1), beta = 1.750(1), gamma = 1.756(2) and 2Vcalc = 39.09 degrees. The calculated and measured density is 5.24 gcm-3 and 5.1 gcm-3 respectively. The empirical formula of anorthoyttrialite-(Y) based on 14 oxygen atoms per formula unit is (Y1.561La0.033Ce0.242Pr0.059Nd0.367Sm0.177Gd0.298Tb0.036Dy0.297Ho0.058Er0.258Tm0.071Yb0.385Lu0.041Ca0.099Mn0.029U0.003Th0.01)Sigma 4.024Si4.011O14.Two polytypic crystal structures have been determined in triclinic space group $P\bar 1$. Anorthoyttrialite-(Y)-1A is isostructural with the B-type structure known for synthetic rare earth element disilicates of composition REE2Si2O7, where REE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm. Unit cell parameters are a = 6.6107(4), b = 6.7139(3), c = 12.2034(9) & Aring;, alpha = 94.819(3), beta = 90.583(3), gamma = 91.742(3)degrees and V = 539.42(5) & Aring;3 with Z = 2. The five strongest lines in the calculated powder diffraction pattern are (d in & Aring; (I) hkl) as follows: 4.369 (72) $1\bar 1\bar 1$, 3.040 (100) 004, 3.040 (71) $02\bar 2$, 2.918 (68) $\bar 202$, 2.814 (59) 211. Anorthoyttrialite-(Y)-2A has unit cell parameters a = 6.6068(6), b = 6.7147(6), c = 24.218(2) & Aring;, alpha = 94.435(6), beta = 90.315(5), gamma = 92.092(5)degrees, V = 1070.41(16) & Aring;3 and Z = 4. The five strongest lines in the calculated powder diffraction pattern are: 4.378 (75) $1\bar 1\bar 2$, 3.027 (79) $0\bar 24$, 3.018 (100) 008, 2.907 (68) $\bar 204$, 2.810 (59) 212. Both polytypes contain linear trisilicate anion groups [Si3O10]8- as well as isolated [SiO4]4- tetrahedra, while the REE are six-, eight-, and nine-coordinated by oxygen. The crystal structures consist of essentially two types of layers with different stacking order. Similar layer stacking is discussed in relation to other rare earth disilicates like percleveite-(Ce) and thortveitite. Anorthoyttrialite-(Y) is also compared to a heat treated metamict Y-silicate from Stetind.

The structural response to high pressures as well as to high temperatures of (1-x)PbTiO3-xBi(Zn0.5Ti0.5)O3 with x = 0.08, (1-x)PbTiO3-xBi(Mg0.5Ti0.5)O3 with x = 0.17, and PbTiO3 single crystals was studied by in situ pressure-/temperature-dependent polarized Raman spectroscopy, complemented by synchrotron x-ray diffraction analysis at ambient pressure and different temperatures. The compositional and pressure dependencies of phonon anomalies indicate the existence of local-scale antiferrodistortive structural entities inside the perovskite (ABO3) polar tetragonal matrix, the fraction of which is enhanced by the substitution of BiMeO3 for PbTiO3. The type of cation (Zn2+ versus Mg2+) replacing Ti4+ at the B site affects the coherence within the dominant singleperovskite polar tetragonal matrix and the coupling between the "defect" double-perovskite antiferrodistortive entities. As a result, on increasing pressure, the partial substitution of Bi(Mg0.5Ti0.5)O3 triggers a structural instability at lower pressure values as compared to pure PbTiO3, whereas that of Bi(Zn0.5Ti0.5)O3 preserves the polar tetragonal phase over a wider pressure range. Furthermore, at ambient pressure B-site Zn2+ favors the coherence between local BO6 tetragonal distortions already above the Curie temperature TC, causing a larger unit-cell tetragonality below TC; in contrast, B-site Mg2+ reduces the local BO6 anisotropy and disturbs the correlation between polar AO12 tetragonal distortions below TC, which results in a considerable reduction of both the unit-cell tetragonality and accumulated volume strain on cooling between TC and room temperature.

The new mineral heimite (IMA2022-019), PbCu2(AsO4)(OH)3 ⋅ 2H2O, was found at the Grosses Chalttal deposit, Mürtschenalp district, Glarus, Switzerland, where it occurs as a secondary mineral associated mainly with bayldonite and chrysocolla. Heimite forms lath-like, prismatic transparent crystals of green or pale-blue colour. It has a pale-green streak and a vitreous-to-silky lustre. The calculated density is 4.708 g cm−3. The empirical formula based on nine O atoms per formula unit is Pb1.04Ca0.03Cu2.10As1.10H6.14O9. Heimite is pseudo-orthorhombic, with monoclinic symmetry; space group P21/n; and unit cell parameters a=5.9132(5), b=7.8478(6) and c=16.8158(15) Å and β=90.007(6)∘, V=780.33(8) Å3 and Z=4. The five strongest lines in the calculated powder diffraction pattern are (d in Å(I)hkl) as follows: 8.425(100)002, 3.713(60)014, 3.276(54)120, 3.221(42)023 and 2.645(61)016. The crystal structure, refined to R1=2.75 % for 1869 reflections with I>3σ(I), is based on chains of edge-sharing, Jahn–Teller-distorted CuO6 octahedra, laterally connected by AsO4 tetrahedra and sixfold coordinated Pb atoms. The resulting layers are stacked along [001]. Interlayer hydrogen bonding is mediated by hydrogen atoms that belong to OH groups and to H2O, mutually participating in the Cu coordination. The crystal structure of heimite is related to that of duftite, and both minerals are found epitactically intergrown at the type locality.

The pressure-induced structural changes in the perovskite-type (ABO 3 ) ferroelectric solid solution (1-x)Na 0.5 Bi 0.5 TiO 3 -xBaTiO 3 (NBT-xBT) at the morphotropic phase boundary (MPB) ( x MPB = 0.048 ) have been analyzed up to 12.3 GPa by single-crystal x-ray diffraction with synchrotron radiation. A pressure-induced phase transition takes place between 4.4 and 5.0 GPa, where the pseudocubic low-pressure phase transforms into an orthorhombic high-pressure phase with space group Pnma. The high-pressure phase is comprised of mixed BO 6 tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti 4 + cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT- x MPB BT structure possesses anti-phase BO 6 tilts with a relatively large correlation length and the same type of polar distortions as those present in pure NBT, but with strongly violated correlation length due to Ba 2 + -induced local elastic-stress fields. For x MPB the effect of Ba on the mesoscopic-scale structure is compensated by a mild external pressure of only 0.7 GPa, resulting in structural features resembling those of pure NBT at ambient conditions.

The pressure-induced structural changes in the perovskite-type (ABO(3) ) ferroelectric solid solution (1-x)Na0.5Bi0.5TiO3-xBaTiO3 (NBT-xBT) at the morphotropic phase boundary (MPB) ( x(MPB )= 0.048 ) have been analyzed up to 12.3 GPa by single-crystal x-ray diffraction with synchrotron radiation. A pressure-induced phase transition takes place between 4.4 and 5.0 GPa, where the pseudocubic low-pressure phase transforms into an orthorhombic high-pressure phase with space group Pnma. The high-pressure phase is comprised of mixed BO6 tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti4+ cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT-x(MPB) BT structure possesses anti-phase BO6 tilts with a relatively large correlation length and the same type of polar distortions as those present in pure NBT, but with strongly violated correlation length due to Ba2+-induced local elastic-stress fields. For xMPB the effect of Ba on the mesoscopic-scale structure is compensated by a mild external pressure of only 0.7 GPa, resulting in structural features resembling those of pure NBT at ambient conditions

We report variable temperature X-ray diffraction (20 K < T < 295 K) and Raman scattering (90 K < T < 400 K) data of malayaite, the tin analog of the mineral titanite, aided by results from density functional perturbation theory. The phase transition from the normal to the incommensurately modulated crystal structure occurs at T-c = 50 +/- 2 K with an almost constant q-vector of 0.27b*. Some first order satellite diffraction maxima are observable up to 55 K, where they increasingly broaden toward the main reflections. Softening of the lowest frequency transverse optical B(g )phonon mode, dominated by antiparallel motion of Ca atoms, is observed on cooling from 400 to 90 K. This confirms the displacive character of the transition to the modulated structure, indicated by the instability of this phonon mode in the zero-temperature approximation of first principle computation. The transition to the incommensurately modulated phase is preceded by a temperature region of anomalous thermal expansion in the normal phase, marked by negative thermal expansion along [010] and consequently a change from hardening to softening phonon modes on cooling below 150 K. The modulated phase of malayaite highlights the potential of density functional perturbation theory for the discovery of hitherto unknown ground state structures of minerals.

Abstract The pressure-induced structural changes in the perovskite-type (ABO $$_3$$ 3 ) ferroelectric solid solution (1-x)Na $$_{0.5}$$ 0.5 Bi $$_{0.5}$$ 0.5 TiO $$_3$$ 3 -xBaTiO $$_3$$ 3 (NBT-xBT) at the morphotropic phase boundary (MPB) ( $$x_{\text {MPB}}=0.048$$ x MPB = 0.048 ) have been analyzed up to 12.3 GPa by single-crystal x-ray diffraction with synchrotron radiation. A pressure-induced phase transition takes place between 4.4 and 5.0 GPa, where the pseudocubic low-pressure phase transforms into an orthorhombic high-pressure phase with space group Pnma. The high-pressure phase is comprised of mixed BO $$_6$$ 6 tilts and anti-polar A-cation displacements, without exhibiting coherent off-centered shifts of the B-site Ti $$^{4+}$$ 4 + cations that can be detected by synchrotron x-ray diffraction. Our results reveal that at ambient pressure and room temperature the NBT- $$x_\mathrm{{MPB}}$$ x MPB BT structure possesses anti-phase BO $$_6$$ 6 tilts with a relatively large correlation length and the same type of polar distortions as those present in pure NBT, but with strongly violated correlation length due to Ba $$^{2+}$$ 2 + -induced local elastic-stress fields. For $$x_{\text {MPB}}$$ x MPB the effect of Ba on the mesoscopic-scale structure is compensated by a mild external pressure of only 0.7 GPa, resulting in structural features resembling those of pure NBT at ambient conditions.

ABSTRACT Climate change poses a significant challenge for life on Earth. Different climate modes have been shown to come along with changes of the magnesium/calcium (Mg/Ca) ratio of seawater, and such changes are believed to control the primary mineral phase of marine authigenic carbonates. However, factors controlling marine carbonate phases other than seawater Mg/Ca ratios exist. Fibrous cements forming at methane seeps in the Black Sea provide new insight into the factors governing elemental and mineral phase compositions of fibrous carbonates. In this study, the distribution of aragonite and fibrous Mg calcite cements from three seep sites in the Black Sea is described as a function of water depth. The Mg/Ca ratio of seawater, as well as the ratio in shallow pore water, is close to four at the examined sites. Fibrous Mg calcite post‐dated aragonite cement in seep carbonates from shallow water depth of 120 to 190 m, whereas Mg calcite is the only cement at a greater depth of ca 2000 m. The primary formation of fibrous Mg calcite is confirmed by its zonation under cathodoluminescence, crystal morphologies agreeing with competitive growth, uniformly distributed MgCO 3 contents and precipitation in equilibrium with local conditions calculated from δ 18 O values. The MgCO 3 contents (4.5 to 12.2 mol%) are negatively correlated with δ 13 C values, indicating that the incorporation of Mg into the calcite crystal structure was favoured by high concentrations of sulphide generated by sulphate‐driven anaerobic oxidation of methane. Unlike open oceanic basins, stratification in the Black Sea leads to euxinic conditions in the deeper water column, favouring fibrous Mg calcite formation. This observation is consistent with sulphide catalysis as a critical agent for the formation of low‐Mg calcite to very high‐Mg calcite at high Mg/Ca ratios and is possibly relevant to carbonate cements forming during times of oceanic euxinia.

Fibrous dolomite widely formed in Neoproterozoic marine sedimentary environments, but apparently disappeared in the Phanerozoic. Here, fibrous dolomite is recognised in a Miocene methane seep limestone (Marmorito, Italy) by synchrotron X-ray diffraction, length slow crystal optics and primary zonation under cathodoluminescence, which is unexpected. Low δ 13 C values and their negative correlation with MgCO 3 contents indicate a formation driven by highly alkaline pore waters and catalysis of dissolved sulphide generated by sulphate-driven anaerobic oxidation of methane. Cementing cavities of reefal carbonate, Neoproterozoic fibrous dolomite might have formed under sulphate-reducing conditions like Quaternary reef microbialites. Since the cavities of Neoproterozoic reefs were restricted microenvironments, the formation of fibrous dolomite was possibly favoured by catalysis similar to its Miocene seep counterpart. Our findings reinforce the concept of penecontemporaneous dolomite formation by sulphide catalysis and contribute to our understanding of the environmental conditions of the Neoproterozoic.

AbstractThe new mineral nafeasite (IMA2021-103), NaFe3+(AsO3OH)2⋅H2O, was found at the Torrecillas mine, Iquique Province, Chile, where it is a secondary alteration phase associated with anhydrite, gypsum, halite, lavendulan, magnesiokoritnigite and natrojarosite. Nafeasite occurs in tightly intergrown aggregates of equant crystals. Crystals are light to medium pink and transparent, with vitreous lustre and white streak. The Mohs hardness is ~2½. The density is 3.23(2) g⋅cm–3. Optically, nafeasite is biaxial (+), with α = 1.679(3), β = 1.682(3), γ = 1.730(5) (white light); 2V = 27(2)°; and slight r < v dispersion. The empirical formulae of the holotype and cotype (based on 9 O atoms per formula unit) are Na0.98K0.02Fe0.92Al0.07As2.00O9H4.01 and Na0.97Fe0.68Al0.33As2.00O9H4.01, respectively. Nafeasite is monoclinic, space group C2, with cell parameters: a = 18.6876(16), b = 8.6769(7), c = 14.8100(10) Å, β = 105.238 (5)°, V = 2317.0(3) Å3 and Z = 12. The structure, refined to R1 = 5.03% for 5979 Io > 2σI reflections, is based on a loose 3D framework of alternating AsO3OH tetrahedra and Fe3+O6 octahedra.

Understanding the thermal behaviour of iron-containing amphiboles (AB 2 C 5 T 8 O 22 W 2 , C 5 = M (1) 2 M (2) 2 M (3)) at atomic-level scale may have important implications in several fields, including metamorphic petrology, geophysics, and environmental sciences. Here, the thermally induced oxidation and decomposition of actinolite are studied by in situ high-temperature Raman spectroscopy and complementary thermogravimetric/mass-spectrometry analyses as well as X-ray diffraction of the products of amphibole decomposition. The effect of C Fe 2+ on dehydrogenation/dehydroxylation is followed by comparing the results on actinolite with those for tremolite. We show that mobile charge carriers, namely polarons (conduction electrons coupled to FeO 6 phonons) and H + cations, exist in actinolite at elevated temperatures ~ 1150–1250 K. The temperature-induced actinolite breakdown is a multistep process, involving (i) delocalization of e − from C Fe 2+ as well as of H + from hydroxyl groups shared by Fe-containing M (1) M (1) M (3) species, which, however, remain in the crystal bulk; (ii) dehydrogenation and ejection of e − between 1250 and 1350 K, where actinolite can be considered as “oxo-actinolite”, as H + also from hydroxyl groups next to M (1,3) (MgMgMg) configurations become delocalized and mostly remain in the crystal bulk; (iii) complete dehydroxylation and consequent structure collapse above 1350 K, forming an Fe 3+ -bearing defect-rich augitic pyroxene. The dehydrogenation of tremolite occurs at 1400 K, triggering immediately a disintegration of the silicate double-chain into single SiO 4 -chains and followed by a rearrangement of the amphibole octahedral strips and B Ca 2+ cations into pyroxene-type octahedral sheets at 1450 K. The result of tremolite decomposition is also a single-phase defect-rich clinopyroxene with an intermediate composition on the diopside–clinoenstatite join.

The new mineral fehrite (IMA 2018-125a), MgCu4(SO4)(2)(OH)(6)center dot 6H(2)O, is a member of the ktenasite group and the Mg-analogue of ktenasite, ZnCu4(SO4)(2)(OH)(6)center dot 6H(2)O. The mineral was found in the Casualidad mine near Banos de Alhamilla, Almeria, Spain, in association with clinoatacamite, kapellasite, gordaite, serpierite, connellite and gypsum. The transparent turquoise-coloured mineral has a vitreous lustre, exhibits a pale blue-green streak and shows distinct pleochroism. It forms radial aggregates of thin lath-like crystals of up to 200 gm in length. Fehrite is not fluorescent. The monoclinic crystals show a perfect cleavage parallel to (001). The mineral has a brittle tenacity and an uneven fracture. The calculated density is 2.73 g/cm(3), the calculated mean refractive index is 1.584. The strongest lines observed in the X-ray powder diffraction pattern are [d in angstrom/I-rel in %/(hkt)] 11.94/100/002, 5.92/31/004, 2.66/12/202, 4.85/11/013, 3.93/11/006 and 2.96/10/008. The chemical composition, measured by means of an electron probe micro-analyser, was determined at (wt.%): MgO 5.31, MnO 0.49, CuO 33.12, ZnO 11.48, SO3 26.01, H2Ocalc. 24.63, total 101.04. The empirical formula based on 20 O pfu., including 6(OH) and 6(H2O), is Mg0.87Cu2.74Zn0.93Mn0.05S2.14O8(OH)(6)center dot 6H(2)O. The simplified end member formula is MgCu4(SO4)(2)(OH)(6)center dot 6H(2)O which requires MgO 5.92, CuO 46.74, SO3 23.52, H2O 23.82, total 100.00 (wt.%). Fehrite is monoclinic with space group P21/c (#14). Unit cell parameters determined by X-ray single crystal diffraction are a = 5.6062(8), b= 6.1294(11), c = 23.834(3) angstrom, beta= 95.29(1)degrees, V= 815.5(2) angstrom(3), Z= 2. The mineral is isotypic with ktenasite with Mg in place of Zn. The name is for the late Karl Thomas Fehr (1954-2014), Professor of Mineralogy at the Depart-ment of Geo- and Environmental Sciences at the Ludwig-Maximilians-University Munich, Germany.

Perrierite-(Ce) crystals from ejecta of the Laacher See volcano, Eifel, in Germany were studied by X-ray single-crystal diffraction and electron microprobe analysis. The composition and crystal structure of this sample is discussed in relation to the known properties of the chevkinite group minerals and related synthetic compounds. Taking into account the modular character of the chevkinite minerals, based on a rutile and a silicate module, the preferred formation of either the perrierite or the chevkinite structure type is correlated with the formal charge of the rutile and silicate modules. The rutile module is expected to carry a negative charge, compensated by a positive charge of the silicate module. On average, the charge modulus is observed to be larger for the chevkinite-type module stacking. It can drop to zero in perrierite-type structures rich in Sr or Ca. In such cases, it is generally not expected to rise above two. The perrierite-(Ce) described in this study crystallizes in space group C2/m. The anisotropic oxygen and rare-earth element displacements observed in this crystal indicate a local domain structure of P2(1)/a symmetry, when compared to the P2(1)/a symmetry of the synthetic perrierite La4Mg2Ti3O8(Si2O7)(2).

The crystal structure of the mineral malayaite has been studied by single-crystal X-ray diffraction at a temperature of 20 K and by calculation of its phonon dispersion using density functional perturbation theory. The X-ray diffraction data show first-order satellite diffraction maxima at positions q = 0.2606 (8)b*, that are absent at room temperature. The computed phonon dispersion indicates unstable modes associated with dynamic displacements of the Ca atoms. The largest-frequency modulus of these phonon instabilities is located close to a wavevector of q = 0.3b*. These results indicate that the malayaite crystal structure is incommensurately modulated by static displacement of the Ca atoms at low temperatures, caused by the softening of an optic phonon with B-g symmetry.

We present the results of a comprehensive study linking the crystal-chemical formulae of amphiboles, a series of extremely complex silicates with the general formula (AB(2)C(5)T(8)O(22)W(2), C-5 = M1(2)M2(2)M3) to variations in the peak positions, widths, and intensities of the Raman-active modes. To this purpose, we have analyzed the Raman scattering generated by the framework vibrations (15-1,215 cm(-1)) and by the OH-stretching modes (3,000-4,000 cm(-1)) of 44 samples, spanning all six major subgroups. We show that, in addition to the information that can be derived from the OH-stretching range (Leissner et al.,Am. Mineral. 2015, 100, 2682), further important features of the amphibole structure, composition, and cationic site population can be directly extracted from the framework Raman spectrum, namely, (a) the distinction between the monoclinic and orthorhombic symmetries; (b) the estimation of(T)Al content, when(T)Al > 0.5 apfu; (c) the estimation of (c) Ti content, when (c) Ti > 0.3 apfu; (d) the estimation of (c) Li content, when (c) Li > 0.3 apfu; (e) the detection of (c) Al, when (c) Al > 0.7 apfu; (f) an estimate of (c) Mg; (g) the estimation of (c) Fe(3+)in the case of Na amphiboles; and (h) the estimation of the Fe(2+)content at theM2 site in the case of Mg-Fe-Mn amphiboles. Additionally, we point out that the TO4-ring-breathing mode near 670 cm(-1), which is commonly used to fingerprint various amphibole species, has to be handled with a great care, because it is sensitive to the site population at all crystallographic sites.

Polymorphism of Cu 2 (OH) 3 Cl coupled with partial substitution of Jahn–Teller active Cu 2+ by other divalent metal cations gives rise to the complex mineralogy of the atacamite family of secondary basic copper chlorides. Herbertsmithite, Cu 3 Zn(OH) 6 Cl 2 , in which Zn substitutes for one quarter of the Cu atoms, provides a lattice of corner-sharing triangles of paramagnetic Cu 2+ (spin ½) cations, rendering the mineral a perfect realization of a kagome antiferromagnet. Geometric frustration of conventional antiferromagnetism is expected to give rise to exotic ground states, with dynamic magnetic structures that might turn out to be physical realizations of quantum spin liquids. In this paper, a synopsis of the key topological, compositional and behavioural features of minerals in the atacamite family is given, with emphasis on the kagome character of the resulting lattice of Cu 2+ cations.

Phonon dispersion of titanite CaTiSiO5 has been calculated using the variational density functional perturbation theory. The experimentally known out-of-center distortion of the Ti atom is confirmed. The distortion is associated with a B-u mode that is unstable for wave vectors normal to the octahedral chain direction of the C2/c aristotype structure. The layer of wave vectors with imaginary mode frequencies also comprises the Brillouin zone boundary point Y (0,1,0), which is critical for the transition to the P2(1)/c ground-state structure. The phonon branch equivalent to the imaginary branch of the titanite aristotype is found to be stable in malayaite CaSnSiO5. The unstable phonon mode in titanite leads to the formation of transoriented short and long Ti-O1 bonds. The Ti as well as the connecting O1 atom exhibit strongly anomalous Born effective charges along the octahedral chain direction [001], indicative of the strong covalency in this direction. Accordingly and in contrast to malayaite, LO-TO splitting is very large in titanite. In the C2/c phase of titanite, the Ti-O1-Ti distortion chain is disordered with respect to neighboring distortion chains, as all chain configurations are equally unstable along the phonon branch. This result is in agreement with diffuse x-ray scattering in layers normal to the chain direction that is observed at temperatures close to the P2(1)/c to C2/c transition temperature and above. The resulting dynamic chains of correlated Ti displacements are expected to order in two dimensions to yield the P2(1)/c ground-state structure of titanite.