Staurolite Structure Research Articles

Crystallographic orbits analysis of staurolite twins

Staurolite is an enigmatic mineral characterized by a high degree of pseudo-symmetry, which frequently occurs twinned. It gives two twins with different occurrence frequency, the 90° or Greek cross (lower frequency) and the 60° or Saint Andrews cross (higher frequency). To date no explanation has been found for their different occurrence frequency. We have analyzed the structure of staurolite in terms of the pseudo-eigensymmetry of the crystallographic orbits building this structure [1]. The union of the crystallographic orbits based on oxygen atoms has a pseudo-cubic eigensymmetry which contains the twin operations of both twins: the operations restore, with good approximation, the whole set of oxygen atoms, which justifies the high frequency of twinning of this mineral but does not discriminate between the two twins. On the other hand, a subset of the tetrahedral cations has a pseudo-eigensymmetry which contains the twin operation of the Saint Andrews cross, but not that of the Greek cross. Also, a subset of the octahedral cations has a pseudo-eigensymmetry which contains the twin operation of the Greek cross and a larger subset has an eigensymmetry which contains the twin operation of the Saint Andrews. The substructure approximately restored by the twin operation is thus more extensive for the Saint Andrews cross, which justifies its higher occurrence frequency.

Hydrogen zoning in zinc-bearing staurolite from a high-P, low-T diasporite (Samos, Greece): A combined EMP-SIMS-FIB-FTIR study

Li-rich zincostaurolite occurs as millimeter-long crystals at the marble footwall of a meta-karst-bauxite on eastern Samos. The Samos rocks have been metamorphosed during an early Alpine high- P , low- T metamorphism (M1) followed by a late Alpine greenschist-grade overprint (M2). Textures and mineral chemistry indicate that staurolite formed from gahnite, cookeite, and pyrophyllite during the early M1 stage. Staurolite crystals show growth zoning with cores enriched in Zn. Concentrations of Fe, Mg, Co, and, to a minor extent, Li increase toward the rims. Hydrogen concentrations were analyzed by SIMS. They are significantly higher in cores (up to 5.97 atoms H per 48 O) compared to rims (3.9 to 4.5 atoms H) and clearly negatively correlated with Al. Synchroton-light polarized FTIR spectra on oriented FIB-prepared foils show the same zonation effect, the absolute hydrogen concentrations being systematically lower by about 25%. The discrepancy is caused by sub-micrometer scale hydrogen loss at the crystal surface during FIB-thinning. This staurolite is unique as from the three available hydrogen sites the H3 site has the highest occupation ever observed, whereas the H2 site is not occupied. This is probably due to the high Li content. The zonation in hydrogen is interpreted as reflecting the two-stage growth. M1-staurolite that formed a low T of about 400–450 °C and high P of >1.5 GPa incorporated nearly the maximum amount of hydrogen allowed by the staurolite structure (6 H pfu) and was subsequently overgrown and marginally replaced during the M2 stage by less hydrous, Fe-Co richer staurolite. Hydrogen zoning in staurolite is facilitated by the sensitivity of its structure to changing P-T conditions. Water in staurolite is maximized at high P and low T . Cores of staurolite from Samos represent the most hydrous staurolite compositions reported to date.

Isothermal compression of staurolite: A single-crystal study

The response of staurolite to pressure was studied by single crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals with composition: (Fe 3 . 3 6 5 Zn 0 . 0 2 5 Li 0 . 1 1 4 Co 0 . 0 0 9 Mn 0 . 0 3 4 )[T2, M4] (Al 2 Mg 0 . 3 0 7 )[M3] (Al 1 5 . 4 9 1 Fe 3 + 0 . 1 0 4 Mg 0 . 3 9 4 Cr 0 . 0 0 4 Ti 0 . 0 7 )[M1, M2] (Si 7 . 5 3 4 Al 0 . 4 6 6 )[T1] O 4 8 H 3 . Lattice parameters, measured at various pressure up to 7.264(6) GPa, were fitted using a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V 0 = 740.85(7) A 3 , K 0 = 180(2) GPa and K' = 4.7(6), a 0 = 7.8723 (2) A, K 0 = 189(2) GPa, and K' a = 4.1 (6), b 0 = 16.62453(1) A, K 0 = 179(2) GPa, K' b = 6.1(6) and c 0 = 5.6604 (4) A, K 0 = 179(5) GPa, K' c = 2(1); whereas the angle β remained almost constant with increasing pressure. These data suggest an almost isotropic compressibility. Structural evolution was studied by comparison of structural refinements carried out with data collected at 0.0001, 2.48, 4.15, 5.43, 6.84, and 8.74 GPa. All refinements were made in the Ccmm space group. Polyhedral evolution with P is a function of occupancy: whereas the T1 tetrahedron and the Ml and M2 octahedra, occupied by Si and Al, are practically incompressible, the T2 tetrahedron, and the M4 and M3 octahedra, only partially occupied principally by Fe (the first two) and by Al (the last), show larger changes as a function of pressure. As a consequence, the two kyanite and Fe-Al hydroxide layers, which can be used to describe the staurolite structure, have different compressibilities.

The natural weathering of staurolite; crystal-surface textures, relative stability, and the rate-determining step

Mineral surface-textures on naturally weathered c of staurolite (monoclinic, pseudo-orthorhombic; Fe4Al18Sis046( rtalS indicate that staurolite weathering is general1 interface-limited. Etch pits on naturally weathered staurolites are disi-shaped, extensive par- allel to (OIO), and thin perpendicular to (010). (010) is the plane of weak bonding and weak cleavage and the orientation of common stacking defects in the staurolite lattice, any of which could account for preferen- tial dissolution in this orientatxon. Staurolite weathers very slowly relative to most other silicate minerals; this may be due to presence of stable kyanite-like ribbons in the staurolite structure or to the low site-energy of the Fe-site in the staurolite structure (compared to other orthosilicates). Staurolite weathering is interface-limited in most weath- erin environments. Although staurolite contains enough Al to coat itsel f completely with a non-porous protective surface layer of Al- hydroxides during weathering, protective surface layers apparently form only in some bauxites, where A1 is abundant. The capacity of staurolite to form protective surface layers around itself in most weath- ering environments is a parently limited by its slow weathering, which prevents the release o !' product-forming elements (especially Al) at rates sufficient to produce local supersaturation with respect to second- ary minerals.

The orientation determination of OH- dipoles in staurolite by polarized infrared spectroscopy.

Polarized infrared absorbance spectra from the oriented plates of staurolite were examined for the orientation determination of OH− dipoles in staurolite by a Fourier transform polarized infrared absorbance spectroscopy (pol-mFTIR). Seven polarized absorbance spectra observed in 3 μm region are due to the OH− dipoles essentially existing in staurolite structure, and all of the corresponding OH− dipoles are oriented parallel to the (010) plane. Three of the seven absorbance spectra are assigned to OH− dipoles due to two sets of the OH bonds, P(1A)–O(1A) and P(1B)–O(1B), and P(2A)–O(1A) and P(2B)–O(1B), directly found by neutron diffraction studies. Unassigned four absorbance spectra indicate that additional hydrogen positions must be present in staurolite, and the unidentified hydrogen may be estimated to be a considerable quantity.

3D structure determination from electron-microscope images: electron crystallography of staurolite.

Resolution of better than 2 A has been obtained in many crystals by high-resolution electron microscopy. Although this resolution is sufficient to resolve interatomic spacings, structures are traditionally interpreted by comparing experimental images with contrast calculations. A drawback of this method is that images are 2D projections in which information is invariably obscured by overlap of atoms. 3D electron crystallography, developed by biophysicists to study proteins, has been used to investigate the crystal structure of staurolite. Amplitudes and phases of structure factors are obtained experimentally from high-resolution images (JEOL ARM 1000 at the National Center for Electron Microscopy at LBL), taken in different directions from thin regions where dynamic scattering is minimal. From images in five orientations (containing 59 independent reflections to a resolution of 1.38 A), a 3D electron potential map is constructed which resolves clearly all cations (Al, Si, Fe, including those with partial occupancy) and all O atoms. This method has great potential in crystal structure determinations of small domains in heterogeneous crystals which are inaccessible to X-ray analysis. It is estimated that 3D structure determinations should be possible on regions only about ten unit cells wide and should resolve not only atom positions but also site occupancies. The method is also applicable to space-group determination.

Electron crystallographic determination of the 3-D structure of staurolite at atomic resolution

With current advances in electron microscope design, high resolution electron microscopy has become routine, and point resolutions of better than 2Å have been obtained in images of many inorganic crystals. Although this resolution is sufficient to resolve interatomic spacings, interpretation generally requires comparison of experimental images with calculations. Since the images are two-dimensional representations of projections of the full three-dimensional structure, information is invariably lost in the overlapping images of atoms at various heights. The technique of electron crystallography, in which information from several views of a crystal is combined, has been developed to obtain three-dimensional information on proteins. The resolution in images of proteins is severely limited by effects of radiation damage. In principle, atomic-resolution, 3D reconstructions should be obtainable from specimens that are resistant to damage. The most serious problem would appear to be in obtaining high-resolution images from areas that are thin enough that dynamical scattering effects can be ignored.

Resolution of oxygen atoms in staurolite by three-dimensional transmission electron microscopy

WITH advances in microscope design, high-resolution electron microscopy has become routine, and resolutions of less than 2 A have been obtained for many inorganic crystals1–4. The images provide a representation of the Coulomb potential (essentially the electron density), but interpretation generally requires comparing experimental images with calculations5. Because the images are two-dimensional representations of the full three-dimensional structure, information is invariably lost. In particular, oxygen atoms are normally not seen. The technique of electron crystallography, in which information from several views of a crystal is combined, has been developed to obtain three-dimensional information on proteins6–8. Here we use this technique to obtain a three-dimensional reconstruction of the crystal structure of staurolite, a silicate mineral. We are able to identify the positions of oxygen atoms in the lattice. These results show the potential of the method for high-resolution structure determination on samples that are too small for the use of X-ray techniques.

On the crystal structure of staurolite. The X-ray crystal structure of staurolite from the Pyrenees and Brittany

The crystal structures of two natural crystals of staurolite, from Heas, the Pyrenees (H), and Scaer, Brittany (S), France, have been refined from X-ray Mo Ka data at 293 K. Monoclinic, C2/m, Z= 1. (H): Fe3.59Ti0.o9Mno.ol Zno.ol Mgo.83All 7.9oSi7.5oO45

On the determination of the structure of staurolite through mössbauer spectroscopy

Mossbauer studies are carried out on the powder and single crystal samples of staurolite silicate mineral at room temperature, 77 K and 4.2 K. The fitted hyperfine parameters are discussed in terms of staurolite structure. Our results suggest only one site for Fe2+ in the staurolite structure. Vzz is found at 45° to crystal a and c axes.

The staurolite story

The history of staurolite (1792-present) demonstrates how long it takes for superseded teachings to be discarded in favor of new results, witness the long estrangement of X-ray crystallographers from morphological mineralogists. The first staurolite structure was described inCcmm (Naray-Szabo, 1929). In 1953Pierre Sauve found, in Fannin-county material, a crystal with strikingly monoclinic morphology. Morphological analysis demanded halving the oldc length, confirmed theC centering, but invalidated thec glide plane (thrice “confirmed” by X-ray diffraction!). NewOkl precession patterns revealed previously “forbidden” reflections, and oblique optical extinction on (010) confirmed the monoclinic character.Ccmm survives as pseudo-space-group,a andc becoming directions of pseudo-symmetry and entailing potential intimate “twinning by lattice quasi-symmetry (ω=0.0°,n=1)”. In 1958Naray-Szabo using formula Fe4Al18Si8O46(OH)2 revised his orthorhombic structure inC2/m. Smith(1968)met great refinement difficulties in assigning atoms to crystallographic sites as partial occupancy or substitution solid-solution (or both) affect every Wyckoff position. Later observations on compositional sector-zoning cast doubt on the specimen monocrystallinity. Neutron diffraction and nuclear magnetic resonance confirmed Smith's conjecture in locating OH groups in O(1A) and O(1 B) positions. Attempts to write a simple structural-chemical formula have been unsuccessful. Dimorphism is not excluded: two monoclinic positions, differing only in occupancy, may combine into a single orthorhombic one for equal occupancies by a given solid solution. Many more refined structures, on different compositions, may be needed before any hope of writing a general formula can be entertained. Just now the problem is hopelessly underdetermined, as shown by the parameters in the structural formula $$\left( {\mathop {Si_{1 - l} Al_l }\limits^{Si} } \right)_8^{iv} \left( {\mathop {\mathop {Al_{1 - m - \in } M_m \square _ \in }\limits^{Al(1A,1B)A1(2)} }\limits_{2 \times 4 + 8 = } } \right)_{16}^{vi} \left( {\mathop {M_{1 - n - \zeta } Al_n \square _\zeta }\limits^M } \right)_4^{iv} \left( {\mathop {\mathop {\square _{1 - p - q} Al_p M_q }\limits^{Al(3A,3B)} }\limits_{2 \times 2 = } } \right)_4^{vi} \left( {\mathop {\mathop {\square _{1 - r} M_r }\limits^{U(1,2)} }\limits_{2 \times 2 = } } \right)_4^{vi} \left( {\mathop {\mathop {O_{48 - z} (OH)_z }\limits^{\mathop {O(2A,2B,3,4,5)}\limits^{O(1A,1B)} } }\limits_{\frac{{2 \times 4 + 5 \times 8 = }}{{48}}} } \right)_1 $$ when M=Fe, Mg, Zn, Ti, Mn.

Staurolite twinning (With Plates V-VI.)

SummaryThe space group Ccmm of the accepted staurolite structure (7·82:16·52:5·63 kX 0·473:1:0·341, Náray-Szabó 1929) is a pseudo-space group; the true one has no c glide plane. Staurolite is monoclinie pseudo-orthorhombic, as shown optically by horizontal dispersion and morphologically by unequal developments of forms r(201) and r′(01). It has variable cell dimensions, 7·83–7·95:16·50–16·82:5·62 5·71 Å., constant β 90° + 3′, aspect C*, and probable space group C2/m (negative pyroelectric test). This leads to the prediction of a new type of twin, ‘by high order merohedry’, a penetration twin that simulates an orthorhombic single crystal. It is known (Friedel, 1922) that the 90°-cross can be accounted for by two different twin laws and the 60°-cross by five. The twin operations are respectively the pseudo-symmetry operations of a pseudo-tetragonal cell, obtained by transformation 010/003/100, and those of Mallard's pseudo-cube, resulting from transformation 013/03/300. By precession methods more than one twin law is established for each type of cross: both laws [100]90° and [013]180° are found for 90°-crosses: [313]180 and [102]120°, for 60°-crosses.