Paul H. Ribbe

Sapphirine I: Crystal chemical contributions

Microprobe analyses of 26 natural sapphirines from 17 localities indicate that the predominant chemical substitutions in this mineral occur along the solid solution joinVI(Mg,Fe)2++IVSi4+=VI(Al, Fe)3++IVAl3+. Chromium and manganese are minor substituents. Evidence for the substitution Si⇄Al+1/2Mg+1/2 vacancy is absent within the limits of analytical error.A partitioning scheme based on electrostatic charge balance considerations has been devised permitting calculation of Fe2+ and Fe3+ from total iron content. Results are in good agreement with previous Mössbauer studies which indicate Fe3+ is sometimes in octahedral and/or tetrahedral coordination.Distribution coefficients for Fe2+-Mg exchange equilibria between sapphirine-spinel and sapphirine-orthopyroxene are similar for most mineral pairs and suggest that most of the assemblages equilibrated at about the same temperature or that the exchange reactions are insensitive to temperature.Compositions of synthetic sapphirines as a function of temperature and pressure are qualitatively predictable from crystal chemical considerations. Changes in sapphirine composition along the MgSi= AlAl solid solution join toward more aluminous compositions stabilize the sapphirine structure at high temperatures and low pressures. The limited extent of MgSi=AlAl solid solution observed in natural sapphirines appears to be related to the requirements of geometrical fit among octahedra and tetrahedra in the almost idealized cubic closest-packed anion framework.

Atomic movements in plagioclase feldspars: Kinetic interpretation

The diffraction, n.q.r. and optical data on plagioclase feldspars are used to derive kinetic interpretations of structural changes induced by laboratory heat treatment and by geological processes.For anorthite, the Si, Al configuration is essentially ordered except for unusual transient processes. Cooperation between Ca atoms, and random nucleation, produces a domain texture in the primitive structure which is highly sensitive to temperature. The rapid inversion from the primitive to the body-centered structure is explained by increasingly rapid “rattling” of the Ca ions in the interstices of the semi-flexible alumino-silicate framework. The weakening of “b” reflections at higher temperatures is ascribed to incipient Si, Al disorder associated with irregular vibration of the alumino-silicate framework and the Ca atoms. Quenching phenomena are explained by variation of the domain boundary texture inherited from disorder at high temperature.For albite, the Si, Al configuration changes sluggishly from an ordered to a disordered pattern, and vice versa. Kinetic data are reinterpreted using a model in which the cell dimensions depend on local rather than distant order: the major change in distant order is deduced to occur at 450–600° C.Sodic plagioclase grown at high temperature shows distant disorder of the atoms, but cell dimensions suggest development of strong local order for calcic compositions.Low-entropy plagioclases of intermediate composition show complex intergrowths and domain structures because of kinetic barriers to atomic diffusion. X-ray diffraction data for slowly-cooled specimens are consistent with nucleation of albite- and anorthite-like regions from a high-temperature disordered phase. Electrostatic energy calculations show that Na and Ca atoms, although they face smaller energy barriers for diffusion, cannot form domains until the Si and Al atoms have moved jointly. The Si, Al ordering patterns of low albite and anorthite are topologically incompatible in a continuous framework if oxygen is not to be bonded to two Al. Therefore domains of low-albite and anorthite must be separated by disordered boundaries.For intermediate compositions, An15-An75, domains remain small. The anorthite-like domains probably form at higher temperatures than the albite-like domains. The latter tend to be about the same size for all bulk compositions. The atomic positions are influenced by neighboring atoms.Upon heating rapidly, Si and Al atoms remain in position and provide a memory for reformation of an identical structure upon cooling. The framework changes shape, and some Na, Ca atoms inter-diffuse to yield a quasi-homogeneous structure with a diffraction pattern which qualitatively approaches that of high albite. Upon prolonged heating at high temperature, Si, Al atoms inter-diffuse producing nonquenchable changes to the high-albite structure.At Na-rich bulk compositions, some domains of low albite grow into large lamellae while others remain small in contact with anorthite domains producing alternate lamellae of intermediate structure type; hence the peristerite intergrowth. A similar but opposite process could cause an intergrowth of lamellae of anorthite structure interposed with an intermediate type structure.A unique low plagioclase series is not expected. Plagioclases of intermediate composition trend towards slightly different endproducts depending on the details of the cooling history. Breaks and bends in plots of physical properties, and intergrowths for certain specimens, depend on special compositional, growth and annealing factors.The intergrowth responsible for iridescence of intermediate plagioclase is ascribed to Na, K segregation prior to development of the complex domain structure. Prolonged annealing at high temperature in a dry environment is suggested.It is futile to attempt to describe low entropy plagioclases in terms of classical thermodynamics: only a kinetic interpretation based on atomic and sub-microscopic textural factors can be viable.

Activity-composition relations in feldspars

Secks (1971a) compositional data on coexisting feldspars in the Or-Ab-An ternary at 650° C and 1 kb were used to calculate the activity-composition relations in binary alkali feldspar and binary plagioclase. The energy constants in Guggenheims expression for excess free energy of mixing are A0=3920 and A1=657 cal/mole for alkali feldspar, in excellent agreement with values obtained by Thompson and Waldbaum (1969), and 1320 and 373 cal/ mole for plagioclase. Using Orvilles (1972) data from ion-exchange experiments between plagioclase and Na—Ca chloride solutions at 700° C and 2 kb, we obtained 967 cal/mole for A0 and 715 cal/mole for A1 in the plagioclase crystalline solution.Activity-composition relations for plagioclase are interpreted in terms of a continuous, random substitution of CaAl for NaSi across the high structural state plagioclase series. This interpretation is consistent with that obtained from a consideration of lattice parameters.

Sapphirine II: A neutron and X-ray diffraction study of (Mg-Al)VI and (Si-Al)IV ordering in monoclinic sapphirine

AbstractThe crystal structure of aP21/a polymorph of sapphirine (a=11.286(3),b=14.438(2),c=9.957(2) Å, β=125.4(2) °) of composition [Mg3.7Fe0.12+Al4.1- Fe0.13+]IV[Si1.8Al4.2]IVO20 was refined using structure factors determined by both neutron and x-ray diffraction methods to conventionalR factors of 0.067 and 0.031. respectively, forFobs>2σ. The results of the two refinements agree reasonably well, but a half-normal probability plot (Abrahams, 1974) comparing the two data sets indicates that the pooled standard deviations of the atomic coordinates have been underestimated by a factor of two.The structure of sapphirine, solved initially by Moore (1969), consists of cubic closest packed oxygens with octahedral and predominantly tetrahedral layers alternately stacked along [100]. The layer in which 70% of the octahedral sites are occupied has an Mg-Al distribution characterized by Mg-rich octahedra sharing edges mainly with Al-rich octahedra. Mean octahedral bond lengths correlate well with Al occupancy determined by neutron site refinement if the relative number of shared octahedral edges is taken into account (see Table 1).The predominantly tetrahedral layer has 10% of the octahedral sites occupied by Al and 30% of the tetrahedral sites occupied by Al-Si in the ratio 2.33∶1. There are single chains of Al-Si tetrahedra parallel toz with corner-sharing wing tetrahedra (T5 andT6) on either side in the (100) plane. The meanT-O distance is highly correlated with Al occupancy, XAl, as determined from the neutron site refinement:

Texture and structural interpretation of the alteration of pyroxene to other biopyriboles

\langle T - O\rangle = 1.656 + 0.105X_{Al} (r^2 = 0.995).

Soft X-Ray Spectroscopy of Ferrous Silicates

Details of the neutron refinement are summarized below.

The variation of tetrahedral bond lengths in sodic plagioclase feldspars

The structural systematics of alkali and plagioclase feldspars and the details of their various space group symmetries are reviewed in relation to A-cation chemistry and Al,Si content, to A1,Si order-disorder, and to temperature. Pseudosymmetry and the site nomenclature arising from it are described in the context of displacive and diffusive phase transformations. The relation between real structures (which may involve doubled unit cells, antiphase domains, and elaborate superstructures, as well as spinodal and submicroscopic exsolution textures and resuloing strained phases) and average structures is established. The average structure concept involves “projecting” the steric details of the real structure onto the Cī albite or C2/m sanidine cell which contain only one A-cation site and 4 or 2 symmetrically nonequivalent tetrahedral sites,respectively. Thus an intensely complex ‘e’-plagioclase or a Pī or transitional anorthite (with c ~ 14.2 A) may be reduced to a simplified, “average” structural model which has its primary usefulness in establishing an understanding of the variation of easily measured metric parameters and optical properties.

The Crystal Structures of the Aluminum-Silicate Feldspars

In a metamorphosed gabbro from Hokkaido, Japan, augite containing exsolved orthohypersthene and minor pigeonite has been altered to a variety of biopyriboles. High resolution transmission electron microscopy of slightly altered augite shows only narrow (010) lamellae of clinoamphibole which always contain even numbers of double chains. In more highly altered regions, all three pyroxenes are changed to double-, triple- and more highly polymerized multiple-chain biopyriboles, with chlorite d001∼ 14.5 Å) found only in orthopyroxene. Several (010) lamellae containing only one double chain have been observed, and their textural relationship to the surrounding single-chain host may explain how the rotated domains of biopyribole initially attain their orientation in the host pyroxene.A structural model is proposed for the polymerization of single chains in the tetrahedral layers of pyroxene which involves small movements of oxygens and tetrahedral atoms (Si, Al) with a minimum of bond breaking and re-forming, concommitant with hydroxylization of certain oxygens and the diffusion of Mg, Fe and/or Ca along the rift in the octahedral layer.