Daniel E. Appleman

The crystal structures of clinoenstatite and pigeonite

The crystal structures of the two monoclinic pyroxenes clinoenstatite, MgSiOj, and ferropigeonite, (Ca,Mg,Fe)Si03 , have been determined from twodimensional x-ray diffraction data, and refined by Fourier and least-squares methods. Bond lengths have been determined with a standard error of 0.05 Â. Deviations in these structures from that of diopside have been elucidated and explained in terms of crystal-chemical principles. The introduction of small bivalent cations into the single-chain silicate structure characteristic of diopside is shown to result in the production of two crystallographically non-equivalent chains in the unit cell, with a corresponding change in the space group from C2/c to P2j/c. In ferropigeonite complete order of Ca and Mg is found between the two metal positions, Mj and Μ π , while F e occupies both sites. Analysis of the coordination environments of the metal atoms in the clinoenstatite, pigeonite and diopside structures indicate that the larger atoms, where present, occupy the The crystal structures of clinoenstatite and pigeonite 121 Mj positions preferentially; thus the coordination number of Mj is eight in diopeide, seven in pigeonite and six in clinoenstatite, while the coordination number of M¡j is six in all three structures. The pigeonite structure appears to be intermediate between those of diopside and clinoenstatite. Introduction The determination of accurate crystal structures of rock-forming minerals of known chemical composition formed under various conditions has become an important field of investigation in mineralogy. From this viewpoint, many excellent cryetallographic studies have been carried out on feldspars and amphiboles. A corresponding study of pyroxene structures, however, has not yet been made, in spite of recent developments in the penological and experimental study of minerals of this group, and in spite of their relatively simple crystal structures compared with those of feldspars and amphiboles. This study has been undertaken as a part of a program to obtain crystallochemical knowledge of pyroxenes in general. Previous work on pyroxene s t ruc tures The natural pyroxenes fall into two classes: orthorhombic and monoclinic. The orthorhombic pyroxenes are known only in the composition range MgSi03—FeSiOs, with less than 10 mol percent of CaSi03 (KUNO, 1954; HESS, 1952). WARBEN and MODELL <1930) have made an x-ray analysis of hypersthene of composition (Fe-30MG70)SiO3. ITO (1950) has re-examined this structure using a hypersthene of composition (Fe ieMg M)Si03, and discussed the relationship between monoclinic pyroxenes and orthorhombic pyroxenes. Protoenstatite, another orthorhombic form of MgSi03, which forms synthetically at high temperatures, was described by THILO and ROGGE (1939), and FOSTER (1951) , and its structure was analyzed by SMITH (1959). This paper is confined to a structure study of the monoclinic group of pyroxenes (clinopyroxenes). WYCKOFF, MERWIN and WASHINGTON (1925) have shown that diopside, CaMgSi^Oj ; synthetic acmite, NaFeSiaOe; jadeite, NaAlSL¡06 ; hedenbergite, CaFeS^C^; and various augites have similar structures and belong to the clinopyroxenes. Among these pyroxenes, diopside was the first mineral to be analyzed by x-ray diffraction methods (WARREN and BRAGG, 1928). WARREN and BISCOE (1931) have compared diopside, hedenbergite, augite, acmite, jadeite, clinoenstatite (MgSiOj), and spodumene (LiAlSi^Og), and concluded that all these substances are members of the monoclinic pyroxene group with 1 2 2 Ν . MORIMOTO, Β . E . APPLEMAN a n d H . T . E V A N S , J R . diopside as type structure. They have also shown that the structure of spodumene is slightly distorted as compared with diopside due to the small size of the Li atoms. MOBIMOTO (1956), and BOWN and GAY (1957) have found that clinoenstatite and pigeonite, (Mg.Fe^S^Og, belong to the space group P2j/c, which is different from that of diopside, C2jc. Although accurate measurement of unit cell dimensions of pyroxenes with various compositions has been made (HESS, 1952; KUNO and

Bonding in eight ordered clinopyroxenes isostructural with diopside

Bond distances and angles in isostructural, ordered clinopyroxenes are compared for eight compositions, based on five new and three published crystal-structure refinements from X-ray diffraction data. Unit-cell parameters and configuration of the silicate chains are directly correlated with cation composition and distribution in the M2 and M1 sites.

Pentaborate Polyanion in the Crystal Structure of Ulexite, NaCaB5O6(OH)6·5H2O

Triclinic ulexite crystals contain isolated borate polyanions [B5O6(OH)6]3- related to the well known pentaborate polyanion [B5O6(OH)4]- by addition of two hydroxyl groups to two opposite B-O triangles. The isolated ulexite polyanions form the [B5O7(OH)4]n3n- chains previously found in crystals of the related mineral probertite, NaCaB5O7(OH)4�3H2O.

Unit-Cell Dimensions of Natural and Synthetic Scapolites

In natural scapolites the cell dimension a shows a regular increase from marialite to meionite composition, while c remains constant. Both a and c of synthetic meionite are larger than the corresponding dimensions of synthetic marialite. The cell volume of both natural and synthetic scapolites is a nearly linear function of composition. Variations in cell dimensions of scapolites may be caused by differences in structural state similar to those in plagioclase feldspars.

Crystallography of Some Lunar Plagioclases

Crystals of calcic bytownite from type B rocks have space group I1 with c ≈ 14 angstroms. Bytownite crystals from type A rocks are more sodic and have space group C1, c ≈ 7 angstroms. Cell parameters of eight bulk feldspar separates from crystalline rocks indicate that the range of angle gamma is about 23 times the standard error of measurement, and its value might be useful for estimation of composition. Cell parameters of seven ilmenites are close to those of pure FeTiO3.

Crystal structure refinement of reedmergnerite, the boron analog of albite

Ordering of boron in a feldspar crystallographic site T1(0) has been found in reedmergnerite, which has silicon-oxygen and sodium-oxygen distances comparable to those in isostructural low albite. If a simple ionic model is assumed, calculated bond strengths yield a considerable charge imbalance in reedmergnerite, an indication of the inadequacy of the model with respect to these complex structures and of the speculative nature of conclusions based on such a model.