Crystal structure of a synthetic, compositionally intermediate, hypersolvus alkali feldspar: evidence for Na, K site ordering
Abstract
The crystal structure of an homogeneous single crystal of composition K0.58Na0.42AlSi3O8 (Or58), C2/m, a = 8.434(1); b = 13.015(2); c = 7.1717(9) Å; β = 116.09(1)°; Δa = −0.11 Å (Stewart and Wright, 1974), grown from a melt in the system Ab–Or–H2O at 700°C and 2.5 kbar, has been refined by least-squares and Fourier methods to a weighted R of 0.029. Structural and chemical homogeneity were verified, within the limits of resolution of the instruments, by transmission electron microscopy and electron-microprobe analysis. The best fit between experimental data and model was obtained using a structural model which split the alkali site into two half-sites, M(1) and M(2), with an ordered arrangement of K and Na, M(1) = 0.5 K, M(2) = 0.08 K + 0.42 Na, and assumed anisotropic thermal motion. Although we are not able to distinguish between the effects of thermal motion and positional disorder because of the small separation (0.08 Å) between the two half-sites, we favor the interpretation that K occupies a single site near the center of the alkali cavity whereas Na occupies two or more sites in the b,c plane displaced from the K position, as was found to be the case for Na in high albite (Ribbeet al., 1969). The orientation of the vector [14 · 0 · 1], separating M(1) and M(2), is approximately parallel to the normals of ([unk]01) and ([unk]01), the planes along which K and Na are commonly intergrown. We therefore suggest that K, Na ordering, as described above, is a possible precursor to the unmixing of intermediate composition alkali feldspars.
A linear trend exists between mean M–O distance and composition for six Al, Si disordered alkali feldspars including Or58, suggesting that structural features can be extrapolated from existing knowledge of end-member alkali-feldspar structures. A simple configurational-entropy calculation shows that the potential contribution of an alkali-site order-disorder relationship to Sconfigurational] (~ 1.3 cal deg−1 mol−1) is roughly one third of that resulting from Al, Si order-disorder.
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- Nyerereite: a new volcanic carbonate mineral from Oldoinyo Lengai, Tanzania
- The magnetic structures of NiWO4 and CoWO4
- A study of the structural chemistry of coesite
- Crystal structure of a synthetic, compositionally intermediate, hypersolvus alkali feldspar: evidence for Na, K site ordering
- Twinning and disorder in zinc perchlorate hexahydrate crystals
- Crystal data for the polymethine dye 1,3-bis (dimethylamino)-trimethinium perchlorate
- Unit-cell dimensions of Hofmann pyridine complexes
Articles in the same Issue
- Zeitschrift für Kristallographie 1877–1977
- The Zeitschrift für Kristallographie and Professor Dr. G. Menzer
- Die Molekular- und Kristallstruktur von N,N′-Disalicyliden-meso-1,2-diphenyläthylendiamin*
- Quadratischer elektrooptischer Tensor des rhombischen Calciumformiats
- Die Kristallstruktur von Cu2As3O6CH3COO
- Die Bestimmung der Kristallstruktur von Trimerit CaMn2(BeSiO4)3 und das Trimeritgesetz der Verzwillingung
- Powder data for the chlorine derivatives of C.I. Pigment Red 9 and C.I. Pigment Brown 1
- Nyerereite: a new volcanic carbonate mineral from Oldoinyo Lengai, Tanzania
- The magnetic structures of NiWO4 and CoWO4
- A study of the structural chemistry of coesite
- Crystal structure of a synthetic, compositionally intermediate, hypersolvus alkali feldspar: evidence for Na, K site ordering
- Twinning and disorder in zinc perchlorate hexahydrate crystals
- Crystal data for the polymethine dye 1,3-bis (dimethylamino)-trimethinium perchlorate
- Unit-cell dimensions of Hofmann pyridine complexes
