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Eggletonite, the Na analogue of ganophyllite

Published online by Cambridge University Press:  05 July 2018

Donald R. Peacor ,
Pete J. Dunn  and
William B. Simmons
Donald R. Peacor
Affiliation:
Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109
Pete J. Dunn
Affiliation:
Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560
William B. Simmons
Affiliation:
Department of Earth Sciences, University of New Orleans, New Orleans, Louisiana 70148
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Abstract

Eggletonite is a new mineral, the Na analogue of ganophyllite. Type material has composition

(Na0.82K0.40Ca0.39□0.39)Σ2.00 (Mn6.61Zn0.08Mg0.16Fe0.61Al0.56)Σ8.02 (Si10.33Al1.67)Σ12.00[(O28.92(OH)3.08]Σ32.00 (OH)4.00·10.66H2O.

It occurs as prismatic crystals elongated along [100] in miarolitic cavities in nepheline syenite at the Big Rock Quarry, Little Rock, Arkansas. Single-crystal X-ray diffraction data are consistent with space group I2/a or Ia; a = 5.554, b = 13.72, c = 25.00Å, β = 93.95° Z = 2; this is equivalent to the substructure of ganophyllite. Physical properties include cleavage: {001} perfect; colour: dark brown; translucent; streak: light brown; hardness (Mohs): 3–4; brittle; observed density 2.76, calculated density 2.76 g/cm3. Optically, eggletonite is biaxial negative, with optical parameters 2Vα = 9(3)° r < ν, weak; indices of refraction α = 1.566(2), β = 1.606(2), γ = 1.606(2); pleochroism is very weak, γ ≃ β > α; α = pale brown to colourless, β ≃ γ = pale yellow brown. Principal lines in the X-ray powder diffraction pattern are (d, I/Io): 12.4, 100; 3.45, 15; 3.13, 30; 2.854, 15; 2.691, 25; 2.600, 20; 2.462, 20; 2.387, 15. The name is in honour of Dr Richard A. Eggleton, Australian National University.

Type
Research Article
Information
Mineralogical Magazine , Volume 48 , Issue 346 , March 1984 , pp. 93 - 96
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1984

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References

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