Eggletonite, the Na analogue of ganophyllite
1984; Cambridge University Press; Volume: 48; Issue: 346 Linguagem: Inglês
10.1180/minmag.1984.048.346.12
ISSN1471-8022
AutoresDonald R. Peacor, Pete J. Dunn, William B. Simmons,
Tópico(s)Plant and Fungal Species Descriptions
ResumoAbstract Eggletonite is a new mineral, the Na analogue of ganophyllite. Type material has composition (Na 0.82 K 0.40 Ca 0.39 □ 0.39 ) Σ2.00 (Mn 6.61 Zn 0.08 Mg 0.16 Fe 0.61 Al 0.56 ) Σ8.02 (Si 10.33 Al 1.67 ) Σ12.00 [(O 28.92 (OH) 3.08 ] Σ32.00 (OH) 4.00 ·10.66H 2 O. 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 I 2/ 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/cm 3 . Optically, eggletonite is biaxial negative, with optical parameters 2 V α = 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.
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