Abstract: Eggletonite is a new mineral, the Na analogue of ganophyllite. Type material has composition (Na_0.82K_0.40Ca_0.39□_0.39)_Σ2.00 (Mn_6.61Zn_0.08Mg_0.16Fe_0.61Al_0.56)_Σ8.02 (Si_10.33Al_1.67)_Σ12.00[(O_28.92(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 2V_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.