Brown acicular crystals of the rare mineral eggletonite from the TL in Arkansas: Granite Mountain Quarry #1, Little Rock, Pulaski County, Arkansas, United States of America. Ex Vandenbroucke Museum collection from Waregem, Belgium. — Photo: David Hospital · CC BY 4.0
Eggletonite
Eggletonite is an IMA-recognized mineral species in the silicate class, with the chemical formula (Na,K,Ca)ₓMn₆(Si,Al)₁₀O₂₄(OH)₄·nH₂O (x = 1-2; n = 7-11).
It crystallizes in the monoclinic crystal system. It is a member of the Clay structural group.
Key properties: Mohs hardness 3–4; specific gravity 2.76; Light brown streak; Vitreous luster; optically biaxial.
IMA symbol Egg, IMA status: approved, named in 1982.
Found in miarolitic pockets in nepheline syenite pegmatite.
It is found associated with Albite, natrolite, apophyllite pyrophanite, kupletskite.
The mineral was named to honor Dr. Richard A. Eggleton, Australian National University, Canberra, Australia.