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Formula: (Na,K,Ca)xMn6(Si,Al)10O24(OH)4.nH2O (x = 1-2; n = 7-11)
Phyllosilicate (sheet silicate), ganophyllite group,
manganese-bearing mineral
Crystal System: Monoclinic
Specific gravity: 2.76 measured, 2.76 calculated
Hardness: 3 to 4
Streak: Light brown
Colour: Brown
Common impurities: Zn,Mg
Environments
Localities
At the type locality, Big Rock Quarry, Granite Mountain area, Little Rock, Pulaski county, Arkansas, USA,
eggletonite occurs in small pegmatite or miarolytic
pockets in nepheline syenite. The
pegmatite pockets are surrounded by a bleached zone of
altered syenite, which grades into
pegmatitic texture and forms the inner surface of the cavities.
Encrusting these wall minerals are albite,
biotite, amphibole,
titanite, apophyllite,
natrolite, aegirine,
zircon, analcime, and in some
instances sprays of eggletonite.
The holotype specimen consists of massive albite with minor amounts of
accessory biotite, aegirine,
titanite and magnetite. The surface
of the specimen, where eggletonite crystals grew freely into pockets, consists of
albite crystals up to 15 mm, with numerous late-stage minerals in the
interstices. Most abundant among these are an unanalysed amphibole and
second-generation albite, with minor amounts of
natrolite, apophyllite and
eggletonite. The sequence of formation appears to be late eggletonite and
albite, followed by amphibole,
and lastly, eggletonite and natrolite
(MM 48.93-96).
Other associated minerals include pyrophanite and
kupletskite
(HOM).
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