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Formula: Al2Si2O5(OH)4.2H2O
Phyllosilicate (sheet silicate),
kaolinite-serpentine group,
paramorph of
dickite, kaolinite and
nacrite
Crystal System: Monoclinic
Specific gravity: 2 to 2.65 measured, 2.14 calculated
Hardness: 1 to 2
Streak: Paler than the color, or white.
Colour: White to tan, sometimes greenish or bluish, also chocolate brown to reddish.
Common impurities: Ti,Ca,Na,K,Fe,Cr,Mg,Ni,Cu
Environments:
Plutonic igneous environments
Volcanic igneous environments
Pegmatites rarely
Hot spring deposits
Halloysite is a product of hydrothermal alteration or surface weathering of aluminosilicate minerals such
as feldspar
(Webmin). Ongoing formation is observed at hot springs (Dana).
It is found as an alteration of basaltic rocks or in hydrothermally altered fissures in
monzonite. It also may be derived from
chlorite, mica,
rhyolite, granite,
volcanic ash, tuff and volcanic glass. It is found rarely in
granite pegmatites and bauxite
and clay. Commonly associated with
kaolinite, muscovite variety illite, minerals
of the smectite group (which includes montmorillonite,
saponite and
nontronite) and
allophane
(Dana).
Localities
Originally reported from Angleur, Liège, Liège Province, Belgium.
Alteration
Halloysite easily weathers to kaolinite, but can also be derived
from it. It is sometimes associated with allophane and may be
derived from it.
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