Formula: Al2Si2O5(OH)4.2H2O phyllosilicate (sheet silicate)
Specific gravity: 2.0 to 2.65
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
Plutonic igneous environments
Volcanic igneous environments
Hot spring deposits
Halloysite is a product of hydrothermal alteration or surface weathering of aluminosilicate minerals such
(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
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,
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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