Halloysite

phyllosilicate

allophane

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. br 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, illite, minerals of the smectite group (which includes montmorillonite and nontronite), allophane (Dana).

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.

Common impurities: Ti,Ca,Na,K,Fe,Cr,Mg,Ni,Cu

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