Allophane

Formula:  Amorphous Al₂O₃.1.3-2.0(SiO₂)^.2.5-3(H₂O) 

Origin of name:  From Greek for another and to appear, referring to a change to appear under the blowpipe.

Crystal system:  Unknown, amorphous

Crystal class:  Unknown

Twinning:

Unit cell:

Colour:  Colourless, white, green, bluish, yellow, brown

Diaphaneity: Transparent to translucent.

Luster:  Vitreous, resinous or waxy.

Habit:  Massive; as crusts resembling hyalite Opal; also stalactitic or powdery aggregates.

Hardness:  2 to 3

Specific gravity:  2.72 to 2.788

Cleavage:  None

Tenacity: Brittle

Streak:  Colourless, white

Synonyms/varieties:

Comments: Formed as a weathering product of volcanic ash. In hydrothermally altered igneous rocks, from the breakdown of feldspars, and in hydrothermal veins, typically related to copper deposits. In sedimentary rocks, including chalk and coal beds.