Pyromorphite

Formula: Pb₅(PO₄)₃Cl

Origin of name: from Greek for fire and morpe = form

Crystal system: hexagonal

Crystal class: 6/m

Twinning: 

Unit cell: a = 9.987 Å, c = 7.330 Å

Colour: green, yellow, orange, brown, gray, colourless, white

Diaphaneity: 

Luster: subadamantine to resinous

Habit: crystals short prisms, hexagonal, often barrel-shaped, cavernous, equant, tabular or pyramidal, granular 

Hardness: 3.5 to 4

Specific gravity: 7.04

Cleavage: 1; {10-11} trace

Tenacity: subconchoidal to uneven, brittle

Streak: white

Synonyms/varieties:

Comments: pyromorphite forms a solid solution series with mimetite as arsenic replaces phosphorus as the anionic group in the formula