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Acanthite (Photo)

Year of discovery: 1855 (Jáchymov, Czech Republic), 1930 (Japan)

Locality list: 22 localities of acanthite

• Ag₂S
• [Ag⁺₂][S]^2-

• Class: Acanthite group
• Class: Sulfide
• Soluble in: HNO₃
• Insoluble in: HCl
• Formed by sulfurization of Silver.
• Paramorph: Replace Argentite.
• Substitution pseudomorph: Replace Pyrargyrite.
• Stable below 177 degree.

• Polymorph: Argentite (Cubic, above 177 degree)

• Ag => Cu: Chalcocite

• Ag₂ => AgCu: Stromeyerite

• Ag_0.5 => Cu_0.5: Jalpaite

• Ag₈ => Ag₅Cu₃: Mckinstryite

• S => Se: Naumannite
• S => Te: Hessite

• S₂ => SeS: Aguilarite
• S₂ => TeS: Cervelleite

• Color: Brown-Iron black
• Streak: Black
• Luster: Metallic
• Diaphaneity: Opaque

• Habit: Pyramidal, Acicular elongated to [001]
• Twin: (-101)

• Cleavage: No
• Hardness: 2-2.5
• Density: 7.3

• System: Monoclinic, Pseudocubic
• Group: 2/m, P2₁/m
• Lattice: a=4.2, b=6.9, c=9.5, β=125.3

Etymology: Thorn in Greek. Aggregates look like a thorn.