Mineral/Sedimentary Interface
Gossan with Copper Carbonate (Chrysocolla/Malachite)
Ferruginous Oxide (Gossan) with Cu2CO3(OH)2 (Malachite) or (Cu,Al)2H2Si2O5(OH)4 (Chrysocolla)
Hardness: 3.5-4 for standard malachite/chrysocolla; color: rusty-brown host rock with pale turquoise-green mineral crusts; luster: earthy to vitreous; structure: amorphous to microcrystalline; cleavage: none (fractures easily).
- Hardness
- 3
- Luster
- earthy to vitreous
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Physical properties
Hardness: 3.5-4 for standard malachite/chrysocolla; color: rusty-brown host rock with pale turquoise-green mineral crusts; luster: earthy to vitreous; structure: amorphous to microcrystalline; cleavage: none (fractures easily).
Formation & geological history
Formed through the oxidation and leaching of primary copper sulfide veins in the 'iron hat' or gossan zone. This occurs when rainwater oxygenates sulfide deposits, causing chemical recrystallization. Age is generally secondary and younger than the primary host rock.
Uses & applications
Gossans are used by geologists as indicators of deep-seated metal deposits (copper, gold, silver). Small specimens are collected for mineralogical study or as copper ore samples.
Geological facts
Historically, prospectors followed these 'rusty' outcrops to discover some of the world's largest copper mines. The green staining is an unmistakable sign of copper oxidation.
Field identification & locations
Identify in the field by looking for the bright green or blue-green 'bloom' against heavy, iron-rich brown rock. Commonly found in arid mining regions like Arizona (USA), Namibia, or Australia.
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