Smithsonite after Calcite

specimen number:
8311244

location:
San Giovanni Mine, Bindua, Iglesias, Sardinia, Italy

description:
A grouping of what were once large scalenohedral crystals of calcite, these replaced now by sparkling crystallized smithsonite. In super condition all around. These are classic pseudomorph specimens for this Sardinian locality.

The San Giovanni Mine, located near Iglesias in the Carbonia-Iglesias province of Sardinia, Italy, is a historic lead-zinc operation situated within the world-famous Iglesiente mining district. Geologically, the deposit is hosted in Paleozoic sedimentary rocks, primarily Cambrian-aged limestones and dolomites of the Gonnesa Formation, which underwent extensive karstification and tectonic brecciation. The ore bodies formed as Mississippi Valley-Type (MVT) deposits and later oxidation zones, where hydrothermal fluids and meteoric water created large underground cavities, including the spectacular Santa Barbara karst cave discovered within the mine. Historically, mining in the region dates back to ancient Roman times, but modern industrial exploitation began in the mid-1800s. The San Giovanni Mine was a major producer through the 20th century under various companies, including Monteponi and Pertusola, before large-scale extraction ceased in the 1990s as the district transitioned from industrial mining to geological heritage and tourism.

This locality is mineralogically famous for its remarkable pseudomorphs, most notably the world-class replacements of smithsonite after calcite. These specimens formed when zinc-rich meteoric waters reacted with pre-existing calcite crystals, gradually replacing the calcium carbonate with zinc carbonate while perfectly preserving the original rhombohedral or scalenohedral crystal forms. The resulting smithsonite often appears as delicate, "rice-grain" or "dry-bone" aggregates that maintain the sharp geometric outlines of the parent calcite. These pseudomorphs are typically translucent to opaque with a pearly or earthy luster and colors ranging from white and cream to pale yellow or grey. In addition to these zinc carbonate replacements, the mine has produced notable pseudomorphs of hemimorphite after calcite and various iron oxides after pyrite, making it a premier European site for studying secondary mineral alteration in oxidized lead-zinc environments.

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