Goethite after Siderite

specimen number:
2062465

location:
2nd Mesabi Claim, Lake George, Park Co., Colorado, USA

description:
Large rhombohedral crystal of what was once siderite, this now replaced by goethite and set up on matrix. In good condition all around. Collected by Ray Berry.

The 2nd Mesabi Claim, located in the Lake George pegmatite district of Park County, Colorado, lacks precise historical records for exact opening and closing dates, as it was operated as a small prospect claim during the twentieth century. Geologically, the site is characterized by late-stage pegmatites within the Mesoproterozoic Pikes Peak batholith. These pegmatites contain miarolitic cavities formed by trapped volatile gases, which created the ideal environment for growing crystallized specimens of microcline, smoky quartz, and associated iron-bearing minerals.

Pseudomorphs form when a secondary mineral replaces an earlier primary mineral while maintaining the original external crystal shape. For goethite after siderite, weathering or hydrothermal alteration causes the original iron carbonate mineral to dissolve or oxidize. Iron oxide minerals then fill or replicate the resulting mold, resulting in a specimen that preserves the exact crystal morphology of the precursor mineral even though its internal composition has completely changed.

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