sedimentary
Quartz Calcite Vug in Limestone
Calcium Carbonate (CaCO3) with Silicon Dioxide (SiO2) inclusions
Hardness: 3 (limestone/calcite) to 7 (quartz); Color: Grey matrix with clear to white crystals; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral in calcite portions.
- Hardness
- 3 (limestone/calcite) to 7 (quartz)
- Color
- Grey matrix with clear to white crystals
- Luster
- Vitreous to dull
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Physical properties
Hardness: 3 (limestone/calcite) to 7 (quartz); Color: Grey matrix with clear to white crystals; Luster: Vitreous to dull; Crystal structure: Trigonal/Hexagonal; Cleavage: Perfect rhombohedral in calcite portions.
Formation & geological history
Formed in marine environments through the accumulation of calcium carbonate shells and skeletons. The crystal vug (cavity) formed later via hydrothermal fluids or groundwater precipitation filling voids within the sedimentary rock, typically during the Paleozoic era.
Uses & applications
Limestone is used extensively in construction, cement production, and as an industrial flux. The crystal specimen itself is primarily for educational or amateur collecting.
Geological facts
Vugs like these are essentially 'mini-geodes' that form when acidic water dissolves a hole in the limestone and mineral-rich water later precipitates crystals into the space. Herkimer diamonds are a famous specialized version of this phenomenon.
Field identification & locations
Identify in the field by checking for the fizz reaction to hydrochloric acid on the grey matrix (calcite) and the inability of a steel knife to scratch the clear crystals (if they are quartz). Commonly found in quarry outcrops and road cuts in karst topographies.
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