mineral
Glendonite (Calcite Pseudomorph after Ikaite)
Calcite (CaCO3) pseudomorph after Ikaite (CaCO3·6H2O)
Hardness: 3 (calcite); Color: Tan, brown, or grey; Luster: Dull to vitreous; Crystal structure: Originally monoclinic (ikaite), now trigonal (calcite); Specific Gravity: 2.71.
- Hardness
- 3 (calcite)
- Color
- Tan, brown, or grey
- Luster
- Dull to vitreous
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Physical properties
Hardness: 3 (calcite); Color: Tan, brown, or grey; Luster: Dull to vitreous; Crystal structure: Originally monoclinic (ikaite), now trigonal (calcite); Specific Gravity: 2.71.
Formation & geological history
Formed as ikaite crystals in cold, organic-rich marine sediments near freezing temperatures. As temperatures rise above 8°C, the ikaite becomes unstable and dehydrates into calcium carbonate (calcite), preserving the original spiked crystal shape.
Uses & applications
Primarily a collector's specimen and geological indicator of ancient cold-water environments (paleoclimatology).
Geological facts
Glendonites are named after Glendon, Australia, where they were first described. They are often called 'pineapple opals' when replaced by opal, or simply 'cold-water indicators' by geologists.
Field identification & locations
Identify by its distinctive stellate, spiked, or 'hedgehog' globular form. Often found in shales or mudstones that represent ancient glacial or deep-sea marine environments.
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