Alnoite

Alnoite is a rare igneous rock type belonging to the lamprophyre family, specifically a variety of mica-rich lamprophyre. It is scientifically notable for its mineralogy, chemistry, and occurrence in alkaline igneous provinces.

🌍 Origins and Naming

• Name Origin: Named after Alnö Island, Sweden, where it was first described.
• Discovery: Identified in the late 19th century during studies of alkaline complexes.
• Type Locality: Alnö Island, Västernorrland, Sweden — famous for carbonatites and lamprophyres.

🔬 Composition and Mineralogy

• Major Minerals:
  • Biotite or phlogopite (mica)
  • Clinopyroxene (diopside, augite)
  • Olivine (sometimes altered)
• Accessory Minerals: Apatite, magnetite, perovskite, calcite.
• Texture: Porphyritic, with large mica phenocrysts in a fine-grained groundmass.
• Chemistry: Rich in potassium and magnesium; belongs to the alkaline lamprophyres.

⚙️ Geological Occurrence

• Found in alkaline igneous complexes, often associated with carbonatites and other unusual rock types.
• Localities:
  • Alnö Island, Sweden (type locality).
  • Other alkaline provinces worldwide, though rare.
• Geological Context: Intrusive dikes and plugs, often linked to deep mantle-derived magmas.

📖 Scientific and Collector Significance

• Petrology: Important for understanding mantle-derived alkaline magmatism and lamprophyre diversity.
• Collectors: Rarely collected as hand specimens; valued academically for its mineralogical associations.
• Economic Note: Sometimes associated with carbonatite complexes that host rare earth elements, though alnoite itself is not an ore.

✨ Conclusion

Alnoite is a mica-rich lamprophyre, first described from Alnö Island, Sweden. It is scientifically significant for its role in alkaline igneous complexes and mantle-derived magmatism, though it remains a petrological curiosity rather than an economic resource.

In short: Alnoite = mica-rich lamprophyre, rare igneous rock, first found at Alnö Island, Sweden.