Acmite is an older name for the mineral aegirine, a sodium iron silicate belonging to the pyroxene group. Today, the International Mineralogical Association (IMA) recognizes aegirine as the official name, but “acmite” is still used historically and in some literature.
🌍 Origins and Naming
- First Description: Acmite was described in 1821 by P.H. Ström from Rundemyr, Øvre Eiker, Buskerud, Norway.
- Name Origin: Berzelius named it achmit (later acmite) from the Greek akhmē meaning “spear point,” referring to its sharp crystal habit.
- Later Revision: In 1835, H.M.T. Esmark described a similar mineral from Låven, Langesundsfjorden, Norway, naming it aegirine after the Norse sea god Ægir.
- Synonymy: Acmite and aegirine were once thought to be distinct species, but later research confirmed they are the same mineral.
🔬 Chemical and Structural Properties
- Formula: NaFe³⁺Si₂O₆
- Mineral Group: Pyroxene (clinopyroxene subgroup)
- Crystal System: Monoclinic
- Color: Dark green, greenish-black, brownish-black, or reddish-black
- Habit: Long, slender prismatic crystals, often with pointed terminations; sometimes fibrous or acicular
- Hardness: 6–6.5 on Mohs scale
- Specific Gravity: 3.50–3.60
- Luster: Vitreous to slightly resinous
- Streak: Light gray to yellowish-gray
⚙️ Geological Occurrence
Acmite/aegirine typically forms in:
- Alkaline igneous rocks such as nepheline syenites and phonolites
- Pegmatites associated with alkali-rich environments
- Metamorphic rocks under high-pressure conditions
Notable localities:
- Norway (Buskerud, Langesundsfjorden) – type localities
- Malawi – fine prismatic crystals
- Canada, Greenland, Russia, and the USA – occurrences in alkaline complexes
📖 Scientific and Collector Significance
- Petrology: Aegirine is an important indicator mineral in alkaline magmatic systems, helping geologists understand geochemical processes.
- Collectors: Its sharp, lustrous crystals are highly prized, especially when associated with feldspar or quartz.
- Historical Note: The dual naming (acmite vs. aegirine) reflects the evolving history of mineral classification in the 19th century.
✨ Conclusion
Acmite is essentially the historic synonym for aegirine, a sodium iron silicate pyroxene. Its dark green prismatic crystals, alkaline rock associations, and sharp spear-like habit make it both scientifically significant and aesthetically appealing. While “acmite” is rarely used today, it remains part of mineralogical history, reminding us how classification evolves with deeper study.
In short: Acmite = aegirine, a sodium iron silicate pyroxene with dark green spear-like crystals, first described in Norway.
