Alnöite
Melilite-bearing ultramafic lamprophyre
A rare dark ultramafic lamprophyre rich in melilite, biotite and olivine, named for Alnö Island in Sweden.
- Mohs hardness
- 5-6
- Color
- Dark gray to black
- Type
- igneous
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Overview
Alnöite is a rare, dark, ultramafic lamprophyre characterized by abundant melilite together with biotite (phlogopite), olivine, and clinopyroxene, set in a fine groundmass. It is a member of the ultramafic lamprophyre family closely tied to carbonatite and kimberlite-style magmatism.
The rock is named after Alnö Island in Sweden, site of a classic alkaline-carbonatite ring complex. Alnöite typically occurs as dikes, sills, and small intrusive bodies cutting older rocks.
Because of its deep mantle origins and association with carbonatites and sometimes diamonds, alnöite is of strong scientific interest despite being uncommon and not used as an ornamental stone.
Formation & geology
Alnöite forms from volatile-rich, silica-undersaturated ultramafic magmas generated deep in the mantle. These melts ascend rapidly along fractures, emplacing as dikes, sills, and diatreme-like bodies. The high content of CO2 and water promotes crystallization of melilite, mica, and carbonate.
It is closely associated with carbonatite and alkaline ring complexes, classically at Alnö (Sweden) and also documented in Canada (Ontario, Quebec), the USA, and Africa. Some alnöite intrusions are linked to kimberlite-like settings and can carry mantle xenoliths, recording melting at great depth beneath stable continental crust.
How to identify it
Alnöite appears as a dense, dark gray to black rock, often with visible glittery flakes of dark mica (biotite/phlogopite) and rounded olivine grains in a fine matrix. It may contain calcite and show a faint reaction (fizz) with dilute acid where carbonate is present.
Hardness is moderate (about 5-6). Distinguish it from basalt by its abundant mica and melilite and its lamprophyric porphyritic texture, and from kimberlite by the presence of melilite. Positive identification of melilite usually requires thin-section study, so field ID relies on the mica-rich, dark, carbonate-bearing character.
Uses & significance
Alnöite has no significant use as a building or gemstone material, but it is geologically valuable. Its mantle origin and association with carbonatites make it useful for studying deep Earth processes, and some alnöite-bearing pipes have been explored for diamonds and other mantle-derived minerals.
For rock collectors, alnöite is a prized rarity representing the ultramafic lamprophyre clan. It has no established metaphysical use and is mainly of academic and exploration interest.
Frequently asked questions
What makes alnöite special?
It is a rare melilite-rich ultramafic lamprophyre formed from deep mantle melts, often linked to carbonatites and occasionally to diamond-bearing settings.
Where does the name alnöite come from?
It is named after Alnö Island in Sweden, home to a classic alkaline-carbonatite complex where the rock was first described.
Does alnöite contain diamonds?
Some alnöite intrusions are emplaced like kimberlite pipes and have been explored for diamonds, but most do not carry economic diamond grades.
How is alnöite different from kimberlite?
Both are deep ultramafic rocks, but alnöite contains melilite and abundant mica, whereas kimberlite is defined by its olivine-rich, carbonate-bearing serpentinized character.
Alnöite guides
In-depth guides for identifying, valuing, and understanding Alnöite.
