Information about Arfvedsonite

Arfvedsonite is a dark, sodium-rich amphibole mineral that forms in unusual alkaline igneous rocks. It is particularly popular among collectors for its striking metallic blue-green flash, which can be spectacular on polished material.

It typically occurs as elongated, prismatic to fibrous crystals, often in radiating or fan-like groups. It is typically greenish-black to bluish-grey in colour, though polished specimens can display a vivid blue, green, or violet sheen depending on the angle of the light. It has a submetallic to vitreous lustre.

Arfvedsonite forms in igneous rocks that are unusually rich in sodium and relatively poor in silica – the same unusual geological environments that produce minerals such as aegirine, eudialyte, and sodalite. It is closely related to riebeckite, and the two minerals can be difficult to distinguish without detailed analysis.

It belongs to the sodium amphibole subgroup of the large amphibole supergroup, and forms a series with fluoro-arfvedsonite.

A note for collectors: Arfvedsonite is one of the more frequently mislabelled minerals in the collector market. It is commonly confused with nuummite and astrophyllite, which can look very similar on polished material, and dark labradorite or other iridescent stones are sometimes sold under its name. If purchasing a specimen described as arfvedsonite, it is worth asking about the locality and verifying with a reputable seller, as genuine localities are relatively few and well documented.

Uses and History

Arfvedsonite has no significant industrial applications and is not used in jewellery in its raw crystalline form due to its cleavage and brittleness. Polished masses and tumbled material do appear in the crystal trade, where its blue-green flash makes it visually striking.

The mineral was first described in 1823 by the Norwegian-Danish geologist Hans Christian Ørsted, from specimens collected at Kangerdluarssuk in the Ilímaussaq complex of southern Greenland, which remains one of its most celebrated localities. It was named in honour of the Swedish chemist Johan August Arfwedson (1792-1841), best known for being the discoverer of lithium.

Notable localities include the Ilímaussaq complex in Greenland, which has produced some of the finest prismatic crystals in the world; the Khibiny and Lovozero massifs on the Kola Peninsula in Russia; Mont Saint-Hilaire in Quebec, Canada; Langesundsfjord in Norway; and various localities in China and Malawi. Arfvedsonite has also been recorded from Mynydd Mawr in Gwynedd, North Wales, where it occurs in microgranite, though the Welsh material is not of specimen quality.

Mineralogy

Hazards and Warnings

No specific health risks have been formally recorded for arfvedsonite. Mineral collectors should wash their hands after handling specimens as a matter of good practice.

Almost all rocks, minerals (and, frankly, almost all other substances on earth) can produce toxic dust when cutting, which can cause serious respiratory conditions including silicosis. When cutting or polishing rocks, minerals, shells, etc, all work should be done wet to minimise the dust, and a suitable respirator or extraction system should be used.

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