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The arfvedsonite Mineral Group

Excellent Arfvedsonite Matrix Crystal

Arfvedsonite was first discovered in 1823. It is named after Johan August Arfwedson (1792–1841), a Swedish chemist credited with the discovery of the element lithium. Arfvedsonite is not a common mineral, and is found only in a limited amount of localities throughout the world. However, several of these deposits have produced outstanding examples of this mineral, making it a very collectible species. There are several variations of Arfvedsonite with different elemental substitutions. Some of these are recognized as individual minerals, thereby making Arfvedsonite a mineral group, with Arfvedsonite the dominating member.

Chemical Formula

NaNa2Fe2+4Fe3+Si8O22(OH)2

Color

Greenish-black, grayish black, reddish-brown, dark brown, black

Crystal System

Monoclinic

Properties

Streak
Dark bluish-gray
Hardness
5.5 - 6
Transparency
Opaque. May be translucent on thin cross-sections under back-lighting.
Specific Gravity
3.1 - 3.5
Luster
Vitreous
Cleavage
1,2 - prismatic
Fracture
Splintery, uneven
Tenacity
Brittle

Crystal Habits

In elongated prismatic crystals, often frozen in a matrix or spraying out of the matrix. Also in stubby crystals, wedge-shaped crystals, and in interconnected groups of crystals. Frequently acicular, radiating, reticulated, and columnar. Crystals are typically striated lengthwise.

Additional Information

Composition
Basic sodium iron silicate, sometimes with some potassium, magnesium, and fluorine
In Group
Silicates; Inosilicates; Amphibole Group
Striking Features
Crystal habits and mode of occurrence.
Environment
In iron-rich alkaline rocks in nepheline syenite deposits.
Rock Type
Igneous, Metamorphic

Varieties

 -   Variety of Arfvedsonite where magnesium partially replaces one the iron, and where fluorine partially replaces the hydroxyl and dominates it. Fluro-magnesio-arfvedsonite is recognized by the IMA as a distinct mineral species with the following chemical formula: NaNa2(Mg,Fe2+)4Fe3+Si8O22(F,OH)2
 -   Manganese-rich variety of Arfvesdonite with a reddish brown to bronze color, found mostly in Tirodi, India. Juddite is named after John Wesley Judd (1840 – 1916), a geologist for the British Geological Survey and later a professor of geology at the Imperial College in London.
 -   Variety of Arfvedsonite where potassium replaces one the sodium molecules. Potassic-arfvedsonite is recognized by the IMA as a distinct mineral species with the following chemical formula: KNa2Fe2+4Fe3+Si8O22(OH)2

Uses

Arfvedsonite is used as a collector's mineral. The highly aesthetic mineral specimens from Malawi are especially popular.

Noteworthy Localities

Some of the best examples of Arfvedsonite, in the form of highly lustrous, slender black crystals, are well known at Mount Malosa, Zomba District, Malawi. They may be individual prismatic crystals, which can exceptionally large, or they can be on an aesthetically contrasting matrix of white feldspar. They also may form a needle-like pseudomorph after Aegirine in that locality.

A manganese-rich form of Arfvedsonite with a reddish color, sometimes known as Juddite, comes from Tirodi, Madhya Pradesh, India. Well-terminated, lustrous Arfvedsonite crystals have recently been coming from Imilchil, Er Rachidia Province, Morocco. In Greenland, elongated crystals associated with feldspar were found in the Ilimaussaq complex, Narsuq.

Arfvedsonite is uncommon in the U.S. The best crystals come from Hurricane Mountain, North Conway, Carroll Co., New Hampshire; and Washington Pass, Okanogan Co., Washington. In Canada, large, thick, and lustrous Arfvedsonite crystals have come from Mont Saint Hilaire, Quebec.

Common Mineral Associations

Microcline, Orthoclase, Nepheline, Albite, Aegirine, Quartz, Zircon

Distingushing Similar Minerals

Aegirine - Crystals have steep pyramidal terminations.
Schorl
- Harder and lacks good cleavage.
Riebeckite - Can be very difficult to distinguish, although usually in thicker crystals.

arfvedsonite Photos



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