THE MINERAL NORBERGITE

Chemistry: Mg₃SiO₄(F, OH)₂, Magnesium Silicate Fluoride Hydroxide.
Class: Silicates
Subclass: Nesosilicates
Group: Humite

Uses: Only as mineral specimens.
Specimens

Norbergite is one of several rare yet collectable minerals from the famous mines at Franklin, New Jersey. It is one of the many minerals there that are fluorescent. Norbergite will glow a tan to yellow color when subjected to shortwave UV light. Norbergite is often associated with other fluorescing minerals such as blue fluorescing diopside and red fluorescing calcite. The combination of different fluorescing colors can be quite attractive. Norbergite was first discovered in Norberg, Sweden from where it gets it name. It forms as small grains in the marbles of contact metamorphic environments.

Norbergite is a member of the Humite Group of minerals. Members of the Humite Group are noted for having a mixture of silicate layers and oxide layers in their structures. The silicate layers have the same structure as olivine. The oxide layers have the same structure as brucite. Norbergite is the only member of the group with just one olivine layer which alternates with the brucite layer. The formula could be written as:

Mg₂SiO₄ - Mg(F, OH)₂

This formula distinguishes the chemistry of the two layers. The most common member of the Humite Group is chondrodite which has two olivine layers between each brucite layer. Humite, the namesake of the group, has three.

PHYSICAL CHARACTERISTICS:

Color is white, yellow, tan, brown, orange or red.
Luster is vitreous to resinous.
Transparency: Crystals are transparent to translucent.
Crystal System: Orthorhombic; 2/m 2/m 2/m
Crystal Habits include small prismatic to rounded crystals, but as is most commonly the case, as embedded indistinct grains.
Cleavage is poor in one direction, basal.
Fracture is subconchoidal.
Hardness is 6 - 6.5.
Specific Gravity is 3.1 - 3.2
Streak is white.
Other Characteristics: Many specimens (usually those that are of a lighter color) fluoresce a tan to yellow color under shortwave UV light.
Associated Minerals include franklinite, fluorite, pyrrhotite, spinel, tremolite, diopside, dollaseite-(Ce), chondrodite and calcite.
Notable Occurrences include the type locality of Ostanmosoa Iron Mine, Norberg, Vastmanland, Sweden (hence the name) as well as Franklin, New Jersey, USA; Kazabazua, Quebec and Bancroft, Ontario, Canada; Pargas, Finland and Mt. Vesuvius, Italy.
Best Field Indicators are color, associations, fluorescence, environment of formation and hardness.