The Mineral ROCKBRIDGEITE

• Chemistry: (Fe, Mn)Fe4(PO4)3(OH)5, Iron Manganese Phosphate Hydroxide.
• Class: Phosphates
• Uses: Only as mineral specimens.
• Specimens

Rockbridgeite is one of the more oddly named and yet interesting phosphate minerals. It has a general green color but that changes with oxidation to a dark brown or black. Never-the-less, rockbridgeite shows classic botryoidal form similar to specimens of limonite. In fact, it produces a nice backdrop to the more colorful minerals that may be attached to the rounded botryoidal landscape, such as the sphericules of the lovely pink phosphate mineral strengite.

Rockbridgeite forms from the alteration (oxidation) of primary iron and manganese phosphates. The chemistry of rockbridgeite is odd in that it includes two different iron atoms. One is ferric (with a plus 3 charge) and one is ferrous (with a plus 2 charge). The ferrous iron occupies the same position as and substitutes with the plus 2 manganese. It is this reason that the formula is written the way it is to show the different irons. Geologist and petrologists are very interested in minerals that contain elements with two different oxidation states. It represents a mineral that formed as the oxidation of the original material was taking place and its temperature and pressure of formation can then give clues to the conditions that were present during oxidation.

Rockbridgeite forms a series with the mineral frondelite. A series occurs when two or more elements can freely substitute for each other without significant alteration of the structure. In this case, rockbridgeite is a ferrous iron rich mineral and frondelite is the manganese rich member of the series.

PHYSICAL CHARACTERISTICS:

• Color is brown, dark green, yellow-brown or black.
• Luster is varied from vitreous to dull or earthy.
• Transparency: Specimens are translucent to transparent.
• Crystal System is orthorhombic; 2/m 2/m 2/m
• Crystal Habits include prismatic to tabular crystals, but often botryoidal, reniform, encrusting, massive and fibrous.
• Cleavage is poor.
• Fracture is uneven.
• Hardness is 4.5
• Specific Gravity is approximately 3.4 (slightly above average).
• Streak is green or brown.
• Associated Minerals include strengite, beraunite, dufrenite and limonite.
• Notable Occurrences include Cornwall, England; Siegerland, Germany; Svappavaara, Norrbotten, Sweden; Black Hills, Custer County, South Dakota and South Mountain, Midvale, Rockbridge County (hence the name), Virginia, USA and Brazil.
• Best Field Indicators are crystal habit, color, associations and streak.