The Mineral ROCKBRIDGEITE


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:

 

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