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Formula: Fe2+Fe3+4(PO4)3(OH)5
Anhydrous phosphate containing hydroxyl, rockbridgeite group, forms
a series with frondelite.
Specific gravity: 3.45 to 3.6
Hardness: 3½ to 4½
Streak: Grey-green, pale brown
Colour: Dark green, olive-green, brown, yellow-brown, greenish black, black; commonly exhibits colour banding; may be bronze-brown
or reddish brown when oxidized (Mindat).
Solubility: Soluble in hydrochloric acid, but not in nitric or sulphuric acid (Mindat, Dana).
Environments:
Pegmatites
Hydrothermal environments
Rockbridgeite is a secondary mineral found as alteration product of
triphylite or other
manganese phosphates in granite pegmatites, associated with
triphylite, hureaulite,
barbosalite, roscherite,
limonite, vivianite,
strengite, phosphosiderite,
heterosite, jahnsite,
laueite, roscherite,
siderite, stewartite,
strunzite and goethite (Dana, HOM, Mindat).
Localities
At the Moculta phosphate deposit and Tom's quarry, Mount Lofty Ranges, South Australia, rockbridgeite is a rare
mineral associated
with dufrénite (AJM 17.1.25).
At the Hagendorf South Pegmatite, Hagendorf, Waidhaus, Neustadt an der Waldnaab District, Upper Palatinate, Bavaria,
Germany, tiny black, lustrous crystals of rockbridgeite have been found on botryoidal, yellowish
fluorapatite
(Mindat photo).
Rockbridgeite from Hagendorf - Image
At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the
pegmatites are beryl-type
rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most
complex zonation and diverse mineralogy of any of the Keyes
pegmatites. Six zones are distinguished, as follows, proceeding
inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase
border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite
wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite
outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite
middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to
1.2 meters in size that were altered to vuggy
albite-muscovite with
fluorapatite crystals. This unit presumably was the source of the
albite, muscovite,
fluorapatite, quartz and other
crystallised minerals found in pieces of vuggy albite
rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including
tourmaline, graftonite,
triphylite, vivianite,
pyrite, pyrrhotite, and
beryl crystals to 30.5 cm long and 12.7 cm across.
Rockbridgeite was identified in specimens from the Keyes No. 1 mine as greenish-black to black masses to cabinet
size, in some cases with triphylite, and as microcrystals associated with
other secondary phosphates. It has also been found at the Keyes
No. 2 mine. Keyes rockbridgeites commonly show the typical radial acicular structure
(R&M 97.4.323-324).
Rockbridgeite from Keyes - Image
At the type locality, the Midvale Mine, Rockbridge county, Virginia, USA, rockbridgeite occurs in a
granite pegmatite (Mindat).
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