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Formula: Fe3+(PO4).2H2O
Hydrated normal phosphate, variscite group,
paramorph of
phosphosiderite, forms a series with
variscite.
Crystal System: Orthorhombic
Specific gravity: 2.84 to 2.87 measured, 2.84 calculated
Hardness: 3½ to 4
Streak: White
Colour: Purple, violet, pink, peach-blossom-red, carmine, greenish white, colourless; Colourless to pale pink in transmitted light.
Solubility: Soluble in hydrochloric acid but not in nitric acid.
Common impurities: Al
Environments:
Pegmatites
Sedimentary environments
Strengite is a late secondary phosphate in complex
granite pegmatites, in iron ores and
gossans, with magnetite
iron ores and rarely
as a cave mineral (HOM). In pegmatites it is a surface
product formed by alteration of iron-bearing phosphates such as
triphylite or dufrénite (Dana).
Associated minerals include beraunite,
hureaulite,
dufrénite, bermanite,
stewartite, cacoxenite,
frondelite/rockbridgeite,
vivianite,
apatite, limonite,
leucophosphite and
phosphosiderite
(HOM, Dana, Mindat).
Localities
At Broken Hill, New South Wales, Australia, strengite is found at Kintore with
corkite-hinsdalite and
pyromorphite, and globules
of strengite/variscite occur with
natrojarosite
on a thin coating of dufrénite on a
garnet
sandstone; The globules are zoned from cores of aluminium-rich
strengite
to rims of iron-rich variscite. At block 14 strengite is associated with
libethenite and corkite
(AJM 3.1.51-52).
At the Moculta phosphate quarry, Mount Lofty Ranges, South Australia, strengite has been found in small cavities in an
iron-rich phosphate rock associated with mitridatite and
leucophosphite
(AJM 17.1.25).
At the Mount Deverell variscite deposit, Milgun Station, Western Australia, strengite
has been identified associated with mitridatite. The
variscite deposits are hosted by marine sedimentary rocks
(AJM 20.2.31).
At the type locality, the Eleonore Mine, Rodheim-Bieber, Hesse, Germany, strengite occurs in a phosphate-bearing iron
ore deposit
(Mindat).
At the Kiirunavaara Mine, Kiruna, Sweden, strengite occurs in magnetite ore
(Dana).
At the John A. Logan mine, Cripple Creek mining district, Colorado, USA, two generations of strengite have been found in thin
limonitic fractures in a partly oxidised
orthoclase-quartz-pyrite rock associated with
cacoxenite and rockbridgeite
(MinRec 36.2.179).
At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, strengite forms millimeter-sized crystals associated with
strunzite, stewartite and
beraunite. The Emmons pegmatite is an example of a highly evolved
boron-lithium-cesium-tantalum enriched pegmatite
(R&M 94.6.516).
At Palermo number 1 mine and the Fletcher mine, North Groton, New Hampshire, USA, strengite occurs as an oxidation product of
triphylite (Dana).
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.
Strengite has been found at the Keyes No. 1 mine as spherical aggregates next to a spray of
individual crystals on rockbridgeite
(R&M 97.4.325).
At the Midvale mine, Rockbridge county, Virginia, USA, strengite is associated with
rockbridgeite (Dana).
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