THE MINERAL STRENGITE

  • Chemistry: FePO4-2H2O, Hydrated Iron Phosphate
  • Class: Phosphates
  • Group: Variscite
  • Uses: As mineral specimens and limited ornamental stone uses.
  • Specimens
Strengite is an attractive and much sought after rare phosphate mineral. It is found in some very nice colors such as pink and violet red and has a nice vitreous luster. Its habits are somewhat variable but the most beautiful habits include the sphericule nodules and botryoidal masses. It is often associated with other phosphates including the rare and attractive green rockbridgeite. Rockbridgeite forms similar spherical nodules and the two together provide for an interesting specimen with multicolored balls attached to a host rock.

Strengite forms as an alteration product of primary phosphate minerals such as triphylite. Strengite will occur if the oxidation conditions are high enough to oxidize the iron of triphylite from a ferrous (+2) state to the ferric (+3) state as is needed for strengite. The presence of strengite is therefore an indicator of the how much oxidation the rock has under gone.

Another interesting occurrence of strengite is found in some caves whose host rocks contain minerals rich in iron. The alteration to strengite involves the reaction of these minerals with bat guano, or droppings. The phosphorus needed to form strengite is provided from the bat guano. The resulting "minerals" are, in a strick sense, not minerals since they indirectly originated from an organic source, bats.

Strengite forms a series with the more common mineral variscite, AlPO4-2H2O. A series is where different minerals will easily allow certain atoms to freely substitute for each other. In this case iron and aluminum can substitute for each other in variscite and strengite without any disturbance of the crystal structure. Usually the two minerals are fairly pure in nature, but some nearly intermediate specimens have been found.

Strengite is a beautiful mineral and is hard to find on the mineral markets. The best samples are usually small and micromounts are seemingly more available than cabinet samples. Once a specimen is obtained, it will no doubt become a real treasure for the owner.

PHYSICAL CHARACTERISTICS:

  • Color is colorless or white, but more commonly pink to violet red.
  • Luster is vitreous.
  • Transparency: Specimens are transparent to translucent.
  • Crystal System is orthorhombic; 2/m 2/m 2/m
  • Crystal Habits include spherical nodules, rosette aggregates, botryoidal masses, tabular to pseudo-octahedral crystals and crusts.
  • Cleavage is good in one direction, poor in another.
  • Fracture is conchoidal.
  • Hardness is variable from 3 - 4.
  • Specific Gravity is approximately 2.87 (average)
  • Streak is white.
  • Associated Minerals are limonite, heterosite, rockbridgeite, variscite, phosphosiderite, strunzite, beraunite, triphylite and many other secondary phosphate minerals.
  • Notable Occurrences include Rio Grande de Norte, Brazil; Bull Moose Mine, Custer, South Dakota, Indian Mountain, Alabama and San Diego Co., California, USA; Svappavaara, Sweden; Eleonore Mine, Germany and Mangualde, Portugal.
  • Best Field Indicators are color, habit, associations and luster.
STRENGITE specimens:
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STRENGITE specimen stn-1
$ 41.00
Dims: 1.6 x 1.6 x 0.6" (4.1 x 4.1 x 1.5 cm)
Wt: 1.10 oz. (31.4 g)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
A small crust of Strengite rests on the rusty iron-bearing base of this specimen. The crust is in fair condition, as it shows considerable damage, but this damage allows one to see its radial, fibrous form. Where intact, it shows a botryoidal habit that has a very dull, almost silky luster. Its color is a pale grayish pink, though its inner material appears to be much lighter. The crust is accompanied by a few random orbs of kidwellite, a related sulfide. It would appear, however, that the kidwellite only makes up a thin skin that covers each orb, and that their cores are made up of much darker-colored rockbridgeite. The base appears to be made up of hematite and shows visible weathering.
no photo
stn-1 ($ 41.00)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
STRENGITE specimen stn-2
$ 54.00
Dims: 2.3 x 2.1 x 1.5" (5.8 x 5.3 x 3.8 cm)
Wt: 5.44 oz. (154.3 g)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
Several small Strengite orbs are scattered on the iron-bearing base of this specimen. These orbs do not exceed 0.1" (3 mm) in diameter, and are generally in good condition, though a few are obviously damaged and incomplete. The broken surfaces show the compact, radial nature of the orbs, and all have a dull, almost silky luster. Their color is a pink-violet, and all are translucent. A few small, pale-green orbs of kidwellite accompany the Strengite, and all rest on what appears to be a crust of goethite. This crust partly coats the base, which appears to me made up mostly of hematite.
no photo
stn-2 ($ 54.00)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
STRENGITE specimen stn-3
$ 48.00
Dims: 1.9 x 1.7 x 1.0" (4.7 x 4.2 x 2.5 cm)
Wt: 1.6 oz. (45 g)
Indian Mountain, Cherokee County, Alabama, U.S.A.
Many tiny clusters of radiating Strengite crystals rest on the rusty base rock of this hand specimen. These crystals do not exceed 1 or 2 mm in length, and though many of them are crushed or decayed, there are a few clusters that are intact and in excellent condition. Their form is difficult to study with a loupe due to their size, but appears to be good. All have a pale to moderate pink coloration and a pearly-to-waxy luster, and are at least translucent.
no photo
stn-3 ($ 48.00)
Indian Mountain, Cherokee County, Alabama, U.S.A.
STRENGITE specimen stn-4
$ 195.00
Dims: 3.06x1.58x1.07" (7.78x4.02x2.73cm)
Wt: 5.83oz (165g)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
More than half of the largest surface on this piece of hematite is coated with lovely pale purple crystals of strengite. They have an unusual appearance, both dull and sparkly at the same time. A loupe reveals that this is caused by the crystal form, as the individual crystals have a vitreous luster, yet they are organized into rough spheres whose crevices absorb most reflections. There are many other minerals present, likely including goethite, kidwellite, and rockbridgite. About half of the strengite is coated with a yellow-brown layer, likely more rockbridgite.
no photo
stn-4 ($195.00)
Leveaniemi Iron Mine, Kiruna, Norbotten, Sweden
STRENGITE specimen stn-5
$ 35.00
Dims: 2.74x1.42x1.65" (6.95x3.61x4.18cm)
Wt: 5.58oz (158g)
Levineami, Sweden
This is a chunk of massive triphylite which has altered to a beautiful patch of strengite at one end. The strengite crystals are a pale lavender color, organized as botryoidal clusters of fine radial crystals. The clusters are rather dull in luster, but under a loupe the vitreous nature of the individual needles is apparent from their sparkle. There are other minerals present, but I have not determined the species.
no photo
stn-5 ($ 35.00)
Levineami, Sweden
 

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