• 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.


  • 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.
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Available STRENGITE specimens:
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