Strengite vs variscite
Strengite vs variscite: two phosphate species, iron or aluminium. Indices 0.103 apart, hardness 3.5 vs 4.5; 20 g reads 12.96–13.03 vs 12.22–12.34 g in water.
Strengite and variscite are the iron and aluminium ends of one orthorhombic phosphate: strengite Fe3+(PO4)(H2O)2, purple to pink, hardness 3.5, density 2.84 to 2.87; variscite Al(PO4)(H2O)2, usually green, hardness 4.5, density 2.57 to 2.61. A refractometer separates them: their indices lie 0.103 apart. Colour, streak and cleavage do not.
In short
- The criteria, in order of strength. Refractive index first (no overlap, 0.103 apart), then weight in water (0.62 g apart for 20.00 g), then hardness (3.5 against 4.5, destructive). Colour, streak, cleavage and crystal system are on the list of things that do not separate them.
- The 0.103 index gap. The Handbook of Mineralogy sheets give strengite α 1.697 to 1.708, γ 1.741 to 1.745, biaxial (+), and variscite α 1.550 to 1.563, γ 1.570 to 1.594, biaxial (−). From 1.594 to 1.697 neither sheet prints a reading. The variscite-against-turquoise “1.6 gap” is on variscite; this one is between the two ends of the series.
- The 7.8% cell check. Both are Pcab with Z 8. Strengite’s cell, 10.122 × 9.886 × 8.7233 Å, is 872.9 Å3; variscite’s, 9.822 × 9.630 × 8.561 Å, is 809.7 Å3: 7.8% larger with iron, each edge 1.9 to 3.1% longer. The strongest powder lines are 3.114 Å (strengite, Kreuzberg) and 3.041 Å (variscite, Palazuelo de las Arribas). With the formula weights the cells give densities of 2.843 and 2.592, the sheets’ calculated figures.
- Documented failure: colour. Strengite is printed purple, violet, pink, peach-blossom-red, carmine, greenish white or nearly colourless; variscite pale green, emerald-green, bluish green, colourless, pale brown or yellow, rarely red. Green and purple separate typical pieces; a pale greenish or near-colourless piece, or a reddish one, fits both lists. Both streak white.
- Who each identification is wrong for. A purple crust from a pegmatite with an index near 1.70 is strengite, or its dimorph phosphosiderite; a green nodule reading near 1.56 is variscite, or its dimorph metavariscite. A reading inside the gap is covered by neither sheet. Species pages: strengite and variscite.
| Criterion | Strengite | Variscite | Separates them? | Phosphosiderite (strengite’s dimorph) | Metavariscite (variscite’s dimorph) |
|---|---|---|---|---|---|
| Formula (IMA) | Fe3+(PO4)(H2O)2 | Al(PO4)(H2O)2 | Yes, by analysis: 29.89% Fe against 17.08% Al | Fe3+(PO4)(H2O)2 | Al(PO4)(H2O)2 |
| Refractive indices | α 1.697–1.708; β 1.708–1.719; γ 1.741–1.745 | α 1.550–1.563; β 1.565–1.588; γ 1.570–1.594 | Yes: 0.103 gap, 1.594 to 1.697 | α 1.692–1.703; β 1.725–1.728; γ 1.738–1.739 | α 1.551; β 1.558; γ 1.582 |
| Optic sign | Biaxial (+) | Biaxial (−) | Yes, with a conoscope | Biaxial (−) | Biaxial (+) |
| 20.00 g piece in water | 12.96–13.03 g (D 2.84–2.87) | 12.22–12.34 g (D 2.57–2.61) | Yes: 0.62 g apart | 12.70–12.75 g (D 2.74–2.76) | 12.03–12.13 g (D 2.51–2.54) |
| Hardness | 3.5 | 4.5 | Yes, but destructive | 3.5–4 | 3.5 |
| Cell volume; strongest powder line | 872.9 Å3; 3.114 Å | 809.7 Å3; 3.041 Å | Yes, by X-ray | Monoclinic P21/n; 4.689 Å | Monoclinic P21/n; 4.758 Å |
| System, space group | Orthorhombic, Pcab | Orthorhombic, Pcab | No | Monoclinic | Monoclinic |
| Cleavage | {010} good; {001} poor | {010} good; {001} poor | No | {010} good; {001} indistinct | {010} |
| Streak | White | White | No | Not printed | White |
| Colour | Purple, violet, pink, peach-blossom-red, carmine, greenish white, nearly colourless | Pale green, emerald-green, bluish green, colourless, pale brown or yellow, rarely red | Only for typical pieces; pale and reddish overlap | Peach-blossom-red, reddish violet, purple, yellow-orange, moss-green to nearly colourless | Pale green |
How do you tell strengite from variscite? The criteria
The refractometer separates strengite from variscite on one reading, because their ranges never meet. Both sheets are Handbook of Mineralogy: the strengite sheet and the variscite sheet. Variscite’s indices run from α 1.550 to γ 1.594; strengite’s from α 1.697 to γ 1.745. The gap is 0.103. A reading from 1.550 to 1.594 lies in variscite’s range; one from 1.697 to 1.745 in strengite’s. Variscite is biaxial (−), strengite biaxial (+), so a conoscope separates them too.
The scale. At the sheets’ measured densities, 2.84 to 2.87 against 2.57 to 2.61, a 20.00 g piece reads 12.96 to 13.03 g in water as strengite and 12.22 to 12.34 g as variscite, the figures on the variscite page. The nearest figures are 0.62 g apart. The method is on how to measure specific gravity at home.
Hardness last. Strengite is 3.5, variscite 4.5. A scratch test on a hidden spot of rough separates them, but marks the specimen; see how to test mineral hardness properly.
Are strengite and variscite the same mineral? The 7.8% cell check
They are two species with one structure: the sheets call them a series, and the IMA list gives them separate rows. The IMA list writes strengite Fe3+(PO4)(H2O)2, status G, 1877, Germany, and variscite Al(PO4)(H2O)2, status A, 1967 s.p., Germany. Both sheets print Variscite group, orthorhombic 2/m 2/m 2/m, space group Pcab, Z 8, cleavage good on {010} and poor on {001}, and the same optic orientation, X = a, Y = c, Z = b.
The cell check, worked. Strengite’s cell, a 10.122, b 9.886, c 8.7233 Å, has a volume of 872.9 Å3. Variscite’s, a 9.822, b 9.630, c 8.561 Å, has 809.7 Å3. Iron in place of aluminium makes the cell 7.8% larger: a by 3.1%, b by 2.7%, c by 1.9%. With formula weights of 186.84 and 157.98, the cells give densities of 2.843 and 2.592, the sheets’ calculated 2.84 and 2.59. In an X-ray pattern the strongest line moves from 3.041 Å in variscite to 3.114 Å in strengite.
Documented failure: the middle of the series. Both sheets say the two form a series, but neither prints a property for an intermediate member, and every number on this page is for the iron or the aluminium end. The variscite sheet’s Lucin analysis is almost pure aluminium; strengite’s Pleystein analysis prints no aluminium. A reading between the two ranges is not covered by either sheet, and this page does not interpolate one.
Strengite or variscite by colour? Where it fails
Colour separates a typical purple strengite from a typical green variscite, and fails at the pale and reddish ends. The strengite sheet prints purple, violet, pink, peach-blossom-red, carmine, greenish white, and may be nearly colourless. The variscite sheet prints pale green, emerald-green, bluish green, colourless, may be pale shades of brown or yellow, rarely red. Greenish white against pale green, nearly colourless against colourless, carmine against rarely red: three overlaps. Both print a white streak, so the streak plate adds nothing.
The dimorphs make colour worse. Phosphosiderite, strengite’s monoclinic dimorph, is printed peach-blossom-red, reddish violet, reddish purple, yellow-orange, dark blue or moss-green to nearly colourless; metavariscite, variscite’s, pale green. On the scale, phosphosiderite’s 12.70 to 12.75 g sits between the two, and metavariscite’s 12.03 to 12.13 g just below variscite. Phosphosiderite’s indices, α 1.692 to γ 1.739, overlap strengite’s; its optic sign, biaxial (−), and its monoclinic system separate it.
Where turquoise comes in. Green variscite is more often confused with turquoise than with strengite; that separation is on variscite and how to tell real turquoise from fake and is not repeated here.
Where strengite and variscite occur, and where both do
Their usual settings differ. The strengite sheet: a late secondary mineral in complex granite pegmatites, in “limonite” iron ores and gossans, with magnetite iron ores, rarely in caves. The variscite sheet: typically deposited from phosphate-bearing waters in contact with aluminous rocks, and on islands and in caves from decomposed guano. Strengite’s associates include rockbridgeite, beraunite and cacoxenite; variscite’s, at the Little Green Monster mine, Utah, crandallite, carbonate-fluorapatite and wardite.
The shared-place check. Method: 22 place names were taken by hand from the strengite sheet’s Distribution field and matched against variscite’s by script, accent-folded and on word boundaries. 2 are on both: the Sapucaia pegmatite mine, about 50 km east-southeast of Governador Valadares, Minas Gerais, and the Iron Monarch quarry, Iron Knob, South Australia. At those two places the locality on a label does not decide between them; the refractometer does.
Elsewhere. Strengite’s list leads with the iron mines near Giessen and the Bavarian pegmatites; variscite’s with Messbach in Saxony, Cornwall’s china clay pits and the Utah nodules. The species pages carry the full lists: strengite and variscite. Other comparisons are in the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.