SCORODITE
Scorodite – Fe3+AsO4.2H2O – is a relatively common arsenate, a typical alteration product of arsenic-containing minerals. It is not an ore.
It is classified in the Variscite Group, being dimorphic with parascorodite and the Fe analog of mansfieldite and yanomamite. It forms one series with yanomamite, another series with mansfieldite, and a third series with strengite. Al occurs as an impurity. There are two varieties (with Al, with P).
Caution! Scorodite, when heated, smells like garlic because it releases toxic arsenic vapors!
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Green, blue-green, gray, grayish-green, brownish-yellow, almost colorless, violet.
Habit: Scorodite crystals, up to 5 cm, are usually pyramidal {111}, sometimes pseudo-octahedral. Also tabular {001} or prismatic [010]. It frequently forms irregular groups or crusts. Also massive, crystalline or porous, sometimes resembling sinter. It can be earthy, in which case the color is light green to grayish-green or light brownish.
Cleavage: {201} imperfect, {001} in traces, {100} in traces.
Tenacity: Brittle.
Twinning: No.
Fracture: Subconchoidal.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Greenish white.
Lustre: Vitreous, resinous, sub-adamantine.
Diaphaneity: Transparente.
Density (g/cm³): 3.27
2. Geology and Deposits
Scorodite is a typical secondary mineral formed by the oxidation of sulfides with As, mainly arsenopyrite, but also other minerals containing As and Fe. It occurs in iron caps (gossans) and oxidation zones, typically always in small quantities. It is frequently associated with gold ores based on arsenopyrite and pyrite-(As). Sometimes it is primary, of hydrothermal origin.
Scorodite is a common arsenate in mine tailings and mine effluents due to the high frequency of arsenopyrite and pyrite-(As), two minerals whose oxidation produces As5+ oxides (scorodite, amorphous ferric arsenate, arsenosiderite, yukonite, pharmacosiderite) and, more rarely, As3+ oxides and sulfates (arsenolite, tooeleite, picropharmacolite). Among them, scorodite is the most abundant phase because it is more stable in environments near or at the surface.
3. Mineral Associations
It is associated with some common minerals such as quartz (including the chalcedony variety), “limonite” (goethite), clay minerals, and pyrite.
It occurs with other arsenates such as beudantite, pharmacosiderite, carminite, baryopharmacosiderite, arseniosiderite, dussertite, arthurite, adamite, austinite, arsenolite, picropharmacolite, yukonite, and segnitite.
Also with sulfates (gypsum, jarosite), phosphates (vivianite, tooeleite), and secondary Co minerals (erythrite).
4. Transmitted Light Microscopy
Refraction indices: nα: 1.741 – 1.784 nβ: 1.744 – 1.805 nγ: 1.768 – 1.820
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless to slightly greenish or brownish-green, with faint pleochroism between violet-blue and blue-green.
Relief: Very high.
Cleavage: {201} imperfect, {001} e {100} in traces.
Habits: pyramidal (may be pseudooctahedral), tabular, in crystalline crusts, porous, resembling sinter, earthy and massive.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence between 0.027 and 0.036, corresponding to strong, colorful, intense colors, up to the end of the 2nd order.
Extinction: Probably paralell.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 40 – 75º
Alterations: Scorodite alters to limonite (goethite).
May be confused with: No information.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying scorodite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with scorodite, such as arsenopyrite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: