PHOSPHOSIDERITE
Phosphosiderite – FePO4.2H2O – is a rather rare phosphate, typically found in very specific paragenetic associations. Recently, a massive pink variety of phosphosiderite has become available on the ornamental stone market in Brazil, where it is used for decorative items and ornaments.
Phosphosiderite belongs to the Metavariscite Group; it is dimorphous with strengite and may contain Al and Mn.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Purple to reddish-violet, dark blue, pink, brownish-yellow, moss green, colorless.
Habit: Short prismatic, tabular. Botryoidal, reniform; crusts with a radial fibrous structure.
Cleavage: {010} good, {001} poor.
Tenacity: Brittle.
Twinning: interpenetration twins, on {101}.
Fracture: Irregular.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: No information available.
Lustre: Vitreous, resinous.
Diaphaneity: Transparent.
Density (g/cm³): 2.74 – 2.76
2. Geology and Deposits
Phosphosiderite is a phosphate formed by the alteration of triphylite (LiFe2+PO4), a mineral characteristic of complex, zoned granitic pegmatites.
It can be a component of soils. It replaces bones and shells.
3. Mineral Associations
It occurs with other rare and very rare phosphates: triplite, barbosalite, strunzite, strengite, rockbridgeite, laueite, bermanite, stewartite, hureaulite, leucophosphite, turquoise, and benyacarite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.692 nβ: 1.725 nγ: 1.738
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless; may exhibit pleochroism ranging from light pink and red to colorless.
Relief: High.
Cleavage: {010} good, {001} poor. In small crystals, it is not visible.
Habits: Short prismatic.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.046 – very high – corresponding to strong colors up to the middle of the 3rd order.
Extinction: Low-angle oblique.
Elongation sign: No information available.
Twins: No information available.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(-)
2V angle: 62º
Alterations: Phosphosiderite is a product of the alteration of triphylite.
May be confused with: Given its paragenesis and highly conspicuous macro-scale colors, it could easily be mistaken for some of the other rare phosphates with which it is associated.
5. Reflected Light Microscopy
Reflected-light microscopy is clearly not the recommended analytical method for identifying phosphosiderite. In pegmatites, where phosphosiderite typically occurs as an alteration product, the preparation of polished sections is more challenging due to the large size of the crystals that make up the rock. Furthermore, many of the ore minerals found in these pegmatites are difficult – if not impossible – to identify under a microscope, whether using transmitted or reflected light. This category includes white to milky, transparent minerals with a vitreous luster (such as amblygonite, petalite, and pollucite), as well as dark, opaque minerals with a metallic to submetallic luster (such as columbite, tantalite, wolframite, and fergusonite).
Sample preparation: When occurring as massive aggregates composed of very small crystals, phosphosiderite slowly acquires a poor polish, full of pits, because the crystals become dislodged during the preparation of the polished section.
PLANE POLARIZED LIGHT – PPL
Reflection color: Dark gray.
Pleochroism: No.
Reflectivity: Very low (<<10%).
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Anisotropy was not perceived.
Internal reflections: Widespread, in various shades of the color of the mineral in hand specimen.
May be confused with: other transparent minerals with colors ranging from pale pink to deep pink. It is not possible to identify phosphosiderite using reflected light. Once its presence is confirmed, however, it is easy to recognize.
