LUDWIGITE
Ludwigite – Mg2Fe3+(BO3)O2 – is a rare borate, member of the Ludwigite Group, dimorph of magnesiohulsite and the magnesium analogue of Vonsenite and Marinaite.
It has two varieties, with Al and with Sn (up to 12.0% w/w). Ludwigite is ferromagnetic and slowly soluble in acids.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Dark-green to black, like very dark tourmaline.
Habit: Rare crystals with a rhombic cross-section and without terminal faces. May be fibrous, radiating to dense felted aggregates, undulating. Sheaf-like rosettes of needle-like crystals. Granular.
Cleavage: No.
Tenacity: Though.
Twinning: No information available.
Fracture: Fibrous.
Mohs Hardness: 5
Parting: No information available.
Streak: Black to blackish green.
Lustre: Silky, sub-metallic,
Diaphaneity: Opaque.
Density (g/cm³): 3.8
2. Geology and Deposits
Ludwigite is a high-temperature mineral that occurs in magnesian Fe skarn and in contact metamorphic deposits.
3. Mineral Associations
Ludwigite occurs together with common carbonates (calcite, dolomite), common oxides (hematite, magnetite, spinel) and with rock-forming silicates (forsterite, lizardite).
Also with rare minerals like szaibélyite and chestermanite.
4. Transmitted Light Microscopy
This does not apply, since ludwigite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Polishing of ludwigite is difficult, because of breaking-out of very fine brittle needles. After impregnation, the polish is usually good.
PLANE POLARIZED LIGHT – PPL
Reflection color: gray with variable tinges. The longitudinal section is brigther and pink in color. Perpendicular to the elongation the color is grayish blue.
Pleochroism: Strong, brownish to bluish.
Reflectivity: Very low, 7.71 – 10.86%.
Bireflectance: Distinct, pinkish gray to dark greenish gray.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Strong, yellowish brown to bluish violet gray, with very conspicuous red and red-violet tinges.
Internal reflections: Numerous. Quantity and color vary, depending on the orientation of the section.
May be confused with: Few other minerals due to its very characteristic anisotropy colors and the strong tendency to form needle-like crystals.
Lievrite has very similar optical properties.
Tourmalines, if very colorful, show lower reflectivity and more abundant internal reflections.
General Characteristics:
Grain shape: Usually fibrous in radiating groups, may form dense felted aggregates. The fibers may be strongly bent, undulating. Granular. Cross-sections are triangular.
Substitutions: Ludwigite may be replaced by magnetite.
Alterations: Ludwigite alters to goethite along the margins or throughout.
