GROUTITE
Groutite – Mn3+O(OH) – is a very rare oxide. It integrates Mn ores, but generally occurs sporadically. It can form larger volumes, but it is never the main ore mineral in the deposit.
It is classified in the Diaspora Group, with which it is isostructural. It is a polymorph of feitknechtite and manganite. May contain Sb.
Macroscopically it is very similar to manganite.
1. Characteristics
Crystal system: Orthorhombic dipyramidal.
Color: Black. It can appear iridescent.
Habit: Crystals up to 1 cm, wedge- or lens-shaped paralell to (001), with curved faces. Acicular-prismatic, in radial groups.
Cleavage: {010} perfect, {100} very good. Striations ocurr.
Tenacity: Brittle.
Twinning: Undefined twins are possible.
Fracture: Irregular, splintery.
Mohs Hardness: 3.5 – 4
Parting: No.
Streak: Dark brown.
Lustre: Submetallic, resinous, dull, etc.
Diaphaneity: Transparent.
Density (g/cm³): 4.144
2. Geology and Deposits
At the type locality (Mahnomen pit, Crow Wing County, Minnesota, USA), it was found as late-stage mineralization in cavities within an altered banded iron formation (BIF).
It has also been found in manganiferous talc schists, in a metamorphosed stratiform Zn deposit (Franklin, New Jersey, USA), and in various hydrothermal deposits.
3. Mineral Associations
At the type locality, it is associated with quartz, manganite, hematite, magnetite, goethite, and lepidocrocite.
It also occurs in association with silicified wood, calcite, aragonite, barite, franklinite, and vesuvianite.
It is associated with other Mn minerals such as hureaulite, kutnohorite, and ramsdellite.
4. Transmitted Light Microscopy
The literature provides some fragmentary data on the behavior of groutite under Transmitted Light.
Refraction indices: nα = 2.100 – 2.200; nγ = 2.100 – 2.200
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Strong pleochroism ranging from very dark brown to black and yellowish-brown.
Relief: Moderate.
Cleavage: {010} perfect, {100} very good.
Habits: Acicular-prismatic, in radial groups.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.1, very high, showing pearly colors, difficult to classify.
Extinction: Paralell.
Elongation sign: No information available.
Twins: Possible.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 40 – 50º.
Alterations: No information available.
May be confused with: No information available.
5. Reflected Light Microscopy
Sample preparation: Groutite takes an excellent polish.
PLANE POLARIZED LIGHT – PPL
Reflection color: Grayish-white to pale brownish-gray. Compared to the color of magnetite, the color of groutite is very similar.
Pleochroism: Somewhere between gray and brownish.
Reflectivity: 12.7 – 18.66%
Bireflectance: Distinct to strong, with the clearest position perpendicular to the elongation.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Very strong anisotropy, but without vivid colors, ranging from dark violet-brown (parallel to the elongation) to pale brownish-gray (perpendicular to the elongation).
Internal reflections: Abundant only in sections parallel to (010) – otherwise rare – and dark reddish-brown in color.
May be confused with: other Mn minerals, but the tabular habit of groutite is characteristic.
Manganite is very similar but exhibits internal reflections much more frequently.
General Characteristics:
Grain shape: occurs as wedge-shaped or lens-shaped crystals up to 1 cm in size, flattened parallel to (001), with curved faces. It may appear prismatic-acicular, in radial groups, or as fine granular aggregates.
Thin crusts of brown iron (goethite) may cover the crystals.