GEDRITE
Gedrite – Mg2(Mg3Al2)(Si6Al2)O22(OH)2 – is a rare amphibole, characteristic of very specific metamorphic parageneses. It is not an ore.
Gedrite is classified in the Gedrite Group, which consists of gedrite and ferrogedrite. The two extreme terms of the Group are rare; variable contents of Mg (gedrite) and Fe (ferrogedrite) generally occur.
Gedrite forms a series with anthophyllite, with which it forms a complete solid solution at high temperatures. Gedrite may contain Na, Ti, Ca, F, Cl, Mn, and K. There is a lilac variety and another without Fe.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Pale greenish to light to medium brown. May be gray.
Habit: Long prismatic, may form subparallel or radial aggregates.
Cleavage: {210} perfect, {100} imperfect, {001} imperfect.
Tenacity: Brittle.
Twinning: No.
Fracture: Splintery.
Mohs Hardness: 5.5 – 6
Parting: On {001}.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.15 – 3.26
2. Geology and Deposits
Gedrites are common in medium- to high-grade mafic metamorphic rocks, such as schists and amphibolites. They can occur in rocks from the greenschist facies to the lower amphibolite facies.
They also occur in metasomatized contact metamorphic rocks, such as skarns and hornfels. They are common in the contact aureoles between ultramafic rocks (such as serpentinized peridotites) and the host rocks.
It occurs in hornfels in the metamorphic aureoles of intermediate intrusive igneous rocks.
3. Mineral Associations
Occurs with common rock-forming minerals such as quartz, plagioclase (albite), amphiboles (hornblende), orthopyroxenes, and micas (biotite, phlogopite).
The associated metamorphic minerals are anthophyllite, cordierite, rutile, staurolite, sillimanite, garnets (almandine, pyrope), cummingtonite, magnetite, spinel, corundum, sapphirine, talc, chlorite, olivine, calcite, and tourmaline.
More rarely with sulphotsumoite and cerite-(Ce).
4. Transmitted Light Microscopy
Refraction indices: nα: 1.671 nβ: 1.681 nγ: 1.690
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: It shows weak pleochroism in various shades of brown, yellowish-brown, greenish-brown, violet-brown, and gray. Pale green may also occur. Darker colors correspond to higher Fe contents.
Relief: High.
Cleavage: {210} perfect, {100} imperfect, {001} imperfect.
The {100} and {001} cleavages are generally not visible.
Longitudinal sections show only one cleavage direction.
Basal sections show two cleavage directions that intersect at 126 and 54º, as in all amphiboles.
Habits: Columnar, tabular, or long prismatic, it can form bundles of subparallel crystals or radial clusters of long prismatic crystals, sometimes acicular.
The basal sections in idiomorphic crystals are rhomboid, as in all amphiboles.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.019, corresponding to interference colors up to the end of the 1st order: gray, white, yellow, orange, and red. Eventually, it reaches the blue or green of the 2nd order.
Extinction: Parallel in longitudinal sections. Symmetrical in basal sections.
Elongation sign: ES(+) in longitudinal sections.
Twins: No. But exsolved anthophyllite lamellae may occur parallel to (010) in gedrite, simulating twinning.
Zoning: No.
CONVERGENT LIGHT
Character: B(+) or (more rarely), B(-).
2V angle: 75º (71 – 98º, varies with the composition).
Alterations: Gedrite, like anthophyllite, alters to a fine-grained mixture composed of serpentine, talc, and other phyllosilicates.
May be confused with: Other amphiboles, but monoclinic amphiboles exhibit oblique extinction.
Anthophyllite is very similar; it is neither easy nor safe to differentiate gedrite and anthophyllite by optical means.
Cummingtonite exhibits higher birefringence and frequently shows twinning.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying gedrite. However, it is important to prepare a polished slide or section to identify opaque minerals that occur associated with gedrite, such as magnetite.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx