GLAUCOPHANE

Glaucophane – Na2(Mg, Fe)3Al2Si8O22 (OH)2 – is an inosilicate of the Amphibole Supergroup, typical of high pressure and low temperature metamorphic rocks of the blueschist facies. It has no economic importance.               

Glaucophane forms a series with riebeckite. “Crossite” is a term now discredited that designated minerals with intermediate compositions between glaucophane and riebeckite. Another series exists between glaucophane (Mg-rich extreme) and ferroglaucophane, the Fe-rich extreme. Glaucophane may contain, as impurities, Cr, Mn, Ca, Ti, Li, K, F and Cl.               

Holmquistite, a lithiniferous orthorhombic amphibole, is extremely similar to glaucophane, both in hand specimen and under the microscope, and has been confused with glaucophane for decades. Its paragenesis, however, is completely different: holmquistite occurs in metasomatic rocks on the periphery of lithiniferous pegmatites.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Gray, lavender blue. Zonation is common.

Habit: Usually long prismatic, can be acicular to fibrous. Also granular or massive.

Cleavage: {110} perfect, like all amphiboles.

Tenacity: Brittle.

Twinning: Single and multiple, // to {100}.

Fracture: Conchoidal, irregular.

Mohs Hardness: 6 – 6.5

Parting: On {010} and {001}.

Streak: Bluish gray.

Lustre: Vitreous to pearly. 

Diaphaneity: Transparent.

Density (g/cm³): 3.08 – 3.22

 

2. Geology and Deposits

Glaucophane occurs in medium-grade, low-temperature, high-pressure metamorphic rocks, originated in regional metamorphic conditions (blue-schists) from gabbros and basalts, usually in subduction zones. It can occur in metamorphic rocks of the greenschist facies.

It forms the “glaucophanite” rock and composes the glaucophane-gneisses and glaucophane-gabbro-gneisses, as well as the glaucophane-cornubianites.

Glaucophane also occurs in eclogites that have undergone retrograde metamorphism.

 

3. Mineral Associations

Glaucophane occurs with the typical low-grade metamorphic minerals: lawsonite, pumpellyite, chlorite, albite and members of the Epidote Group (clinozoisite, zoisite). In addition, quartz, jadeite, rutile, garnet, micas (muscovite, fuchsite, phengite), carbonates (calcite, aragonite), benitoite, natrolite, joaquinite-Ce and omphacite.

In eclogites it occurs with garnet (almandine), omphacite and hornblende.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.595    nβ: 1.614 – 1.650    nγ: 1.620 – 1.652

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Lavender blue, blue, dark blue, gray to black. Very distinct pleochroism:

X = colorless, pale blue, yellow;

Y = lavender blue, bluish green,

Z = blue, greenish blue, violet.

Colors become darker with higher Fe contents.

Relief: Moderate to moderate high.

Cleavage: {110} perfect, like all amphiboles.

In the longitudinal (prismatic) sections there is only one cleavage.

In the basal (rhombic) sections there are two cleavages that intersect at angles of 56º and 124º, which is typical of amphibole.

Habits: Usually prismatic, it can be acicular or columnar. More rarely fibrous or granular. Basal sections have a rhombic shape.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: High birefringence, ranging from 0.006 to 0.023: 1st and 2nd order colors, usually well colored. Its own blue color somewhat masks the interference colors.

Extinction: Oblique, with an angle of 0 – 22º (can simulate being parallel!).

In the basal sections the extinction is symmetric.

Elongation sign: ES(+).

Twins: Rare, on {100}.

Zoning: Frequently zoned.

CONVERGENT LIGHT

Character: B(-) ou B(+), usually B(-)

2V angle: 10-80º or 0-50º or 10-45º (discrepant data in references).

Alterations: by retrometamorphism, to amphibole (actinolite), barroisite or chlorite.

May be confused with: few other minerals, as the blue color, both in hand sample and in thin section in PPL, is very diagnostic.

Riebeckite has ES(-), deeper blue color and somewhat different habits.

Arfvedsonite shows much darker blue-green colors, sometimes look like an opaque mineral.

Dumortierite show a much weaker blue, has ES(-) and occurs in another paragenesis.

Hauyne is isotropic.

Tourmaline has a parallel extinction and no cleavage.

Holmquistite, a lithiniferous orthorhombic amphibole, is extremely similar but occurs in a completely different paragenesis.

Glaucophane 01: in PPL, in the center of the image, vertically, glaucophane crystal with one of its pleochroism colors, a strong blue. Around it, other glaucophane crystals.
Glaucophane 01: the same crystal as of the previous image, rotated 90 degrees and showing the second color of pleochroism, almost cololess. PPL.
Glaucophane 02: in PPL, glaucophane crystal with the first pleochroism color, a definite blue. The second color can be colorless, a lighter blue or a lavender color.
Glaucophane 02: again in PPL, the same crystal as of the previous image, now rotated 90 degrees. Its second pleochroism color is lavender. Shades of lavender or violet best seen in large crystals. In small crystals, only bluish colors can be noticed.
Basal section of glaucophane in PPL. Note the two cleavage directions crossing at 56º to 124º, typical for amphiboles. Upon rotating the stage, the section changes the color to colorless.
Glaucophane crystals in XPL, showing their strong colors. The crystals show colors ranging from light gray to intense second-order colors (yellowish-green), passing through strong 1st-order final colors (orange to deep blue).
Glaucophane in very small crystals in PPL, with its typical prismatic habit and characteristic blue color, which changes to almost colorless at the 90º turn of the stage.
Glaucophane 03: In PPL, with the 40x objective, rhombic basal section of very small glaucophane crystal showing its pleochroism. Around it, muscovite (long grain), garnet (high relief grain) and quartz. In such very small crystals, the typical amphibole cleavage is rarely visible.
Glaucophane 03: the same section as in the previous figure, now rotated 90 degrees. It is a very small crystal seen with the 40x objective. No cleavage is present.
In PPL, glaucophane basal section showing the typical amphibole cleavages despite of its small size.
Twinned glaucophane in a meta-basic rock, with many opaque minerals (black). In PPL, this glaucophane shows pleochroism between definite blue and almost colorless. In XPL as here, some lamellar twins can be seen.
In PPL, very fine grained schist with glaucophane (blue), garnet (round, idiomorphic), muscovite (lamellar) and quartz (low relief). Here, the glaucophane crystals can be seen clearly only with the 40x objective.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of glaucophane. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with glaucophane, like pyrite, chromite and others.

Sample preparation: the polishing of glaucophane is simple and follows the polishing of the associated feldspars. When in very small grains, can form holes and other imperfections.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, but lighter than quartz and feldspars.

Pleochroism: No.

Reflectivity: Low (<8%?).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy is not perceived.

Internal reflections: Generalized, very dark, almost black, blue colors are not perceived.

When the crystals are small, points of intense light to white luminosity are associated from the imperfections of the polishing.

May be confused with: many other minerals with dark internal reflections. Knowing of its presence, habit and size, glaucophane is easy to recognize in the polished section.

Glaucophane 01: in PPL, glaucophane crystals (long prismatic, lighter gray) in quartz (darker gray) and garnet (rounded to polygonal, very small, lighter gray). In black, holes in the polished section.
Glaucophane 01: the same section as of the previous figure. In XPL, recognition of glaucophane is very difficult or not possible.
In PPL, detail of a fine grained glaucophane schist. Quartz is dark, glaucophane (long prismatic crystal, horizontal) is lighter and garnet (idiomorphic or rounded crystals) is the clearest.
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