VOLCANIC GLASS

Volcanic glass, also called obsidian, is a very common, amorphous material that occurs only in lavas and hypabyssal intrusive rocks (sills and dykes), as a fundamental mass and containing small crystals and phenocrysts. Its formula depends on the type of rock and is complex, very variable.

4. Transmitted Light Microscopy

Refraction indices:  Not applicable.

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless, gray, brown, red.

Relief: Low positive in basic rocks, low negative in acidic rocks.

Cleavage: Not applicable.

Habits: Without definite shape. It occurs as interstitial material or making up the largest percentage of the rock. It is usually massive, may have flow structures or irregular spherical fractures (perlitic texture), formed by shrinkage during cooling. May have shard textures (typical of ignimbrites) and feathery aggregates due to devitrification.

CROSSED POLARIZED LIGHT – XPL

Isotropic.

CONVERGENT LIGHT

Isotropic.
 

Alterations: can devitrify or alter, all stages are possible. Devitrification propagates from fractures; the glass is replaced by finely crystalline aggregates composed of the minerals that would have formed if the magma had cooled more slowly. There are a number of textures generated by the alteration: acicular crystals (~needles), dendritic structures (~trees), feathery structures (~feathers); spherulites (radial aggregates), etc. Use specialized literature to work on this subject.

May be confused with: constituting the matrix of the rock, it cannot be confused with any other mineral. “Flow” textures are not actually flow, but generated by compressing the hot material. Pearlitic (circular) fractures are very characteristic but not always present. If it occurs as interstitial material in small amounts and is colorless, it is difficult to recognize.

Opal and fluorite can be very similar, but have lower refractive indices.

Analcime, leucite and minerals from the Sodalite Group show crystalline forms and/or cleavage.

Volcanic Glass 01: in PPL, brown volcanic glass with some plagioclases (colorless + low relief) and pyroxenes (colorless + medium relief) crystals with some fractures.
Volcanic Glass 01: The same section as of the previous image, now in XPL. Volcanic glass is isotropic.
In PPL, brown volcanic glass with perlitic fractures, which are cracks that tend to be circular.
In PPL, detail of perlitic fractures in brown volcanic glass.
Volcanic Glass 02: Devitrification spherulite in acidic volcanic rock. In PPL, the fine radial structure of the spherulite can be seen.
Volcanic Glass 02: The same section as of the previous image, now in XPL. The gray/white interference colors of the feldspars that form the spherulite can be observed. There may be a crystal in the center of the structure.
Basaltic rock with high glass content in PPL. The glass is brown and is permeated with devitrification structures. In CPL, this glass is black.
Volcanic glass in PPL with radial devitrification structures between feldspar (colorless) and magnetite (black).
Volcanic glass in PPL with flow structures, which are these roughly horizontal alignments. The image was obtained with the diaphragm partially closed to highlight these flows. In XPL, the image is black, as the glass is isotropic.
Volcanic glass in PPL with well-marked flow structures. In XPL, the glass is completely black (isotropic).
Volcanic Glass 03: Devitrification structure in volcanic glass, marketed as “snowflake obsidian” ornamental stone, from Mexico. Macroscopically the structures are white and the obsidian is black.
Volcanic Glass 03: The same section as of the previous image, now in XPL. Glass is isotropic and the structure is composed of very small crystals, indistinguishable under the microscope.
Volcanic Glass 04: Vitreous volcanic rock with abundant devitrification spherulites. These are small, rounded structures with a radial fibrous structure, irregularly distributed throughout the rock.
Volcanic Glass 04: The same section as of the previous image, now in XPL. Each one of the spherulites has a radial structure.
In PPL, volcanic glass with devitrification spherulites with radial structure. In gray, quartz. Spherulites tend to form spherical structures, but most often develop only partially.
Volcanic Glass 05: In PPL, phonolite with devitrification spherulites. In PPL, the spherulites with their typical radial structure are easy to see.
Volcanic Glass 05: The same section as of the previous image, now in XPL. The identification of the spherulites in XPL is is much more difficult due to the relatively coarse granulation of the rest of the rock.
Volcanic Glass 05: From the same phonolite of the two previous images, now the spherulites in detail. PPL.
Volcanic Glass 05: The same section as of the previous image, now in XPL. The spherulite of the right has a crystal in its center.
Volcanic Glass 06: In rhyolite, in XPL, idiomorphic quartz crystals and, in the rock mass, feldspathic spherulites, which may be very well developed.
Volcanic Glass 06: From the same rock shown in the previous image, now a devitrification spherulite in detail. XPL.

5. Reflected Light Microscopy

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

Sample preparation: The polishing of volcanic glass is simple and is of excellent quality.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspar.

Pleochroism: No.

Reflectivity: Low (<10%)

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  Complete isotropy.

Internal reflections: Generalized in the macro color of the glass, which can be black or brown.

May be confused with: transparent minerals with low reflectivity and very dark colors.

General Characteristics: 

Polishing scratches may be present.

Holes and small vesicles may occur.

Magnetite in very small crystals is likely to be common.

 

Volcanic Glass 01: Black obsidian, commercially available, in PPL. The dark left is the edge of the polished section. There are some small holes (black) and, in the center of the image, a white dot (magnetite). Some scratches are present.
Volcanic Glass 01: The same section as of the previous image, now in XPL. Black internal reflections do not allow any other observations.
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