Glassy or vitreous rock texture describes the texture of amorphous solid volcanic glasses. Rocks with this texture form when magma or lava cools rapidly or is quenched, preventing the formation of minerals.
Most of the glassy rocks form from highly viscous silicic or acidic lava. Some may have acidic magmas, but they are uncommon.
Rocks with vitreous or glassy textures are said to be vitric and comprise all the natural volcanic glasses. These vitreous rocks include obsidian, pumice, tachylite, perlite, sideromelane, and hyaloclastite. Others are apache tears, palagonite, pitchstone, Limu o Pele, Pele’s hair, and Pele’s tears.
Lastly, paints, ceramics, tiles, or enamel may have a vitreous texture besides rocks. But don’t confuse this texture with the vitreous or glassy luster.
What is a vitreous or glassy texture?
A glassy or vitreous texture is an extrusive or volcanic igneous rock texture characterized by an amorphous solid structure. This texture lacks internal structure and forms when lava is rapidly quenched or cooled so that crystallization doesn’t occur.
Rocks with a glassy or vitreous texture have the appearance of natural or artificial glass, hence the name glassy. Breaking them doesn’t form any planes and crystal symmetry. Also, some of these rocks may have a vitreous or glassy luster, such as obsidian.
You may meet terms like hypocrystalline. It describes rocks consisting of a mixture of glass and crystals with a crystal-to-glass ratio of more than 3:5. If the glass ratio is higher, the stone is said to be holohyaline. However, holohyaline or hyaline usually describes an entirely glassy rock texture.
Lastly, vitreous or glassy rocks don’t imply a specific composition. Instead, it is a purely textural term.
Appearance and characteristics of the glassy texture
Rocks with a vitreous texture come in various colors, depending on the inclusions. Also, they may have the following features.
- Homogenous – They have no microlites, crystallites, or any inclusion. Such rocks are very rare.
- Banded – Some glassy rocks may have bands or lamination separated by different color tints, layers, or strings of granular matter that may occur as lenticular streaks.
- Damascened – It refers to the confused, twisted, contorted threads or streaks created by a slight agitation of magma flows of two different densities. It resembles a damascening gun barrel or Damascus sword blade.
- Spherulitic – Spherulites are small spherical or rounded bodies common in glassy texture. They often have tiny fiber structures radiating outwards and are common in silica-rich rhyolitic glasses.
- Axiolitic – Zirkel proposed the term axioliate. According to (Definition of axiolite, n.d.), axiolite describes a “variety of elongated spherulite in which there is an aggregation of minute acicular crystals arranged at right angles to a central axis rather than from a point.”
- Vesicular – Some of the rocks may appear froth-like or have a cavity.
Glassy or vitreous texture vs. luster
Vitreous or glass texture should not be confused with vitreous or glassy luster. Vitreous texture describes the amorphous solid structure of rocks and other materials that make them appear glass-like.
In contrast, vitreous or glassy luster describes mineral surfaces that shine or are reflective, like broken glass. Examples of vitreous or glassy minerals are apatite, fluorite, topaz, calcite, and quartz. Others are beryl, tourmaline, augite, and amphibole.
Remember, luster is one of the physical properties used to identify minerals. It describes the light-reflective qualities or characteristics of mineral surfaces, i.e., it shows how well or how much a mineral reflects light and depends on the diaphaneity (level of transparency), structure, and refractive power of a given mineral.
Also note that minerals’ luster can be metallic, submetallic, or non-metallic. Vitreous is just an example of non-metallic luster. Other non-metallic luster types are greasy, waxy, silky, resinous, adamantine, pearly, or dull.
Lastly, besides luster, the other physical properties we use to identify minerals are hardiness, color, streak, melting point, cleavage, crystal habit, and tenacity.
How are glassy textured rocks formed?
Rapid cooling and quenching (with water or ice) of magmas, especially highly viscous ones, forms glassy rocks. This rapid cooling and high viscosity prevent atoms, ions, or molecules from moving to nucleation and crystal growth sites.
Remember, the ions, atoms, and molecules must move and cryptographically arrange themselves to form a pattern characteristic of certain minerals, as seen in crystalline rocks. Since this doesn’t happen, you will end up with an uncrystallized or amorphous solid structure that resembles glass.
It happens because the rapid cooling will result in supercooled magma, i.e., below crystallization temperature. Any further cooling will transition the molten magma to glass, an amorphous solid with unorganized atoms, ions, or molecules.
Both fine crystalline (aphanitic) and glassy igneous rocks indicate rapid cooling on or near the Earth’s surface. However, finely crystalline has mineral crystals, while glass is uncrystallized except for a few crystallites and inclusions.
Also, the cooling rates are slower in aphanitic textured rocks, allowing for crystallization. However, crystals don’t grow large.
Lastly, a much slower cooling rate will result in coarse texture or what we refer to as phaneritic texture. It usually occurs deep inside the Earth’s crust.
Mineral composition
The rapid cooling of the lava and, at times, high viscosity don’t allow the orderly arrangement of atoms to form minerals. Therefore, in a strict sense, these rocks do not have minerals, i.e., even a microscope cannot identify minerals.
But if you consider pristine magma, most rocks with a glassy texture form from rhyolitic or granitic magma. These are high-silica and viscosity magmas.
Besides granitic and rhyolitic lavas, you may also have glassy textured rocks from basaltic magmas, but they are rare.
Please note that the chemical composition of glassy rocks may differ from the equivalent finely crystalline rocks. For instance, in peralkaline lavas, chloride concentration in glassy and finely crystalline rocks shows a variation in trachytes and pantellerites.
Besides the rhyolitic and basaltic magma, glassy rocks may have dispersed minute inclusions. These inclusions include crystallites, microlites (minute crystals), or incipient crystals. Also, these rocks may have iron-titanium oxide and empty or fluid-filled vesicles.
That is not all. Some may have larger grains, including what we see in vitrophyre, i.e., a porphyritic textured rock with glass groundmass.
Lastly, since they can absorb oxygen, the composition of vitreous or glassy rocks may not be the same as the original or pristine lava. For instance, according to (Carmichael, 2015), “pitchstone has 2-7% water while erupted with 0.09-0.29% water. Therefore, in pitchstone, water may have been incorporated or percolated later”.
Examples of vitreous or glassy volcanic rocks
1. Obsidian – smooth dark or black glassy rock or stone
Obsidian is a shiny, smooth black to dark-brown volcanic glassy rock and by far the most common volcanic glass. However, depending on the inclusion or impurities, such as iron oxide, chromium, etc., you may have red, yellow, brown, gray, green, or even dark greenish obsidian rocks. Also, trapped glass bubbles may contribute to the various colors.
Like other vitreous rocks, obsidian’s glassy texture indicates that it formed from rapid cooling or quenching of viscous rhyolitic (silica-rich or felsic) magma.
Obsidian has a mineral-like appearance, is shiny, vitreous, and has a conchoidal fracture like quartz. So, some people confuse it with a mineral. However, it is not a mineral but a mineraloid, i.e., an amorphous substance that resembles minerals. Other common mineraloids are pearl, opal, and calcite.
2. Pumice – a froth-like, bubbly, or porous glassy rock
Pumice is another common glassy volcanic rock with a froth-like, sponge-like, or vesicular appearance. It is usually light-colored, i.e., whitish, gray, or cream. But it may be bluish, dark green, or black, depending on the inclusions.
How does it form? Pumice forms from the explosive eruption of mainly rhyolitic or andesitic magma, and the vesicles form as steam escapes from the rapidly cooling magma. However, basaltic pumice glasses do exist in form but are rare.
3. Tachylite or tachylyte – glassy basalt
Tachylite is mafic brown to black glassy rock formed from the rapid cooling of basaltic magma, especially on the edges of chilled or quenched veins, sills, dikes, etc. This rock has a greasy, resinous, or pitchlike luster and is brittle and fusible. Also, it reacts with acids, i.e., acids decompose it.
4. Pitchstone
Pitchstone is dull black and sometimes red, green, gray, or brown glassy rock formed from the rapid cooling of silica-rich magma. It may be uniform, streaked, and mottled and has a resinous texture.
5. Palagonite
Palagonite is a water-altered basaltic glassy rock or rocks formed as resultant steam reacts with basaltic magma if water quenched. This glassy rock may be brown, orange-gray, or yellowish, depending on the minerals present.
6. Sideromelane
Sideromelane is a yellow-brown type of tachylite. It forms from hotter magma and under faster cooling than tachylite in a wet environment. This transparent and pure vitreous rock may occur alongside tachylites and palagonite tuff.
7. Perlite
Perlite is a transparent to grayish (sometimes glossy black, greenish, brown, red, or blue) natural volcanic glassy high in water quantity (2-5 wt.%). However, when expanded by heat treatment, it is grayish white to snowy white.
It forms when silica-rich, high-viscosity rhyolitic magma cools rapidly and then hydrated. Its luster is waxy to pearly and shows concentric fractures forming small spherical rocks.
8. Other glassy rocks
Besides the above, the other examples of glassy or vitreous rocks include Pele’s tears (jet black), Pele’s hair (golden brown), and Limu o Pele (brownish-green). Apache tears (black or dark-colored) and hyaloclastite (mainly greyish green or brown-grey also have a glassy texture.
References
- Carmichael, I. S. E. (2015, December 31). Chapter 8. Glass and the glassy rocks. Evolution of the Igneous Rocks: Fiftieth Anniversary Perspectives, 233–305. https://doi.org/10.1515/9781400868506-009
- Igneous rocks. (n.d.). The University of Columbia. Retrieved September 28, 2022, from http://www.columbia.edu/%7Evjd1/igneous.htm
- Central Geological Survey, M. O. E. A. (24, February 25.). What are glassy, crystalline, and holocrystalline? https://www.moeacgs.gov.tw/eng/faqs/faqs_more?id=6b2fe7f0e2414535905f6893640275b1
- Definition of axiolite. (n.d.). Mindat.org. Retrieved October 16, 2022, from https://www.mindat.org/glossary/axiolite