What kind of texture does obsidian have

Occasionally two colors of obsidian will be swirled together in a single specimen. The most common color combination is black and brown obsidian swirled together - that's called "mahogany obsidian" see photo. As a "glass," obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations, the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian.

When cut and polished, these specimens are referred to as "snowflake obsidian" see photos. Rarely, obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas. These colored specimens are known as "rainbow obsidian," "golden obsidian," or "silver obsidian," depending upon the color of the sheen or iridescence.

These specimens are very desirable for the manufacture of jewelry. Snowflake obsidian: A tumble-polished specimen of "snowflake obsidian. Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian.

Rarely, volcanic glasses are found with a composition similar to basalt and gabbro. These glassy rocks are named "tachylyte. Pumice , scoria , and tachylyte are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt.

Tachylyte differs in composition - it has a composition similar to basalt and gabbro. Obsidian outcrop: Obsidian along the edge of a lava flow in central Oregon. Obsidian knife blade: A knife blade manufactured from mahogany obsidian. The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity.

Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes. In the United States it is not found east of the Mississippi River, as there is no geologically recent volcanic activity there. Most obsidian used in the jewelry trade is produced in the United States. Although the newly formed obsidian has a low water content, usually less than 1 percent by weight, it is slowly hydrated to form perlite when exposed to groundwater.

How can you identify obsidian? The lack of a crystalline structure indicates that obsidian is not a true mineral and causes extreme sharpness of the fracture surfaces. Since prehistoric times, obsidian has been used in cutting tools and is still used today in surgical scalpels.

Explore obsidian where cooling is rapid in the margins of lava flows. Glass Buttes in central Oregon is one of the best places to find obsidian in the U. Pieces of fist size can be found on the surface in abundance here. It has a distinctive appearance of smooth glass. Obsidian is a frozen liquid that contains small amounts of mineral impurities. See the color Because pure obsidian is usually dark, on rare occasions it may also be almost white. Consider the effect of impurities on the obsidian color For examples, iron and magnesium may make obsidian dark green.

Hematite or limonite add a red or brown color to the obsidian. A lot of microscopic rock and mineral particles usually cause the jet black color most closely associated with obsidian. It can cause the obsidian to have a gold or silver shine if the bubbles were spread almost flat. The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite.

The most common igneous compositions can be summarized in three words: mafic basaltic , intermediate andesitic , and felsic granitic. Felsic composition is higher in silica SiO 2 and low in iron Fe and magnesium Mg. Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions.

Composition influences the color of igneous rocks. Felsic rocks tend to be light in color white, pink, tan, light brown, light gray. Mafic rocks tend to be dark in color black, very dark brown, very dark gray, dark green mixed with black.

The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors green, gray, brown. The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition.

If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic.

This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly. The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color.

If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. Igneous rocks with quartz in them are usually felsic. Igneous rocks with olivine in them are usually mafic. Igneous rocks with neither quartz nor olivine in them are most commonly intermediate. Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed.

The intrusion of large bodies of granite—batholiths—is usually part of the origin of a mountain range. Improve this page Learn More.

Skip to main content. Module 3: Rocks and the Rock Cycle. Obsidian starts with lava very high in silica more than about 70 percent , such as rhyolite. The many strong chemical bonds between silicon and oxygen make such lava very viscous, but equally important is that the temperature range between fully liquid and fully solid is very small.

Thus, obsidian does not need to cool especially fast because it solidifies especially quickly. Another factor is that low water content may inhibit crystallization. View pictures of obsidian in this gallery. Big obsidian flows display the rugged surface of the highly viscous lava that forms obsidian. Obsidian flows develop a blocky surface as their outer shell quickly solidifies. Obsidian may display complex folding and segregation of minerals in bands and round masses consisting of feldspar or cristobalite high-temperature quartz.

Obsidian flows may contain droplets of fine-grained feldspar or quartz. These are not amygdules , as they were never empty.

Instead, they are called spherulites. Usually black, obsidian can also be red or gray, streaked and mottled, and even clear. The shell-shaped conchoidal fracture on this obsidian cobble is typical of glassy rocks, like obsidian, or microcrystalline rocks, like chert. Obsidian combines with water and begins to break down into a frosty coating. Internal water can convert the whole rock into perlite.