Obsidian-

is the result of volcanic lava coming in contact with water. Often the lava pours into a lake or ocean and is cooled quickly. This process produces a glassy texture in the resulting rock. Iron and magnesium give the obsidian a dark green to black color.

Obsidian has been used by ancient people as a cutting tool, for weapons, and for ceremonial purposes and is sometimes found by archaeologists in excavations.

Obsidian has several varieties. Obsidian can contain small bubbles of air that are aligned along layers created as the molten rock was flowing just before being cooled. These bubbles can produce interesting effects such as a golden sheen, known as Sheen Obsidian or a rainbow sheen called Rainbow Obsidian. Inclusions of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern producing Snowflake Obsidian. Small nuggets of obsidian that have been naturally rounded and smoothed by wind and water are called Apache Tears.

Often confused with smoky quartz, obsidian has similar properties to quartz because of a similar chemistry. However, many properties dependant on a crystal structure are altered or absent in obsidian because it lacks any crystal structure of its own. The piezoelectric and optical properties in quartz are thus absent in obsidian. Smoky quartz usually has a splotchy or zoned distribution to its color while Obsidian’s color is more uniformly distributed.

PHYSICAL CHARACTERISTICS:

• Color is dark green to dark brown and black, also can show sheens of gold or green, yellow, blue and/or purple coloration. Sometimes with white inclusions (Snowflake Obsidian).
• Luster is vitreous.
• Transparency: Obsidian is translucent in any stone of appreciable size.
• Crystal System does not apply because obsidian is amorphous.
• Habits include compact nodules or as massive layers between other volcanic rocks
• Fracture is conchoidal.
• Hardness is 5 – 5.5 (much softer than quartz).
• Specific Gravity is approximately 2.6 (average)
• Streak is white.
• Other Characteristics: Generally lacks open voids or large bubbles like other volcanic rocks.
• Notable Occurrences include Italy; Mexico; Scotland; Arizona, Colorado, Texas, Utah and Idaho, USA.
• Best Field Indicators are color, fracture, flow bubbles, softness, association with other volcanic rocks and lack of crystal faces.

Cristobalite

The mineral cristobalite is a high-temperature polymorph of quartz and tridymite. It occurs as white octahedra in acidic volcanic rocks. Cristobalite is stable only above 1470 degrees Celsius, but can crystallize and persist metastably at lower temperatures.

The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is “reconstructive”, requiring the breaking up and reforming of the silica framework. These frameworks are composed of SiO₄ tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO₂. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy and may not happen on a human time frame. Framework silicates are also known as tectosilicates.

There is more than one form of the cristobalite framework. At high temperatures the structure is cubic. A tetragonal form of cristobalite occurs on cooling below ca. 250 degrees Celsius at ambient pressure, and is related to the cubic form by a static tilting of the silica tetrahedra in the framework. This transition is variously called the low-high or α − β transition. It may be termed “displacive”, i.e., it is not generally possible to prevent the cubic β-form from becoming tetragonal by rapid cooling. Under rare circumstances the cubic form may be preserved if the crystal grain is pinned in a matrix that does not allow for the considerable spontaneous strain that is involved in the transition, which causes a change in shape of the crystal. This transition is highly discontinuous. The exact transition temerature depends on the crystallinity of the cristobalite sample, which itself depends on factors such as how long it has been annealed at a particular temperature.

The cubic β-phase consists of dynamically disordered silica tetrahedra. The tetrahedra remain fairly regular and are displaced from their ideal static orientations due to the action of a class of low-frequency phonons called Rigid Unit Modes. It is the “freezing” of one of these Rigid Unit Modes that is the soft mode for the α − β transition.

In the α − β phase transition only one of the three degenerate cubic crystallographic axes retains a four-fold rotational axis in the tetragonal form. The choice of axis is arbitrary, so that various twins can form within the same grain. These different twin orientations coupled with the discontinuous nature of the transition can cause considerable mechanical damage to materials in which cristobalite is present and that pass repeatedly through the transition temperature, such as refractory bricks.

When devitrifying silica, cristobalite is usually the first phase to form, even when well outside of its thermodynamic stability range. The dynamically disordered nature of the β-phase is partly responsible for the low entropy of fusion of silica.

The micrometre-scale spheres that make up precious opal are made of cristobalite, crystallized metastably at low temperature.

CHEMISTRY SiO₂ Silicon oxide

CRYSTALLOGRAPHY Tetragonal (pseudo cubic)

CRYSTAL GROWTH AND HABITS Cristobalite is found as dendritic or spherulitic masses, it is also found as fibrous to massive.

COLOR AND OTHER OPTICAL PROPERTIES White to light gray; translucent to opaque

HARDNESS 6 -7

SPECIFIC GRAVITY 2.3 – 2.4

LUSTER Vitreous to dull

STREAK White

BREAKABILITY Uneven to splintery fracture; brittle

OCCURRENCE Cristobalite is found in siliceous volcanic rocks.

ASSOCIATED MINERALS Quartz, Tridymite, Sanidine, Opal