A habit displaying a flattened form.

 

Nearly 75% of the Earth's crust is made up of minerals of the tectosilicate class. The silicate structures of this class are characterized by a very strong and stable three-dimensional framework.

 

Glass objects which are formed by the impact of large meteorites on Earth's surface. The impact melts material from Earth's surface and catapults it up to several hundred kilometers away from the impact site. The molten material cools and solidifies to glass.

 

The resistance of a mineral to breakage.

 

Crystal faces at the end of a crystal axis.

 

Lateral or panel openings.

 

Crystal system having two equal horizontal axes at right angles to each other, and a vertical axis of a different length at right angles to the other two.

 

Specimens that fit inside a 1¼ x 1¼ x 1¼ inch plastic container known as "perky boxes". Specimen has features that can be seen under mild or no magnification. However, a micrmount specimen may be indicated to have "thumbnail sized matrix", but it would still have tiny crystals that needed 10x or higher magnification.

 

A collective term for underground wooden supports.

 

The setting of timber supports in mine workings or shafts for protection against falls from roof, face, or rib.

 

A timber frame to support the roof, sides, and sometimes the floor of mine roadways or shafts.

 

A brilliant white metal found in most igneous or sedimentary rocks.

 

A short or net ton is equal to 2,000 pounds; a long or British ton is 2,240 pounds; a metric ton is approximately 2,205 pounds.

 

A mine roof; same as "back."

 

Difficult to break or fracture.

 

A vertical or inclined connection between two or more levels and used as an ore pass.

 

Occurs when a preexisting crystal undergoes a transformation due to a change in pressure or temperature.

 

Capable of transmitting some light, but cannot be seen through.

 

Capable of being seen through.

 

The trigonal crystal system is really part of the hexagonal crystal system and only has one minute difference (in that true hexagonal minerals have sixfold symmetry, whereas trigonal minerals have threefold symmetry). The trigonal crystal system, like the hexagonal crystal system, has: four axes, three are equal in length and lie at an angle of 120° from each other. The fourth is either longer or shorter but must be at a right angle toward the other corners.

 

Crystal system having three unequal axes at oblique angles to each other.

 

A cyclical twin made up of three crystals, generally forming a hexagonal outline.

 

Trilobites are hard-shelled, segmented creatures that existed over 300 million years ago in the Earth's ancient seas. They went extinct before dinosaurs even came into existence, and are one of the key signature creatures of the Paleozoic Era, the first era to exhibit a proliferation of the complex life-forms that established the foundation of life as it is today.

 

A train of mine cars.

 

Minerals containing the tungstate (WO₄) radical as a major component.

 

• Contact Twins - have a planar composition surface separating two individual crystals. These are usually defined by a twin law that expresses a twin plane (an added mirror plane).
• Penetration Twins - have an irregular composition surface separating two individual crystals. These are defined by a twin center or twin axis.
• Polysynthetic twins - the compositions surfaces are parallel to one another. Plagioclase commonly shows this type of twinning, called the Albite Twin Law. Such twinning is one of the most diagnostic features of plagioclase.
• Cyclical twins - the composition surfaces are not parallel to one another.

Twin Laws determine the shape of the more common types of twinned crystals. The atomic structures of twins are arranged in specific geometric patterns, which may be as simple as mirroring of structures, or as complex as rotations of atomic structure.

 

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