Topaz
- Chemistry: Al 2 SiO 4 (F,
OH) 3, Aluminum silicate fluoride hydroxide.
- Class: Silicates
- Subclass: Nesosilicates
- Uses: Gemstones and optical properties are useful in industry.
Topaz is a common gemstone that has been used for centuries
in jewelry. Its golden brown to yellow color is classic but is confused with the
less valuable citrine, which is sold under the name topaz. The blue topaz that
is often confused with aquamarine is rarely natural and is produced by
irradiating and then heating clear crystals. Topaz is the November Birthstone.
The structure of Topaz is controlled by a chain like
structure of connected irregular octahedrons. These octahedrons have an aluminum
in the middle surrounded by four oxygens. Above and below the aluminum are the
hydroxide or fluoride ions. The chains of octahedrons are held together by
individual silicate tetrahedrons but it is the octahedron chains that give topaz
its crystalline shape. Topaz is the hardest silicate mineral and one of the
hardest minerals in nature. However it has a perfect cleavage which is
perpendicular to the chains and is caused by planes that break the weaker Al-O,
Al-OH and Al-F bonds. None of the stronger Si-O bonds cross these planes. Topaz
crystals can reach incredible size of several hundred pounds. Topaz can make
very attractive mineral specimens due to their high luster, nice colors and well
formed and multifaceted crystals.
PHYSICAL CHARACTERISTICS:
- Color is clear, yellow, orange, red, blue and green.
- Luster is adamantine to vitreous.
- Transparency crystals are transparent to translucent.
- Crystal System is orthorhombic; 2/m 2/m 2/m
- Crystal Habits include a prismatic crystal with usually two
different prisms that produce a rounded or sharp diamond-shaped
cross-section. The termination is typically capped by a dome forming a roof
like top. Another dome can modify the termination producing a point at the
juncture of the two domes. A basal pinacoid can flatten the prisms
termination or truncate the top of the domes. The pinacoid, multiple domes
and occasionally orthorhombic pyramid faces can produce a complex,
multifaceted and well formed termination. Topaz can be granular and massive.
- Cleavage is perfect in one direction, basal.
- Fracture is conchoidal.
- Hardness is 8.
- Specific Gravity is approximately 3.4 - 3.5+ (above average)
- Streak is white.
- Associated Minerals include quartz, tourmalines, micas, brookite,
cassiterite, and fluorite.
- Other Characteristics: index of refraction is 1.61 - 1.64. Prism
faces maybe striated lengthwise.
- Notable Occurrences include Minas Gerias, Brazil; Pakistan; San
Diego Co, California; Ural Mountains, Russia; Mexico and the Thomas Range,
Utah.
- Best Field Indicators are crystal habit, color, density and
hardness.
Serendipity
Ranch Gem Mine