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TOPAZ - Al2 SiO4(F,OH)2
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Examples of typical habits of Topaz from the Thomas Range.
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Crystal Description:
Orthorhombic
Physical Properties:
Color- Varies from water clear to a rich sherry amber to light
pink; some crystals (Maynard mine) are dichroic showing a dark
amber color when vied perpendicular to the C axis and a deep
cinnamon red color when viewed directly down the C axis.
Luster- Glassy
Hardness- 8
Specific Gravity- 3.5
Cleavage- Perfect basal
Crystals are elongated prisms, most of which are terminated; may form clusters or even occasionally doubly terminated crystals. Three types occur within the Thomas Range: Transparent, rough opaque and smooth opaque. Most crystals are 0.5 inches or less in length; larger crystals are uncommon. Often they occur in association or combination with other minerals.
In recent years, the Thomas Range has become one of the most famous topaz mineral locations in the world. Topaz crystals from the Thomas Range are considered among the finest found anywhere on earth. For crystal form, perfection, and luster, they are unrivaled. Crystals of topaz sheltered from direct sunlight are always colored. When viewed directly through the crystal the color varies from a golden-yellow to deep sherry color. However, when viewed straight down the C-axis, they are a deep cinnamon-red color, somewhat comparable to the highly prized topaz of Brazil.
Unfortunately, the color of the Thomas Range topaz is not stable when the crystals are left exposed to direct sunlight for extended periods of time.
When exposed to direct sunlight, the crystals change from the rich sherry color to a light pink and eventually to a clear "silver" white color; it takes about a week to ten days for the change to take place. This is why all of the countless millions of crystals so plentiful on the hillsides and in the washes are of a clear, silver-white color.
The crystals can, however, regain their original color upon exposure to strong radiation for a short period of time. The color of the unexposed crystals is a direct result of exposure of the crystals to natural ground radiation for millions of years, probably from trace amounts of uranium in the rhyolite. The radiation causes electrons to be displaced to a higher energy state giving the crystal a temporary "color center". Exposure to sunlight causes the electrons to return to their normal sites, thereby eliminating the color center, resulting in a change from sherry to colorless.
For some reason, people are under the misconception that once the crystals are exposed to daylight the color is automatically doomed. It has been my experience, over the past 25 years, that if proper care is taken, and long periods of direct sunlight is avoided, the color will remain almost indefinitely. Artificial lighting, including halogen lighting, does not appear to have much if any negative effect on the color stability of the topaz.
There are a few places within the Thomas Range where topaz is found that changes permanently to a rich rose-pink to red color instead of the usual silver-white. At each of these localities, pseudobrookite is associated directly with the topaz.
Topaz crystals from the Thomas Range can be divided into three different groups: transparent, rough-opaque and smooth-opaque.
Large crystals of transparent topaz are rare, most being under 1/2 inch in length. There are certain localized areas, however, that produce topaz crystals up to 4 inches (10cm) in size. Most are single crystals, but from time to time good clusters and even occasional doubly terminated crystals can be found. Over the years, collectors have gathered up any obvious surface material and it is now necessary to resort to hard labor in order to break up the rock to free these beautiful sherry-colored treasures.
Both the transparent and opaque topaz are found in very localized areas where voids or structures occur i.e. cavities, seams and fractures. They generally occur as singles or in groups of two or more crystals attached to the cavity or seam walls. Occasionally, cavities are found filled with a fine clay-like sand formed when the pockets were subjected to tectonic forces during or shortly after consolidation of the rhyolite. These forces caused the surrounding rock to fracture, thus allowing meteoric water access to these openings and causing the original pocket material to be altered and reduced to clay. In these cavities the topaz crystals are found lying loose in the sandy, clay- filled openings. As many as 2000 crystals have come from a single cavity system. The "transparent" topaz have the most complex forms, while the opaque topaz tend toward more simple forms. More than thirteen different forms have been described to date.
Most topaz found in the Thomas Range is of the rough opaque variety. The opaque topaz are usually much larger than the clear type (crystals up to three to four inches are not uncommon). They usually form rough clusters with little or no terminal faces. They are normally found in solid to near solid rhyolite, having formed within the solid rhyolite from the reaction of fluorine rich vapors with feldspar in the host rock. At a few isolated locations within the Thomas Range there are some very localized areas where opaque topaz clusters occur that are very unlike the rough opaque clusters. They form perfectly smooth, well-terminated crystals, that in all aspects resemble the transparent type, except that they are full of small microscopic quartz crystals, giving them a sandy appearance. They are sometimes referred to by collectors as "sand topaz." They form in a very fine, clay-like ashy rock, forming complete "floater" (no points of attachment) like clusters. Sometimes crystals of this type are found where parts of the crystal have grown into an open void forming a combination of transparent and opaque. Some people consider smooth-opaque topaz as pseudomorphs (one mineral replaces another, molecule by molecule, but retains the crystal form of the original mineral), but microscopic examination has shown that they are not pseudomorphs, but rather topaz crystals with quartz inclusions.
There are a few localized areas within the Thomas Range where topaz can be found that is suitable for cutting into gemstones. The best source is the Maynard Topaz Mine which has produced crystals capable of cut gems up to 15 carats. Note that my criteria for being suitable for gemstones means that the crystals are totally flawless, i.e. with no feathers, vails, or visible inclusions. I realize that this might sound like I am too picky, but gem topaz deserve to be perfect. At a few localized areas, topaz crystals occur in direct association with other minerals. Combinations of topaz-bixbyite, topaz-garnet, topaz-beryl, topaz-pseudobrookite, topaz-hematite and all variations thereof have been found. These will be described in greater detail later.
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