Talc is known for being the softest mineral on earth. It is number 1 on the Mohs hard... More
Talc is known for being the softest mineral on earth. It is number 1 on the Mohs hardness scale, and can be easily scratched by a fingernail. Talc is not commonly seen in collections, as it is usually uninteresting and fairly common, although a few deeply colored and crystallized examples are known and well sought after. Also very popular are the Talc pseudomorphs. Talc forms some very interesting pseudomorphs after many different minerals, and certain localities are known for the specific minerals replaced by Talc. Health concerns including a rare respiratory disease are known to be associated with Talc in its powder form. Although solid forms of Talc are assumed to be safe, and most concerns are only after prolonged exposure, it is advisable to wash hands after handling Talc. Hide
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div Tantalite forms a solid solution series with Columbite known as the Columbite Tan... More
Tantalite forms a solid solution series with Columbite known as the Columbite-Tantalite series. Tantalite is the tantalum-rich member, and Columbite is the niobium-rich member. Tantalite is named after its tantalum content, and, together with Columbite, are the most important minerals containing the rare element tantalum.
The name Tantalite has been discredited by the IMA, with the more specific designations of Tantalite-(Fe) (also known as Ferrotantalite), and Tantalite-(Mn) (also known as Manganotantalite) being used for this mineral's IMA-accepted nomenclature. Ferrotantalite is always dark and opaque; Manganotantalite is usually reddish-brown and may exhibit transparency. An even rarer member of this series is Tantalite-(Mg) (Magnesiotantalite), a magnesium-rich member. In general, despite being discredited, the name Tantalite is still used without further designation, and often it is simply labelled as Columbite-Tantalite, since the exact species can be difficult to determine.
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Native Tellurium is very rare. Only small quantities have been found in a natural sta... More
Native Tellurium is very rare. Only small quantities have been found in a natural state, and it is only of interest to the collector. Due to its rarity, Tellurium specimens are highly sought after by collectors. Hide
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div Tennantite is the nbsp arsenic rich end member nbsp of the Tetrahedrite series . ... More
Tennantite is the arsenic-rich end member of the Tetrahedrite series. Tennantite is the less common member of this series, and is sometimes indistinguishable from Tetrahedrite by common methods. Tennantite was named in 1819 in honor of English chemist Smithson Tennant (1761-1815), who was best known for his discovery of the rare elements iridium and osmium, as well as proving the identity of Diamond and Graphite from the element carbon.
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Tephroite is uncommon member of the Olivine group. It is the manganese counterpart of... More
Tephroite is uncommon member of the Olivine group. It is the manganese counterpart of Forsterite, and forms a series with Forsterite. Tephroite gets it name from the Greek word "tephros", meaning ash-colored, referring to its ash-gray color habit. Hide
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div The Tetrahedrite series is composed of two member minerals Tetrahedrite, the anti... More
The Tetrahedrite series is composed of two member minerals: Tetrahedrite, the antimony-rich end member, and Tennantite, the arsenic-rich end member. Tetrahedrite is the more common member of this series, and is also the most common sulfosalt mineral. This page discusses Tetrahedrite; Tennantite has its own dedicated page. The most common crystal habit for Tetrahedrite is tetrahedral, which describes an unusual triangular crystal form. Its name is derived from this interesting crystal shape.
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Thenardite is a delicate sulfate mineral that comes from arid dry or saline lake depo... More
Thenardite is a delicate sulfate mineral that comes from arid dry or saline lake deposits. It is similar to Mirabilite, a chemically similar mineral to Thenardite that, unlike Thenardite, contains water in its structure. Mirabilite is an unstable mineral, and will dehydrate and lose its water when exposed to air, thereby changing to Thenardite. These altered specimens are in fact Thenardite pseudomorphs after Mirabilite. Some of these crystals, especially those of outstanding crystal form, are artificially grown as by-products of borax mining operations. Thenardite is named after the French chemist Louis Jacques Thenard (1777-1826), a professor at the University of Paris. Hide
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Thomsonite is an uncommon and desirable member of the zeolite group , forming in uniq... More
Thomsonite is an uncommon and desirable member of the zeolite group, forming in unique and interesting crystal aggregates. In 1997,
the Zeolite Subcommittee of the IMA divided this mineral into
two individual sub-species, thereby regarding Thomsonite as a series of two members. The series is defined by calcium and strontium end members. Common Thomsonite, which is calcium-dominating, is called Thomsonite-Ca, and is usually pure without any strontium. Strontium-dominating Thomsonite is known as Thomsonite-Sr, and is extremely rare. Almost all Thomsonite specimens in
collections are Thomsonite-Ca. A distinction among the two Thomsonite types is rarely made, and the term Thomsonite is generally used without any further breakdown. An interesting habit of Thomsonite is as rounded, concentric bands, which are polished into aesthetic rounded stones. This form is most well-known at Thomsonite Beach at Grand Marais, Minnesota, on the shoreline of Lake Superior. It usually forms in association with other zeolites and green Chlorite. Thomsonite is named in honor of Scottish mineralogist and chemist Thomas Thomson. Hide
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Thorite is named for its content of the rare element thorium. It is isomorphous with ... More
Thorite is named for its content of the rare element thorium. It is isomorphous with Zircon and occurs in the same crystal habits. Thorite is the most prevalent mineral containing the radioactive element thorium. However, it is not as important as the primary thorium ore, Monazite, since the extraction process of the ore from Thorite is more difficult then that of Monazite. Thorite, being radioactive, must be stored with all the precautions exercised with radioactive minerals. The radioactivity of Thorite often brings it to a state of metamiction, in which its crystal structure becomes unstable thereby causing the crystal edges to become smooth and rounded. When this happens, the Thorite may gain hydroxyl (OH) molecules in place of the silica (SiO 4) molecules and transform into a different mineral, Thorogummite. Hide
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Thorogummite is always a replacement pseudomorph after Thorite , and is sometimes tho... More
Thorogummite is always a replacement pseudomorph after Thorite, and is sometimes thought as being a variety of that mineral. However, Thorogummite is regarded by the IMA as a distinct mineral species with its own chemical formula. It forms when Thorite starts decaying from the radioactive metamiction process. When this happens, its crystal edges become rounded and smooth, and it eventually loses some silica (SiO 4) molecules in its structure. The areas in the structure where the silica molecules were are replaced by hydroxyl (OH) molecules, and thus begins the transformation into Thorogummite.
Thorogummite is a highly radioactive mineral, and should be stored with all the precautions exercised with radioactive minerals. Hide
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Most Borax specimens lose water in their structure if stored in dry areas and alter t... More
Most Borax specimens lose water in their structure if stored in dry areas and alter to Tincalconite. Although Tincalconite is found in a natural state, almost all specimens were transformed from Borax after being taken from the mine. The name Tincal in ancient times applied to the mineral Borax, but now it is synonymous with Tincalconite. Hide
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div Titanite is frequently called by the name Sphene, which was the more popular term... More
Titanite is frequently called by the name Sphene, which was the more popular term for this mineral prior to 1982. In 1982, the IMA adopted the official name as Titanite and discredited Sphene. The name Sphene is still frequently used, especially in Europe. Sphene also persists as the more prevalent name for faceted Titanite gemstones or transparent crystal forms.
Titanite is known for its high luster and dispersion rate. This results in transparent Titanite crystals being brilliant, and opaque Titanite being highly reflective. It is also pleochroic, with different transparent crystals exhibiting different hues when viewed at different angles.
Titanite from a few localities contains thorium within its chemical structure. Such Titanite is slightly radioactive and will be metamict, resulting in those crystals having slightly rounded crystal edges and interior structural deficiencies.
Titanite is named for its titanium content. Its alternate name Sphene is named from the Greek term "sphenos", which means wedge, in allusion to the typical wedge-shaped crystals exhibited by this mineral.
For additional information, see the gemstone section on Sphene. Hide
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Topaz is a well known mineral, occurring in large and beautifully shaped and colored ... More
Topaz is a well-known mineral, occurring in large and beautifully shaped and colored crystals. It naturally occurs in many colors, but rarely in pink, purple, and deep blue. Those colors are formed from lighter-colored stones that undergo irradiation and heat treatment. Topaz from certain localities may also turn pink or colorless upon heating. The color of some Topaz specimens, namely those from Siberia, fade upon prolonged exposure to light. Topaz crystals in a matrix are rare and very much desired, since the perfect basal cleavage of Topaz causes it to separate from its base and form loose crystals. Precautions must be taken not to damage specimens, for pressure or improper faceting can cause a crystal to cleave and become ruined. For additional information, see the gemstone section on Topaz. Hide
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The pictures below show just one example of each individually named type of Tourmaline. For more images, see the specific pages on Elbaite, Schorl, Dravite, Uvite, Liddicoatite, and Buergerite.
Tourmaline is not a single mineral, but a group of several closely related minerals. ... More
Tourmaline is not a single mineral, but a group of several closely related minerals. The three most well-known members are Elbaite, Schorl, and Dravite. Other lesser known members include Uvite, Liddicoatite and Buergerite. There are yet more members of the Tourmaline group, but these are extremely rare and not discussed in this guide.
Tourmaline is extremely popular among collectors and is a well-known gemstone. It is the most multicolored mineral type known, occurring in virtually every color of the spectrum. Individual stones are often multicolored and are unsurpassed in their beauty.
The color of some Tourmaline can be enhanced through heat treatment. Some greenish stones can be made deep green, some brownish-red stones can be made red, and some light pink stones can be made colorless through heating.
Tourmaline has many interesting optical properties. Many green and blue specimens are strongly pleochroic. When viewed through their vertical axis, such specimens appear darker in color than when seen through their horizontal axis. In other Tourmalines, the color may actually be different when viewed at different angles because of the pleochroism. Certain Tourmalines exhibit a cat's eye effect when polished into cabochons.
Tourmaline is both pyroelectric and piezoelectric. If a specimen
is put under a pressure or temperature change, it will generate an
electrical charge. When this happens, dust particles become attached
to the crystal ends. For additional information, see the gemstone section on Tourmaline. Hide
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Tremolite and Actinolite are two very similar minerals that form a series with each o... More
Tremolite and Actinolite are two very similar minerals that form a series with each other and
essentially share the same chemical formula. Tremolite has a greater
presence of magnesium over iron, whereas Actinolite has a
greater presence of iron over magnesium. Tremolite and Actinolite share several recognized varieties.
Mountain Leather, a thickly fibrous and leathery variety, has a
silky luster, a soft felt-like feel, and elastic fibers.
Nephrite, another fibrous variety, is made up of tough, interlocking
fibers, so dense that the fibers are not discernible. Actinolite
and Tremolite both contain a form of asbestos which is made of
movable and elastic fibers. Actinolite asbestos is less common;
most forms are in fact Tremolite. This form of the mineral contains
significant health hazards and should never be brought near the mouth. If its fibers or particles enter the lungs, they can cause asbestosis. Asbestosis is a lung disease caused by inhalation of asbestos particles, which causes several cancers, particularly lung cancer and mesothelioma. Symptoms of asbestosis do not arise until about 20 years after the inhalation. Due to the hazards, washing hands after handling specimens is highly recommended. Many mineral collectors avoid collecting asbestos minerals out of safety concerns. Hide
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Tridymite is a rare polymorph of the mineral Quartz . However, its crystals are very ... More
Tridymite is a rare polymorph of the mineral Quartz. However, its crystals are very distinct and form very different habits from Quartz. Many Tridymite specimens are in fact pseudomorphs of Quartz after Tridymite, as the Tridymite often alters to the more common Quartz in many environments. After Quartz replaces the Tridymite, it still retains the original, distinct crystal form of Tridymite. True Tridymite and Quartz pseuomorphs are virtually indistinguishable without locality knowledge or x-ray analysis. The name "Tridymite" was derived from its most common crystal habit as pseudohexagonal trillings. Hide
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div Triphylite describes both a mineral series , as well as an individual mineral wit... More
Triphylite describes both a mineral series, as well as an individual mineral within the series. Triphylite is the iron-rich end member of the Triphylite series, and Lithiophilite is the manganese-rich end member. Minerals of the Triphylite series commonly alter to secondary phosphates such as Vivianite, as well as to manganese and iron oxides.
Triphylite is named from the Greek words "tria", for threefold, and "phylon", family, alluding to it as being a mineral group with three cations (Fe, Li, and Mn).
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Turquoise is an antique ornamental stone, highly regarded for its unique and striking... More
Turquoise is an antique ornamental stone, highly regarded for its unique and striking namesake color. Valued in both the ancient Persian and Native American civilizations, it still retains its popularity today. Turquoise specimens are often polished or sliced for collectors, and may even be dyed. Turquoise is porous and has a naturally waxy luster; it is sometimes impregnated with a plastic lubricant for to enhance its luster and increase its stability. Many Turquoise fakes exist. The most prevalent is white Howlite dyed blue to resemble Turquoise. Turquoise often contains embedded shiny Pyrite flakes, or may contain black oxide veins running through it. It is sometimes intergrown together with other secondary copper materials, especially Chrysocolla. Turquoise may also form as a pseudomorph of other minerals such as Apatite, Beryl, and feldspars. For additional information, see the gemstone section on Turquoise. Hide
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