Scheelite

Key takeaways

Scheelite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Scheelite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Scheelite from memory before moving on to harder problems.

Reference excerpt

Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten (wolfram). Scheelite is originally named after Swedish chemist Carl Wilhelm Scheele (1742–1786). Well-formed crystals are sought by collectors and are occasionally fashioned into gemstones when suitably free of flaws. Scheelite has been synthesized using the Czochralski process; the material produced may be used to imitate diamond, as a scintillator, or as a solid-state lasing medium. It was also used in radium paint in the same fashion as was zinc sulphide, and Thomas Edison invented a fluoroscope with a calcium tungstate-coated screen, making the images six times brighter than those with barium platinocyanide; the latter chemical allowed Röntgen to discover X-rays in early November 1895. The semi-precious stone marketed as 'blue scheelite' is actually a rock type consisting mostly of calcite and dolomite, with occasional traces of yellow-orange scheelite.

Properties

Its crystals are in the tetragonal crystal system, appearing as dipyramidal pseudo-octahedra. Colors include golden yellow, brownish green to dark brown, pinkish to reddish gray, orange and colorless. Transparency ranges from translucent to transparent, and crystal faces are highly lustrous (vitreous to adamantine). Scheelite possesses distinct cleavage, and its fracture may be subconchoidal to uneven. Its specific gravity is high at 5.9–6.1 and its hardness is low at 4.5–5. Aside from pseudo-octahedra, scheelite may be columnar, granular, tabular or massive in habit. Druzes are pretty rare and occur almost exclusively at Cínovec, Czech Republic. Twinning is also commonly observed, and crystal faces may be striated. Scheelite has a white mineral streak and is brittle. Gems cut from transparent material are fragile. Scheelite's refractive index (1.918–1.937 uniaxial positive, with a maximum birefringence of 0.016) and dispersion (0.026) are both moderately high. These factors combine to result in scheelite's high lustre and perceptible "fire", approaching that of diamond. Scheelite fluoresces under shortwave ultraviolet light, the mineral glows a bright sky-blue. The presence of molybdenum trace impurities occasionally results in a green glow. Fluorescence of scheelite, sometimes associated with native gold, is used by geologists in the search for gold deposits.

Occurrence Scheelite occurs in contact metamorphic skarns; in high-temperature hydrothermal veins and greisen; less commonly in granite pegmatites. Temperature and pressure of formation is between 200 and 500 °C (400 and 900 °F) and from 200 to 1,500 bars (2,900 to 21,800 psi). Typical mineral association includes cassiterite, wolframite, topaz, fluorite, apatite, tourmaline, quartz, grossular–andradite, diopside, vesuvianite and tremolite. Scheelite usually occurs in tin-bearing veins and is sometimes found in association with gold. Fine crystals have been obtained from Caldbeck Fells in Cumbria, Cínovec and Loket in the Czech Republic, Guttannen in Switzerland, the Giant Mountains in Silesia, Dragoon Mountains in Arizona and elsewhere. At Trumbull in Connecticut and Kimpu-san in Japan, large crystals of scheelite completely altered to wolframite have been found: those from Japan have been called “reinite.” It was mined until 1990 at King Island, Australia, Glenorchy in Central Otago and Macraes Flat in North Otago and also at The Golden Bar mine at Dead Horse Creek during World War I in Nelson, New Zealand. There is a high concentration of scheelite in the Northeast of Brazil, mainly in the Currais Novos mine in Rio Grande do Norte State. One of the world's largest scheelite mining companies is in Luoyang, China.

History

Scheelite was first described in 1751 for an occurrence in Mount Bispbergs klack, Säter, Dalarna, Sweden, and named for Carl Wilhelm Scheele (1742–1786). Owing to its unusual heaviness, it had been given the name tungsten by the Swedes, meaning “heavy stone.” The name was later used to describe the metal, while the ore itself was given the name scheelerz or scheelite.

Synthetics

Scheelite as a diamond imitation has been surpassed by more convincing products, like cubic zirconia and moissanite. Synthetic scheelite is occasionally offered as natural scheelite, and collectors may thus be fooled into paying high prices for it. Gemologists distinguish natural scheelite from synthetic material mainly by microscopic examination: Natural material is very seldom without internal growth features and inclusions (imperfections), while synthetic material is usually spotless. Distinctly artificial curved striae and clouds of minute gas bubbles may also be observed in synthetic scheelite. The visible absorption spectrum of scheelite, as seen by a hand-held (direct-vision) spectroscope, may also be of use: most natural stones show several faint absorption lines in the yellow region of the spectrum (~585 nm) due to praseodymium and neodymium trace impurities. Conversely, synthetic scheelite is often without such a spectrum.

Applications Scheelite is widely used in phosphors, particularly in scintillators for X-ray and gamma-ray detection. The second and third iterations of the Cryogenic Rare Event Search with Superconducting Thermometers dark matter detector experiment use calcium tungstate as a scintillator as well. It is also utilized in fluorescent lighting systems for its ability to convert ultraviolet light into visible light. In some cathode-ray tubes (CRTs), calcium tungstate (scheelite) is used as a phosphorescent screen material.

In popular culture Scheelite figures in the manga series Dr. Stone, as a precursor to tungsten, and for its fluorescence.

References

Illustrations

Worked examples

Example 1 — a first encounter with Scheelite

Start with the simplest possible case. Write down what Scheelite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Scheelite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Scheelite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Scheelite

In research: Scheelite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Scheelite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Scheelite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Calcium minerals, Gemstones, Laser gain media, so understanding it makes those chapters shorter.
In everyday life: Look for Scheelite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Scheelite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Scheelite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Scheelite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Scheelite in simple terms?

Scheelite is a calcium tungstate mineral with the chemical formula CaWO4. It is an important ore of tungsten (wolfram).

Why does Scheelite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Scheelite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Scheelite.