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Frequently Asked Questions

Answers to Common Questions


From Wikipedia, the free encyclopedia



Aragonite from Los Molinillos, Cuenca, Spain, sample width about 4 cm


Category Carbonate minerals

Formula(repeating unit)CaCO3 IMA symbol Arg[1] Crystal system Orthorhombic

Unit cell a = 4.9598(5) Å, b = 7.9641(9) Å, and c = 5.7379(6) Å at 25 °C [2]


Color Can come in a variety of colors, but commonly red or white

Crystal habit Commonly dendritic or pseudo-hexagonal; can also be acicular, tabular, prismatic, coral-like

Twinning Cyclic on {110}, forms pseudohexagonal aggregates. If polysynthetic, forms fine striations parallel to [110].

Cleavage Good on [110], Poor on {110}.

Fracture Subconchoidal

Tenacity Very brittle

Mohs scale hardness3.5–4

Luster Vitreous, waxy, resinous

Streak White

Diaphaneity Transparent to opaque

Specific gravity2.94

Optical properties Biaxial (-)

Refractive indexnω = 1.550 nε = 1.650

Birefringenceδ = 0.155

2V angle Measured 18–19°

Dispersion Weak

Extinction Parallel

Ultraviolet fluorescence Faint white-blue to blue-violet

Solubility Soluble in acids, and saltwater (but takes longer)

Common impurities Commonly strontiumzirconiumlead

Other characteristics Thermodynamically unstable, Morphs slowly back into calcite


Aragonite is a carbonate mineral and one of the three most common naturally occurring crystal forms of calcium carbonate (CaCO3), the others being calcite and vaterite. It is formed by biological and physical processes, including precipitation from marine and freshwater environments.


Aragonite Crystal Structure

The crystal lattice of aragonite differs from that of calcite, resulting in a different crystal shape, an orthorhombic crystal system with acicular crystal.[5] Repeated twinning results in pseudo-hexagonal forms. Aragonite may be columnar or fibrous, occasionally in branching helictite forms called flos-ferri ("flowers of iron") from their association with the ores at the Carinthian iron mines.[6]


The type location for aragonite is Molina de Aragón in the Province of Guadalajara in Castilla-La ManchaSpain, for which it was named in 1797.[7] Aragonite is found in this locality as cyclic twins inside gypsum and marls of the Keuper facies of the Triassic.[8] This type of aragonite deposit is very common in Spain, and there are also some in France.[6]

An aragonite cave, the Ochtinská Aragonite Cave, is situated in Slovakia.[9]

In the US, aragonite in the form of stalactites and "cave flowers" (anthodite) is known from Carlsbad Caverns and other caves.[10] For a few years in the early 1900s, aragonite was mined at Aragonite, Utah (now a ghost town).[11]

Massive deposits of oolitic aragonite sand are found on the seabed in the Bahamas.[12]

Aragonite is the high pressure polymorph of calcium carbonate. As such, it occurs in high pressure metamorphic rocks such as those formed at subduction zones.[13]

Aragonite forms naturally in almost all mollusk shells, and as the calcareous endoskeleton of warm- and cold-water corals (Scleractinia). Several serpulids have aragonitic tubes.[14] Because the mineral deposition in mollusk shells is strongly biologically controlled,[15] some crystal forms are distinctively different from those of inorganic aragonite.[16] In some mollusks, the entire shell is aragonite;[17] in others, aragonite forms only discrete parts of a bimineralic shell (aragonite plus calcite).[15] The nacreous layer of the aragonite fossil shells of some extinct ammonites forms an iridescent material called ammolite.[18]

Aragonite also forms naturally in the endocarp of Celtis occidentalis.[19]

The skeleton of some calcareous sponges is made of aragonite.

Aragonite also forms in the ocean inorganic precipitates called marine cements (in the sediment) or as free crystals (in the water column).[20][21] Inorganic precipitation of aragonite in caves can occur in the form of speleothems.[22] Aragonite is common in serpentinites where magnesium-rich pore solutions apparently inhibit calcite growth and promote aragonite precipitation.[23]

Aragonite is metastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite in fossils. Aragonite older than the Carboniferous is essentially unknown.[24]

Aragonite can be synthesized by adding a calcium chloride solution to a sodium carbonate solution at temperatures above 60 °C (140 °F) or in water-ethanol mixtures at ambient temperatures.[25]

Physical properties

Aragonite is not the thermodynamically stable phase of calcium carbonate at any pressure below about 3,000 bars (300,000 kPa) at any temperature.[26] Aragonite nonetheless frequently forms in near-surface environments at ambient temperatures. The weak Van der Waals forces inside aragonite give an important contribution to both the crystallographic and elastic properties of this mineral.[27] The difference in stability between aragonite and calcite, as measured by the Gibbs free energy of formation, is small, and effects of grain size and impurities can be important. The formation of aragonite at temperatures and pressures where calcite should be the stable polymorph may be an example of Ostwald's step rule, where a less stable phase is the first to form.[28] The presence of magnesium ions may inhibit calcite formation in favor of aragonite.[29] Once formed, aragonite tends to alter to calcite on scales of 107 to 108 years.[30] Comparing to the calcite, aragonite

The mineral vaterite, also known as μ-CaCO3, is another phase of calcium carbonate that is metastable at ambient conditions typical of Earth's surface, and decomposes even more readily than aragonite.[31][32]


In aquaria, aragonite is considered essential for the replication of reef conditions. Aragonite provides the materials necessary for much sea life and also keeps the pH of the water close to its natural level, to prevent the dissolution of biogenic calcium carbonate.[33]

Aragonite has been successfully tested for the removal of pollutants like zinccobalt and lead from contaminated wastewaters.[34]

Claims that magnetic water treatment can reduce scaling, by converting calcite to aragonite, have been met with skepticism,[35] but continue to be investigated.[36][37]


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​From Wikipedia, the free encyclopedia

This article is about the mineral. For other uses, see Quartz (disambiguation).



Quartz crystal cluster from Brazil


Category Silicate mineral[1]

(repeating unit)SiO2

IMA symbol Qz[2]

Strunz classification4.DA.05 (oxides)

Dana classification75.01.03.01 (tectosilicates)

Crystal systemα-quartz: trigonal
β-quartz: hexagonal

Crystal classα-quartz: trapezohedral (class 3 2)
β-quartz: trapezohedral (class 6 2 2)[3]

Space groupα-quartz: P3221 (no. 154)[4]
β-quartz: P6222 (no. 180) or P6422 (no. 181)[5]

Unit cella = 4.9133 Å, c = 5.4053 Å; Z = 3


Formula mass60.083 g·mol−1

Color Colorless through various colors (pink, orange, purple, dark brown) to black

Crystal habit6-sided prism ending in 6-sided pyramid (typical), drusy, fine-grained to microcrystalline, massive

Twinning Common Dauphine law, Brazil law, and Japan law

Cleavage{0110} Indistinct

Fracture Conchoidal

Tenacity Brittle

Mohs scale hardness7 – lower in impure varieties (defining mineral)

Luster Vitreous – waxy to dull when massive

Streak White

Diaphaneity Transparent to nearly opaque

Specific gravity2.65; variable 2.59–2.63 in impure varieties

Optical properties Uniaxial (+)

Refractive indexnω = 1.543–1.545
nε = 1.552–1.554

Birefringence+0.009 (B-G interval)


Melting point1670 °C (β tridymite); 1713 °C (β cristobalite)[3]

SolubilityInsoluble at STP; 1 ppmmass at 400 °C and 500  lb/in2 to 2600 ppmmass at 500 °C and 1500 lb/in2[3]

Other characteristicsLattice: hexagonalpiezoelectric, may be triboluminescentchiral (hence optically active if not racemic)


Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical formula of SiO2. Quartz is the second most abundant mineral in Earth's continental crust, behind feldspar.[10]

Quartz exists in two forms, the normal α-quartz and the high-temperature β-quartz, both of which are chiral. The transformation from α-quartz to β-quartz takes place abruptly at 573 °C (846 K; 1,063 °F). Since the transformation is accompanied by a significant change in volume, it can easily induce microfracturing of ceramics or rocks passing through this temperature threshold.

There are many different varieties of quartz, several of which are classified as gemstones. Since antiquity, varieties of quartz have been the most commonly used minerals in the making of jewelry and hardstone carvings, especially in Eurasia.

Quartz is the mineral defining the value of 7 on the Mohs scale of hardness, a qualitative scratch method for determining the hardness of a material to abrasion.


The word "quartz" comes from the German Quarz,[11] which is of Slavic origin (Czech miners called it křemen). Other sources attribute the word's origin to the Saxon word Querkluftertz, meaning cross-vein ore.[12][13]

Quartz is the most common material identified as the mystical substance maban in Australian Aboriginal mythology. It is found regularly in passage tomb cemeteries in Europe in a burial context, such as Newgrange or Carrowmore in Ireland. The Irish word for quartz is grianchloch, which means 'sunstone'. Quartz was also used in Prehistoric Ireland, as well as many other countries, for stone tools; both vein quartz and rock crystal were knapped as part of the lithic technology of the prehistoric peoples.[14]

While jade has been since earliest times the most prized semi-precious stone for carving in East Asia and Pre-Columbian America, in Europe and the Middle East the different varieties of quartz were the most commonly used for the various types of jewelry and hardstone carving, including engraved gems and cameo gemsrock crystal vases, and extravagant vessels. The tradition continued to produce objects that were very highly valued until the mid-19th century, when it largely fell from fashion except in jewelry. Cameo technique exploits the bands of color in onyx and other varieties.

Roman naturalist Pliny the Elder believed quartz to be water ice, permanently frozen after great lengths of time.[15] (The word "crystal" comes from the Greek word κρύσταλλος, "ice".) He supported this idea by saying that quartz is found near glaciers in the Alps, but not on volcanic mountains, and that large quartz crystals were fashioned into spheres to cool the hands. This idea persisted until at least the 17th century. He also knew of the ability of quartz to split light into a spectrum.[16]

In the 17th century, Nicolas Steno's study of quartz paved the way for modern crystallography. He discovered that regardless of a quartz crystal's size or shape, its long prism faces always joined at a perfect 60° angle.[17]

Quartz's piezoelectric properties were discovered by Jacques and Pierre Curie in 1880.[18][19] The quartz oscillator or resonator was first developed by Walter Guyton Cady in 1921.[20][21] George Washington Pierce designed and patented quartz crystal oscillators in 1923.[22][23][24] Warren Marrison created the first quartz oscillator clock based on the work of Cady and Pierce in 1927.[25]

Efforts to synthesize quartz began in the mid-nineteenth century as scientists attempted to create minerals under laboratory conditions that mimicked the conditions in which the minerals formed in nature: German geologist Karl Emil von Schafhäutl (1803–1890) was the first person to synthesize quartz when in 1845 he created microscopic quartz crystals in a pressure cooker.[26] However, the quality and size of the crystals that were produced by these early efforts were poor.[27]

By the 1930s, the electronics industry had become dependent on quartz crystals. The only source of suitable crystals was Brazil; however, World War II disrupted the supplies from Brazil, so nations attempted to synthesize quartz on a commercial scale. German mineralogist Richard Nacken (1884–1971) achieved some success during the 1930s and 1940s.[28] After the war, many laboratories attempted to grow large quartz crystals. In the United States, the U.S. Army Signal Corps contracted with Bell Laboratories and with the Brush Development Company of Cleveland, Ohio to synthesize crystals following Nacken's lead.[29][30] (Prior to World War II, Brush Development produced piezoelectric crystals for record players.) By 1948, Brush Development had grown crystals that were 1.5 inches (3.8 cm) in diameter, the largest at that time.[31][32] By the 1950s, hydrothermal synthesis techniques were producing synthetic quartz crystals on an industrial scale, and today virtually all the quartz crystal used in the modern electronics industry is synthetic.[33]

  • Rock crystal jug with cut festoon decoration by Milan workshop from the second half of the 16th century, National Museum in Warsaw. The city of Milan, apart from Prague and Florence, was the main Renaissance centre for crystal cutting.[34]

  • Synthetic quartz crystals produced in the autoclave shown in Western Electric's pilot hydrothermal quartz plant in 1959

  • Fatimid ewer in carved rock crystal (clear quartz) with gold lid, c. 1000


The word "quartz" is derived from the German word Quarz, which had the same form in the first half of the 14th century in Middle High German and in East Central German[35] and which came from the Polish dialect term kwardy, which corresponds to the Czech term tvrdý ("hard").[36]

The Ancient Greeks referred to quartz as κρύσταλλος (krustallos) derived from the Ancient Greek κρύος (kruos) meaning "icy cold", because some philosophers (including Theophrastus) understood that the mineral to be a form of supercooled ice.[37] Today, the term rock crystal is sometimes used as an alternative name for transparent coarsely crystalline quartz.[38][39]

Crystal habit and structure


Crystal structure of α-quartz (red balls are oxygen, grey are silicon)


Crystal structure of β-quartz


A chiral pair of α-quartz

Quartz belongs to the trigonal crystal system at room temperature, and to the hexagonal crystal system above 573 °C (846 K; 1,063 °F). The ideal crystal shape is a six-sided prism terminating with six-sided pyramids at each end. In nature quartz crystals are often twinned (with twin right-handed and left-handed quartz crystals), distorted, or so intergrown with adjacent crystals of quartz or other minerals as to only show part of this shape, or to lack obvious crystal faces altogether and appear massive.[40][41]

Well-formed crystals typically form as a druse (a layer of crystals lining a void), of which quartz geodes are particularly fine examples.[42] The crystals are attached at one end to the enclosing rock, and only one termination pyramid is present. However, doubly terminated crystals do occur where they develop freely without attachment, for instance, within gypsum.[43]

α-quartz crystallizes in the trigonal crystal system, space group P3121 or P3221 (space group 152 or 154 resp.) depending on the chirality. Above 573 °C (846 K; 1,063 °F), α-quartz in P3121 becomes the more symmetric hexagonal P6422 (space group 181), and α-quartz in P3221 goes to space group P6222 (no. 180).[44]

These space groups are truly chiral (they each belong to the 11 enantiomorphous pairs). Both α-quartz and β-quartz are examples of chiral crystal structures composed of achiral building blocks (SiO4 tetrahedra in the present case). The transformation between α- and β-quartz only involves a comparatively minor rotation of the tetrahedra with respect to one another, without a change in the way they are linked.[40][45] However, there is a significant change in volume during this transition, and this can result in significant microfracturing in ceramics[46] and in rocks of the Earth's crust.[47]

  • Prismatic quartz (6 facets)

  • Sceptered quartz

  • Sceptered quartz (as aggregates: "Elestial quartz")

  • Bladed quartz

  • Druse quartz

  • Granular quartz

  • "Herkimer diamond"

  • Twinned quartz

  • Massive quartz

Varieties (according to microstructure)

Although many of the varietal names historically arose from the color of the mineral, current scientific naming schemes refer primarily to the microstructure of the mineral. Color is a secondary identifier for the cryptocrystalline minerals, although it is a primary identifier for the macrocrystalline varieties.[48]

Major varieties of quartz

Type Color and description Transparency

Herkimer diamond Colorless Transparent

Rock crystal Colorless Transparent

Amethyst Purple to violet colored quartz Transparent

Citrine Yellow quartz ranging to reddish-orange or brown (Madera quartz), and occasionally greenish yellow Transparent

Ametrine A mix of amethyst and citrine with hues of purple/violet and yellow or orange/brown Transparent

Rose quartz Pink, may display diasterism Transparent

Chalcedony Fibrous, variously translucent, cryptocrystalline quartz occurring in many varieties.
The term is often used for white, cloudy, or lightly colored material intergrown with moganite.
Otherwise more specific names are used.

Carnelian Reddish orange chalcedony Translucent

Aventurine Quartz with tiny aligned inclusions (usually mica) that shimmer with a vitrescence Translucent to opaque

Agate Multi-colored, curved or concentric banded chalcedony (cf. Onyx)Semi-translucent to translucent

Onyx Multi-colored, straight banded chalcedony or chert (cf. Agate)Semi-translucent to opaque

Jasper Opaque cryptocrystalline quartz, typically red to brown but often used for other colors Opaque

Milky quartz White, may display diasterism Translucent to opaque

Smoky quartz Light to dark gray, sometimes with a brownish hue Translucent to opaque

Tiger's eye Fibrous gold, red-brown or bluish colored chalcedony, exhibiting chatoyancy.

Prasiolite Green Transparent

Rutilated quartz Contains acicular (needle-like) inclusions of rutile

Dumortierite quartz Contains large amounts of blue dumortierite crystals Translucent

Prase Green Translucent

Varieties (according to color)


Quartz crystal demonstrating transparency

Pure quartz, traditionally called rock crystal or clear quartz, is colorless and transparent or translucent and has often been used for hardstone carvings, such as the Lothair Crystal. Common colored varieties include citrine, rose quartz, amethyst, smoky quartz, milky quartz, and others.[49] These color differentiations arise from the presence of impurities which change the molecular orbitals, causing some electronic transitions to take place in the visible spectrum causing colors.

The most important distinction between types of quartz is that of macrocrystalline (individual crystals visible to the unaided eye) and the microcrystalline or cryptocrystalline varieties (aggregates of crystals visible only under high magnification). The cryptocrystalline varieties are either translucent or mostly opaque, while the transparent varieties tend to be macrocrystalline. Chalcedony is a cryptocrystalline form of silica consisting of fine intergrowths of both quartz, and its monoclinic polymorph moganite.[50] Other opaque gemstone varieties of quartz, or mixed rocks including quartz, often including contrasting bands or patterns of color, are agatecarnelian or sard, onyxheliotrope, and jasper.[40]



Rock crystal




Blue quartz


Dumortierite quartz


Citrine quartz (natural)


Citrine quartz (heat-altered amethyst)


Milky quartz


Rose quartz


Smoky quartz



Amethyst is a form of quartz that ranges from a bright vivid violet to a dark or dull lavender shade. The world's largest deposits of amethysts can be found in Brazil, Mexico, Uruguay, Russia, France, Namibia, and Morocco. Sometimes amethyst and citrine are found growing in the same crystal. It is then referred to as ametrine. Amethyst derives its color from traces of iron in its structure.[51]

Blue quartz

Blue quartz contains inclusions of fibrous magnesio-riebeckite or crocidolite.[52]

Dumortierite quartz

Inclusions of the mineral dumortierite within quartz pieces often result in silky-appearing splotches with a blue hue. Shades of purple or grey sometimes also are present. "Dumortierite quartz" (sometimes called "blue quartz") will sometimes feature contrasting light and dark color zones across the material.[53][54] "Blue quartz" is a minor gemstone.[53][55]


Citrine is a variety of quartz whose color ranges from pale yellow to brown due to a submicroscopic distribution of colloidal ferric hydroxide impurities.[56] Natural citrines are rare; most commercial citrines are heat-treated amethysts or smoky quartzes. However, a heat-treated amethyst will have small lines in the crystal, as opposed to a natural citrine's cloudy or smoky appearance. It is nearly impossible to differentiate between cut citrine and yellow topaz visually, but they differ in hardness. Brazil is the leading producer of citrine, with much of its production coming from the state of Rio Grande do Sul. The name is derived from the Latin word citrina which means "yellow" and is also the origin of the word "citron". Sometimes citrine and amethyst can be found together in the same crystal, which is then referred to as ametrine.[57] Citrine has been referred to as the "merchant's stone" or "money stone", due to a superstition that it would bring prosperity.[58]

Citrine was first appreciated as a golden-yellow gemstone in Greece between 300 and 150 BC, during the Hellenistic Age. Yellow quartz was used prior to that to decorate jewelry and tools but it was not highly sought after.[59]

Milky quartz

Milk quartz or milky quartz is the most common variety of crystalline quartz. The white color is caused by minute fluid inclusions of gas, liquid, or both, trapped during crystal formation,[60] making it of little value for optical and quality gemstone applications.[61]

Rose quartz

"Rose Quartz" redirects here. For other uses, see Rose Quartz (disambiguation).

Rose quartz is a type of quartz that exhibits a pale pink to rose red hue. The color is usually considered as due to trace amounts of titaniumiron, or manganese in the material. Some rose quartz contains microscopic rutile needles that produce asterism in transmitted light. Recent X-ray diffraction studies suggest that the color is due to thin microscopic fibers of possibly dumortierite within the quartz.[62]

Additionally, there is a rare type of pink quartz (also frequently called crystalline rose quartz) with color that is thought to be caused by trace amounts of phosphate or aluminum. The color in crystals is apparently photosensitive and subject to fading. The first crystals were found in a pegmatite found near RumfordMaine, US and in Minas Gerais, Brazil.[63] The crystals found are more transparent and euhedral, due to the impurities of phosphate and aluminum that formed crystalline rose quartz, unlike the iron and microscopic dumortierite fibers that formed rose quartz.[64]

Smoky quartz

Smoky quartz is a gray, translucent version of quartz. It ranges in clarity from almost complete transparency to a brownish-gray crystal that is almost opaque. Some can also be black. The translucency results from natural irradiation acting on minute traces of aluminum in the crystal structure.[65]


Prase is a green variety of quartz.[66] The green color is caused by inclusions of amphibole.[67]


Not to be confused with Praseolite.

Prasiolite, also known as vermarine, is a variety of quartz that is green in color.[68] The green is caused by iron ions.[67] It is a rare mineral in nature and is typically found with amethyst; most "prasiolite" is not natural – it has been artificially produced by heating of amethyst. Since 1950[citation needed], almost all natural prasiolite has come from a small Brazilian mine, but it is also seen in Lower Silesia in Poland. Naturally occurring prasiolite is also found in the Thunder Bay area of Canada.[68]

Synthetic and artificial treatments


A synthetic quartz crystal grown by the hydrothermal method, about 19 cm long and weighing about 127 grams

Not all varieties of quartz are naturally occurring. Some clear quartz crystals can be treated using heat or gamma-irradiation to induce color where it would not otherwise have occurred naturally. Susceptibility to such treatments depends on the location from which the quartz was mined.[69]

Prasiolite, an olive colored material, is produced by heat treatment;[70] natural prasiolite has also been observed in Lower Silesia in Poland.[71] Although citrine occurs naturally, the majority is the result of heat-treating amethyst or smoky quartz.[70] Carnelian has been heat-treated to deepen its color since prehistoric times.[72]

Because natural quartz is often twinned, synthetic quartz is produced for use in industry. Large, flawless, single crystals are synthesized in an autoclave via the hydrothermal process.[73][40][33]

Like other crystals, quartz may be coated with metal vapors to give it an attractive sheen.[74][75]



Granite rock in the cliff of Gros la Tête on Aride IslandSeychelles. The thin (1–3 cm wide) brighter layers are quartz veins, formed during the late stages of crystallization of granitic magmas. They are sometimes called "hydrothermal veins".

Quartz is a defining constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks such as sandstone and shale. It is a common constituent of schistgneissquartzite and other metamorphic rocks.[40] Quartz has the lowest potential for weathering in the Goldich dissolution series and consequently it is very common as a residual mineral in stream sediments and residual soils. Generally a high presence of quartz suggests a "mature" rock, since it indicates the rock has been heavily reworked and quartz was the primary mineral that endured heavy weathering.[76]

While the majority of quartz crystallizes from molten magma, quartz also chemically precipitates from hot hydrothermal veins as gangue, sometimes with ore minerals like gold, silver and copper. Large crystals of quartz are found in magmatic pegmatites.[40] Well-formed crystals may reach several meters in length and weigh hundreds of kilograms.[77]

Elemental impurity incorporation strongly influences the ability to process and utilize quartz. Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare. These high-purity quartz are defined containing less than 50 ppm of impurity elements.[78] A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States.[79] Quartz may also be found in Caldoveiro Peak, in Asturias, Spain.[80]

The largest documented single crystal of quartz was found near ItaporeGoiaz, Brazil; it measured approximately 6.1 m × 1.5 m × 1.5 m and weighed 39,916 kilograms.

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