cordierite

n.
A dichroic violet-blue to gray mineral silicate of magnesium, aluminum, and sometimes iron. Also called dichroite.

[French, after Pierre L. Cordier (1777-1861), French geologist.]

An orthorhombic magnesium aluminosilicate mineral of composition Mg₂[Al₄Si₅O₁₈]. The crystal structure is related to beryl. Limited amounts of Fe²⁺ may substitute for Mg²⁺, and Fe³⁺ for Al³⁺. The hardness is 7 (Mohs scale); specific gravity 2.6; luster vitreous; cleavage poor; and color greenish-blue, lilac blue, or dark blue, often strongly pleochroic colorless to deep blue. Transparent pleochroic crystals are used as gem material. The disordered cordierite structure, Mg₂[(Al,Si)₉O₁₈], is called indialite and is hexagonal, isotypic with beryl. Osumilite, KMg₂Al₃[(Al,Si)₁₂O₃₀] · H₂O, is also related but has a structure built of double six-membered rings of tetrahedrons. Cordierite, indialite, and osumilite are difficult to distinguish. Pale colored varieties are often misidentified as quartz, since these minerals have many physical properties in common. See also Beryl; Silicate minerals.

Cordierite possesses unusually low thermal expansion, and synthetic material has been applied to thermal-shock-resistant materials, such as insulators for spark plugs and low-expansion concrete.

Cordierite frequently occurs associated with thermally metamorphosed rocks derived from argillaceous sediments. It may occur in aluminous schists, gneisses, and granulites; though usually appearing in minor amounts, cordierite occurs at many localities throughout the world.

---

(Mg,Fe)
Orthorhombic -- Rhombic bipyramidal

Environment

Usually a mineral of metamorphic rocks, considered indicators of intense heat and pressure.

Crystal description

Crystals rare, and embedded, often more or less altered to a mica or chlorite. Usually in grains or masses embedded in rock without crystal outlines.

Physical properties

Gray and blue in the same grain. Luster glassy; hardness 7-7Ɖ; specific gravity 2.6-2.7; fracture subconchoidal; cleavage poor pinacoidal (side pinacoid best). Brittle; gemmy to translucent; strong transformation of hue in different crystal directions, changing from violet-blue to grayish as it is turned (the inspiration for one of its alternate names, "dichroite").

Composition

Magnesium, aluminum silicate; plus iron, calcium, and hydroxyl (OH): 10.2% MgO, 33.6% Al ₂ O ₃ , 49.4% SiO ₂ , perhaps 5.3% FeO, and 15% H ₂ O.

Tests

No blowpipe test is necessary. The color change from blue to gray, which will be seen through most flakes, is sufficiently distinctive. In case of doubt, look through chip at light reflecting from a polished table top or glass sheet and turn it to see the two colors.

Distinguishing characteristics

Color and dichroism (directional change in color) are very characteristic; there is no other very common violet-blue schist mineral with this appearance.

Occurrence

The largest (altered) crystals have been found in Bodenmais, Germany. Good embedded glassy masses are found at Orijärvi, Finland; Kragerø, Norway; and Mt. Tsilaizina, Madagascar. Gemmy waterworn pebbles are found in the Sri Lanka (Ceylon) gem gravels and in India. Recently found in Yellowknife, Northwest Territories.

In the U.S. it is found near Haddam, Connecticut, on the w. side of the Connecticut River.

Also known as iolite for its violet color and as dichroite because of the remarkable directional color change. It has been used as a jewelry stone but is lacking in brilliance, since it is rarely really clear and usually too dark. Valued as a geological thermometer and used as a guide to the grade of metamorphism. It has been suggested that this mineral was used in the Norsemen's Atlantic navigation to locate the sun's position in cloudy weather (the strongest polarization of sky light, and the bluest hue in the iolite, is at 90° from the sun).

---

Note: click on a word meaning below to see its connections and related words.

The noun has one meaning:

Meaning #1: a blue mineral of magnesium and iron and aluminum and silicon and oxygen; often used as a gemstone

---

This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)