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[named for French geologist P. Cordier (1777–1861)], also iolite or dichroite, a mineral, a complex aluminosilicate of aluminum, magnesium, and iron; chemical composition, (Mg, Fe)2Al3[AlSi5O18]. Its crystal structure is characterized by a ring construction similar to that of beryllium. Cordierite crystallizes in the orthorhombic system. It occurs in the form of prismatic crystals, irregular clusters, and grains of blue-gray, blue, violet, or brown color; sometimes it is colorless. It has a hardness of 7.0–7.5 on the mineralogical scale and a density of 2,600–2,660 kg/m3. Dichroism is a characteristic feature and is highly pronounced in transparent grains. The mineral effloresces easily to form talc, mica, and other secondary minerals. Cordierite is formed during contact metamorphosis of rocks rich in aluminum and magnesium. Transparent varieties of cordierite are used as gems.