Cordierite - (Mg,Fe)2[Si5Al4O18]* nH2O

Cordierite Occurs as a characteristic, usually accessory, phase in many types of felsic peraluminous extrusive and intrusive igneous rocks (rhyolites, granitoids, pegmatites). Such rocks commonly have high A/CNK (> 1.1) [A/CNK = mol. Al2O2/(CaO + Na2O + K2O)].

Cordierite is an orthorhombic tectosilicate with the general formula: X0-1M2T9018, where X = H20, CO2, Ar, Xe, Na, K occupying large channels parallel to the c-axis created by the pseudohexagonal rings; M = Mg2+, Fe2+ (with possibly some Li+ in octahedral coordination; and T = Al4Si5 (with possibly some Be2+ in tetrahedral coordination.
Cordierite has a structure similar to that of beryl, forming prismatic crystals commonly showing a prominent (001) parting, and it may develop simple, lamellar, or cyclic twinning. Indialite is the high-temperature hexagonal polymorph, with a continuum of AI:Si ordering linking it to cordierite.

Origin of Cordierite

Metamorphic
Xenocrystic: The former, and mistaken, notion that cordierite was an exclusively metamorphic mineral led to the venerable interpretation that all cordierite in felsic igneous rocks was xenocrystic. Indeed, granitic magmas commonly intrude low-to-medium-grade metamorphic rocks of semi-pelitic to pelitic compositions, and incorporation of cordierite-bearing fragments of these country rocks into the invading magmas is almost inevitable. The combination of heating of an enclave leading to melting along its grain boundaries, and differential thermal expansion of the minerals of the xenolith or country rock, may result in disaggregation of the xenolith.
Thus, cordierites of metamorphic origin may be released into a silicate melt and, in general, they would be out of chemical equilibrium with that melt. Their subsequent history then depends on the degree to which they are out of equilibrium with the magma, and on the chemical kinetics of the new environment. Xenocrystic cordierite may disappear rapidly in a high- temperature, well-mixed, relatively fluid metaluminous melt, survive largely unmodified in a near-solidus, static, viscous peraluminous melt, or grow in a highly peraluminous melt.

Expressed as a simple equation:

• Crd1 + L → Crd2

where Crd1 is the original metamorphic cordierite, L is the silicate melt phase, and Crd2 is the re-equilibrated cordierite in the melt. Crd2 is almost certainly different in size, shape, and chemical composition compared with Crd1.

Magmatic
Peritectic: This category includes cordierite that appears only as the result of a melt-producing reaction in response to rising temperature. In other words, cordierite is not part of the subsolidus mineral assemblage at the onset of partial melting, but it appears as melting begins in a peritectic reaction such as:

• Als + Bt + Qtz → L + Kfs + Crd

Spatially, this cordierite may form along the contact between pelitic xenoliths and melt, or associate with the melt phase (initially as leucosomes) rather than the refractory residuum (restite).

A second type of peritectic magmatic reaction occurs in which cordierite appears in response to falling temperature and/or pressure. drops of pressure can produce cordierite, possibly as reaction rims on other phases (particularly garnet and aluminosilicate) in rapidly ascending magmas, or even in isobafically cooling granites. Some reactions are:

• L + Kfs + Grt → Crd + Bt + Qtz
• L+Grt → Crd+Bt+Qtz+V
• L + Grt + Qtz + Als → Crd + Bt
• L + Als → Crd + Kfs + Qtz + V

Metasornatic:
Some workers suggested that this cordierite is the product of the reaction:

• 4Ab + 2MgO → Crd + 7Qtz + 2Na20

in which the source of the MgO could be closed system within the pegmatite (autometasomatic), or open system (originating from the MgO-rich serpentinite wall rocks).

Optical Properties:
Color: Colorless
Form: Euhedral to subhedral pseudohexagonal crystals
Birefringence: Usually I order gray-white and never exceed I order red
Twinning: Cordierite may be untwinned, or may have polysynthetic twinning like plagioclase; however most crystals are complexly twinned by polysynthetic or cyclic forms. Twin lamellae, in basal section, commonly radiate from a common center to yield cyclic star patterns of three, six or twelve rays with composition planes 60° or 30° to position of extinction.
Alteration: Cordierite alters rather quickly, beginning on basal parting planes and causing the entire cordierite crystal to assume a yellowish cast. The alteration product is usually a fine-grained, yellow-greenish aggregate (Chlorite+biotite+muscovite) called Pinite.

Bibliography



• D. B. Clarke (1995): Cordierite in felsic igneous rocks; a synthesis Mineralogical Magazine. • Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). PPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)
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Altered Cordierite crystal with cyclic twinning in a peraluminous Granite from Porto Azzurro, Elba Island (Italy). XPL image, 2x (Field of view = 7mm)