Phengite is a series name for dioctahedral

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into fragile elastic plates. This characteristic is described as ''perfect basal cleavage''. Mica is co ...

s of composition K(Al,Mg)₂(OH)₂(Si,Al)₄O₁₀. It is common for Mg or Fe²⁺ to substitute for Al on the Y site and a corresponding increase in Si on the Z site in the mica group formula ''X''₂''Y''_4–6''Z''₈ O₂₀( OH, F)₄. Similar to

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula KAl2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavage y ...

but with addition of

magnesium Magnesium is a chemical element; it has Symbol (chemistry), symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 ...

. It is a non- IMA recognized mineral name representing the series between muscovite and

celadonite Celadonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide with formula . It crystallizes in the monoclinic system and usually forms massive aggregates of prismatic crystallites or, ...

. The silica content of phengite has been proposed as a geobarometer for the

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

of low grade

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s.M. E. Fleet and R. A. Howie, ''Rock-Forming Minerals, Vol. 3A: Micas'', Geological Society of London; 2nd ed, 2006, p. 132, (accessed through Google books)

References

* Glossary of Geology, Fifth Edition, Eds. K.K.E. Neiendorf, J.P. Mehl, J.A. Jackson, American Geological Institute, 2005 * von Kobell, Franz (1853) Tafeln zur Bestimmung des Mineralien, 5th edition, Munchen. * Guidotti, Charles V. (1984) Micas in metamorphic Rocks, reviews in Mineralogy, 13: 357-467. * Rieder et al. (1998): Nomenclature of the micas. Canadian Mineralogist 36: 905-912. * Mookherjee, M., and Redfern, S.A.T. (2002) A high-pressure Fourier-transform infrared study of the interlayer and Si-O stretching region in phengite-2M 1. Clay Minerals: 37: 323-336. * Cibin, G., G. Cinque, A. Marcelli, A. Mottana, & R. Sassi (2008): The octahedral sheet of metamorphic 2M1-phengites: a combined EMPA and AXANES study: American Mineralogist 93, 414-425. Phyllosilicates Mica group {{silicate-mineral-stub