The Barrovian facies series, named for George Barrow, who described them around 1900, is the "normal" metamorphic facies series. These are the rocks formed in a typical geothermal gradient of about 25 C per km. The typical pressures under which these rocks form are 5-10 kb (mid-crustal depths). All temperatures are approximate and dependent on pressure.
|The phyllosilicate stilpnomelane is present:
Stilp = K1-x(Fe",Mg,Al)3-xSi4O10(OH)2.nH2O
Carbonate-rich rocks may contain dolomite or calcite along with chlorite (dashed lines). Magnesite may be present in magnesium rich rocks.
|Stilpnomelane breaks down:
Stilp + musc = Bi + Chl + Qz
K-feldspar no longer coexists with chlorite:
Chlorite and calcite are no longer compatible:
|Chlorite begins to break down, releasing iron to form almandine:
(Fe,Mg)-Chl + Qz = Alm + Mg-Chl
It also releases iron to form hornblende:
|The breakdown of chlorite is complete, chloritoid disappears and
Fe-Chl + Musc = St + Bi + Alm + H2O
Ctd + Chl + Mus = St + Bi + Alm + H2O
If there are appreciable volatiles, scapolite may form in addition to plagioclase. The scapolites are a solid solution series of marialite (3albite + NaCl) and meionite (3anorthite + CaCO3 or CaSO4.
|Staurolite is no longer stable and disappears:
3St + 2Qz = Alm + 5Ky + H2O
St + Musc + (Mg,Fe)-Bi = Ky/Sil + Fe-Bi + H2O (Staurolite breaks down and the reaction yields more iron-rich biotite)
|Kyanite transforms to sillimanite
Microcline transforms to orthoclase
Muscovite becomes unstable:
Epidote is no longer stable:
Created 22 Sept 1997, Last Update 22 Sept 1997
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