Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 247

MINERAL CHEMISTRY AND REEATIONSHIPS
243
the studied samples. This breakdown has been described above as falling
into two categories: 1) a relatively weak marginal alteration, resulting in a
rim of small opaque oxides and new silicates, and 2) a strong alteration
affecting the whole grain in such a way that the original biotite is totally
replaced by the opaque oxides and new silicates. In the f'ormer case, some of'
the silicates are still biotite but this biotite has changed composition towards
the pure phlogopite end member of the solid solution system. This is shown
by analyses of the secondary mica patches in the sample Jan 81.
The stability of annite, the Ti- and Mg free biotite, has been studied by
Eugster & Wones (1962) and that of the Ti free biotite-phlogopite solid
solution by Wones & Eugster (1965). These experimental studies reveal an
oxidation breakdown pattern of Ti free biotites resulting in dehydration and
formation of opaque oxides and silicates. The observed breakdown of the
compositionally more complex biotites of these trachytes is most probably
identical to that of the experiments.
The composition of the secondary mica patches of Jan 81 may theore-
tically (thermodynamics not tested) be reached by extracting some sanidine,
ilmenite+hematite (sol. sol.) and water from the primary biotite of samples
like Jan 168 through oxidation thus:
^fKKg^Mg^.syFeltgHTio.soHTio.isSis^Ah.eoOzo.ooKOHKJ+O.SOa-
Ti rich biotite
3[K! 9o(Mg3 43Fe'f 5o)( 1 Ío.6oAlo.l3)[SÍ5.25Al2.75020.0o] (OH)4] +
Ti poor phlogopite
1.75 (K AlSi308) + 2 (FeTi03) + Fe203+2 (HzO).
sanidine ilmenite hematite
The total breakdown of this same original Ti rich biotite (Jan 168) results in
increased amounts of opaque phases (compared to the partial breakdown),
the appearance of forsterite which takes up Mg, substitution of the sanidine
by an undersaturated aluminosilicate and a trace of an A1 rich phase to
match the peraluminous nature of this biotite. An equation for this break-
down of the biotite ofjan 168 may be as follows (thermodynamics not
tested):
4[K.i.86(Mg2.57l,e2.t9)(rI Ío.8o)[TÍo.l5SÍ.5.25Al2.6oE)20.0o](OH)4]+ 1.202 =
Ti rich biotite
3.8(FeTi03)+2.48(Fe203)+5.14(Mg2Si04) + 7.44(KAlSi206) +
ilmenite hematite forsterite leucite
0.49(AlgSi2O13)+8(H2O).
mullite
Oxidation of the primary opaque oxides of these rocks has taken place as
well, but at what stage and under what conditions in the history of these
rocks, is not certain.