Systematic mineralogy - Mineralogy for Petrologists - page 65

feldspars field: two feldspars crystallize until the end of the evolution; this is
a subsolvus evolution.
In plutonic rocks , plagioclase crystallizes in a low temperature form
and potassium feldspar is orthoclase or microcline. In the rocks that have
crystallized at relatively low temperatures (granites, granodiorites. etc.),
solid solutions between K-feldspar and plagioclase remain limited and
slow cooling leads to exsolutions, observable under a microscope, the
perthites . In other rocks, especially the alkaline rocks (syenite, nephe-
line syenite, etc.) solid solutions between K-feldspar and plagioclase are
much larger and exsolution leads to mesoperthites. Perthites show differ-
ent habits: from very fine and regular film perthites since the, to thicker,
irregular rods perthites . This habit depends on the initial proportion of
K-feldspar and albite in the initial mineral and of the cooling history.
Antiperthites occur in plagioclase of plutonic rocks that crystallized
at high temperature under conditions of granulite facies (charnockite,
norites, etc.).
Graphic textures are mainly observed in pegmatites. They result from a
syncrystallization quartz and feldspar in the late magmatic evolution.
Rapakiwi textures appear in the development of albite around K-feld-
spar phenocrysts. The albite may be monocrystalline or polycrystalline,
and it sometimes forms radial aggregates. There may be alternating layers
of albite and K-feldspar. Interpretations call upon surface phenomena and
mainly to changes in water pressure that shift slightly the liquidus.
Myrmekites are bud-or wart-like textures of acidic plagioclase con-
taining quartz vermiculi developed at the contact between K-feldspar and
plagioclase from the plagioclase into the K-feldspar. Myrmekite are inter-
preted by a subsolidus reaction: Na and Ca diffuse from plagioclase towards
K-feldspar and replace potassium in the K-feldspar; the latter is then trans-
formed into plagioclase with release of silica (quartz).
In volcanic rocks plagioclase are high temperature forms and alkali
feldspar are sanidine or anorthoclase. In the latter the exsolutions remain
invisible under the microscope and these cryptoperthites can only be detect-
able by X-ray. Feldspars are strongly zoned and show more numerous and
varied twinnings than in the plutonic rocks due to faster cooling. In evolved
rocks (phonolitic, rhyolites, trachytes some) there is a single (mixed): feld-
spar: the liquid has passed the final point of the cotectic line.
3.1.2.4 Alteration of the feldspars
Red clouding is a common and distinctive alteration of orthoclase: the
feldspar is commonly stained into brown or reddish brown by a pervasive
development of a very fine grained mineral which cannot be determined
under the microscope: it may be kaolinite or sometimes hematite.