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K-feldspars include microcline,
orthoclase and
sanidine, all of which have the formula
KAlSi3O8 and are tectosilicates (framework silicates)
Adularia is a more ordered low-temperature variety of orthoclase or
partially disordered microcline.
Specific gravity: 2.54 to 2.63
Hardness: 6 to 6 1/2
Streak: White
Colour: Colourless, white, grey, greyish yellow, yellowish, tan, pink, bluish green, greenish white, reddish white
Melting point: About 1,300oC at atmospheric pressure
(JVW p275)
Common impurities: Fe,Ca,Na,Li,Cs,Rb,H2O,Pb
Environments
Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Metamorphic environments
Hydrothermal environments
K-feldspars are primary minerals; they are essential constituents of
rhyolite and
common constituents of
quartzolite.
They also may be found in
diorite.
K-feldspars are minerals of the hornblende-hornfels,
greenschist and
amphibolite facies.
Structure and Colour
The feldspars are made up of linked chains of four-membered tetrahedral rings.
In the K-feldspars each ring has three tetrahedra of Si surrounded by four O's and one tetrahedron where Al
replaces Si, hence the AlSi3 group in the formula. The degree of disorder increases from
microcline, which is fully ordered with the Al tetrahedron in an equivalent
position in each ring, through orthoclase to
sanidine, which is the most disordered. The degree of disorder affects the
absorption spectrum of visible light, and hence the perceived colour of the mineral.
Amazonite is a green to blue-green variety of K-feldspar, usually
microcline, but sometimes
orthoclase. Green K-feldspars have a disordered Al-Si arrangement,
and blue K-feldspars are ordered, but this alone does not account for the colour.
All blue or green amazonites contain some lead as an impurity, less than
2% and sometimes as little as 0.5%, probably in the potassium site, where one Pb2+ would substitute for two
K+.
However, some lead-bearing feldspars are
not coloured, and Pb2+ is not known as a chromophore.
In addition, all coloured amazonite contains water bound in the structure.
Heating and irradiation experiments indicate that the colour is due to radiation damage centres. This is plausible
because 0.01% of potassium is a weakly radioactive isotope, so there is a source of radiation within the
K-feldspar itself.
Radiation reacts with the molecules of water to form H+ and (OH)-. The (OH)- is a
strong oxidant and it oxidises the Pb2+ to form Pb3+, which is the cause of the blue colour.
Order/disorder can affect the colour, blue ordered, green disordered, but in all blue or green K-feldspar the
colour is a radiation colour involving lead, water molecules and natural radiation
from within the K-feldspar
(https://www.youtube.com/watch?v=ejucgGmeJMA).
Alteration
K-feldspar is a major alteration phase in many ore deposits, but most common in porphyry (rock with coarse phenocrysts in a
finer groundmass)
metal deposits, usually formed early in the sequence.
In high temperature alteration the K-feldspar that forms is usually orthoclase,
and at lower temperatures
it is usually microcline.
dolomite, K-feldspar and H2O to
phlogopite, calcite and
CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O =
KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of
phlogopite according
to the above equation
(DHZ 5B p213).
enstatite-ferrosilite,
K-feldspar and H2O to biotite and
quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌
K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage
(DHZ 2A 139).
K-feldspar and H+ to muscovite,
quartz and K+
3KaAlSi3O8 + 2H+ ⇌
KAl2(AlSi3O100(OH)2 + 6SiO2 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction
(KB p99).
montmorillonite and K-feldspar to muscovite variety
illite,
SiO2 and H2O
Al2Si4O10(OH)2.nH2 +
KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O
(JVW p328)
muscovite to corundum,
K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 +
K(AlSi3O8) + H2O
(JVW p102)
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure
(hornblende-hornfels facies) to about
720oC at pressure above 4 kbar
(amphibolite facies)
(MOM p517).
muscovite, biotite and SiO2 to
K-feldspar, cordierite and H2O
6KAl2(AlSi3O10)(OH)2 +
2K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 15SiO2
→ 8KAlSi3O8 +
3(Fe2+,Mg)3Al4Si5O18 + 8H2O
At the high-grade end of the amphibolite facies
biotite is no longer stable and reacts with
muscovite according to the above reaction
(DHZ 3 p73).
muscovite, biotite and SiO2 to
K-feldspar, garnet and H2O
KAl2(AlSi3O10)(OH)2 +
K(Fe2+,Mg)3(AlSi3O10)(OH)2 + 3SiO2
→ 2KAlSi3O8 + (Fe2+,Mg)3Al2(SiO4)3
+ 2H2O
(DHZ 3 p23)
muscovite and
quartz to sillimanite,
K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌
Al2SiO5 + KAlSi3O8 + H2O
At 5 kbar pressure the equilibrium temperature is about 690oC
(amphibolite facies)
(SERC).
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat),
hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down
(KB p17).
Although the muscovite-quartz assemblage is stable over a large part of the
PT range of regional metamorphism,
at temperatures around 600 to 650oC it is replaced by
sillimanite and
K-feldspar
(DHZ 3 p24).
phlogopite, calcite and silica to
diopside, K-feldspar, H2O
and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 =
3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter
according to the above reaction
(DHZ 2A p272).
The association of phlogopite and
calcite
is stable only in the absence of
excess silica
(DHZ 3 p51).
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