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
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
Plutonic igneous environments
Volcanic igneous environments
K-feldspars are primary minerals; they are essential constituents of
common constituents of
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).
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
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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