What is the difference between potassium feldspar sodium feldspar and calcium feldspar?

Answer 1

The main difference is in their chemical composition, their temperature of formation, density, colour, cleavage and mineral habit.

Calcium and Sodium Rich Feldspars:

There can be a mix of calcium and sodium (more correctly termed solid solutions) in Feldspars between Albite and Anorthite which are known generally as Plagioclase feldspars.

Depending on the relative sodium and calcium contents they are given differing names:

Albite = 100 - 90 % Na, 0 - 10 % Ca

Oligoclase = 90 - 70 % Na, 10 - 30 % Ca

Andesine = 70 - 50 % Na, 30 - 50 % Ca

Labradorite = 50 - 30 % Na, 50 - 70 % Ca

Bytownite = 30 - 10 % Ns, 70 - 90 % Ca

Anorthite = 10 - 0 % Na, 90 - 100% Ca

Alkali Feldspars (containing Potassium and Sodium)

There is also a limited solid solution series between Albite and K-Feldspars which takes the following form which is further complicated by the temperature dependent stability of some of the mineral forms:

Albite = 100 - 90 % Na, 0 - 10 % K

Anorthoclase = 90 - 64 % Na, 10 - 36 % K

Sanadine: Contains varying amounts of Na. Stable at high temperature. Occurs in rapidly cooled rocks.

Orthoclase: Predominately K, minor amounts of Na. Stable at moderate temperatures.

Microcline: Predominately K, minor amounts of Na. Stable at lowest temperatures. Common in the most slowly cooled rock types such as granites and pegmatites.

The chemical formulas of the Plagioclase Feldspars are as follows:

Albite = NaAlSi3O8

Oligoclase = (Na,Ca)(Al,Si)AlSi2O8

Andesine = NaAlSi3O8 - CaAl2Si2O8

Labradorite = (Ca,Na)Al(Al,Si)Si2O8

Bytownite = (NaSi,CaAl)AlSi2O8

Anorthite = CaAl2Si2O8

The chemical formulas of the Alkali Feldspars are as follows:

Albite = NaAlSi3O8

Anorthoclase = ((Na,K)AlSi3O8)

Sanadine = (K,Na)(Si,Al)4O8

Orthoclase and Microcline = KAlSi3O8

Colour:

Albite - White

Oligoclase - Off-white or grey or pale shades of green, yellow or brown

Andesine - White or grey

Labradorite - Grey to smoky black

Bytownite - White, grey or colourless but can be pale shades of other colours

Anorthite - White, grey or colourless

Anorthoclase - Colourless, green, grey, grey pink, yellow

Sanadine - Off-white, yellow or pale shades of other colours

Microcline - Off-white,yellowish, flesh pink, brown or green

Orthoclase - Off-white, yellow, or shades of red, orange to brown

Density (g/cm3) :

Albite - 2.61

Oligoclase - 2.64 - 2.68

Andesine - 2.68 - 2.71

Labradorite - 2.70 - 2.74

Bytownite - 2.74 - 2.76

Anorthite - 2.76

Anorthoclase - 2.57 - 2.6

Sanadine - 2.56 - 2.53

Microcline - 2.5

Orthoclase - 2.53 - 2.56

Crystal Habit:

Albite - triclinic

Oligoclase - triclinic

Andesine - triclinic

Labradorite - triclinic

Bytownite - triclinic

Anorthite - triclinic

Anorthoclase - triclinic

Sanadine - monoclinic

Microcline - triclinic

Orthoclase - monoclinic

Cleavage:

Albite - Perfect in one direction, good in second.

Oligoclase - Perfect in one direction, good in second.

Andesine - Perfect in one direction, good in second.

Labradorite - Perfect in one direction, good in second.

Bytownite - Perfect in one direction, good in second.

Anorthite - Perfect in one direction, good in second.

Anorthoclase - Perfect in one direction, good in second.

Sanadine - Good in 2 directions.

Microcline - Perfect in one direction, good in second.

Orthoclase - Good in 2 directions.