Alkali feldspars | SSHADE

Mineral phase

IMA name: Alkali feldspars

Name and identifiers

Secondary names: Potassium feldspars, K-feldspars

Class: silicate mineral
Compound type: tektosilicate
Comments: K-Na series of Feldspar

Compositional phase diagram of the different minerals of the feldspar solid solution (Wikipedia - By Muskid - Own work, CC BY-SA 4.0)

Chemical composition

Formula: $(K,Na,Ca)(Al,Si)_4O_8$ - Ca < 0.1-0.3
Chemical formula: (K,Na,Ca)(Al,Si)4O8
Elemental formula: (K)0-1 (Na)0-1 (Ca)0-0.3 Al Si3 O8
Isotope mixture type: terrestrial abundance

	Formula	Name	Family	Number min-max	Isotope mole fraction	State	Relevance
	$K$	Potassium	element	0 - 1		constituent element	main
	$Na$	Sodium	element	0 - 1		constituent element	main
	$Ca$	Calcium	element	0 - 0.3		constituent element	main
	$Al$	Aluminium	element	1		constituent element	main
	$Si$	Silicon	element	3		constituent element	main
	$O$	Oxygen	element	8		constituent element	main

Composition comments: Ca has limited solid solution - Common replacements: Ba - Common impurities: Mg, Fe, Rb, H2O

Oxides composition

Oxides

Formula	$K_{2}O$	$Na_{2}O$	$CaO$	$Al_{2}O_{3}$	$SiO_{2}$
Mass fraction (%)

Classifications

Strunz

Class: silicates (09)
Division: tektosilicate without zeolitic H2O (09.F)
Family: without additional non-tetrahedral anions, (Al,B):Si = 1:1 (09.FA)
Code: 09.FA.30

Dana

Major class: silicates (VIII)
Class: tectosilicate Al-Si framework (76)
Type: with Al-Si frameworks (76.01)
Group: N/A (N/A)

Endmembers and polymorphs

End-members

Members

Orthoclase (Na (Ab) = 0-10% - Ca = 0-10% - K (Or) = 90-100% (monoclinic))
Microcline (Na (Ab) = 0-10% - Ca = 0-10% - K (Or) = 90-100% (triclinic))
Sanidine (Na (Ab) = 10-64% - Ca = 0-10% - K (Or) = 36-90% (monoclinic))
Anorthoclase (Na (Ab) = 64-90% - Ca = 0-10% - K (Or) = 10-36% (triclinic))
Albite (Na (Ab) = 90-100% - Ca = 0-10% - K (Or) = 0-10% (triclinic))

Crystallography

Phase type: crystalline
Crystal system: various
Crystal class: various
Crystal class symbol: various
Crystal spacegroup: various

Physical properties

Molar mass: 263.02 - 278.33 $g/mol$
Density: 2.56 - 2.63 $g/cm^3$

Thermodynamical properties

State NTP: solid

Optical properties

Refringence type: biaxial
Refringence sign: positive or negative
Birefringence: 0.005 - 0.010
Refraction index $n_{\alpha}$: 1.518 - 1.533
Refraction index $n_{\beta}$: 1.522 - 1.537
Refraction index $n_{\gamma}$: 1.523 - 1.542
Dispersion V: weak to relatively strong
Color: colorless, white, gray, greenish, bluish green, grayish yellow, white, pink
Diaphaneity: transparent to subtranslucent
Luster: vitreous

Links and Publications

Links