Feldspars

Alkaline

Sanidine (monoclinic) K[AlSi₃O₈]

	Colour: colourless Pleochroism: non pleochroic Habit: Tabular crystals, massive habit Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
	Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey Twinning: simple, Carlsbad twins (as seen on the left)

Orthoclase (monoclinic) K[AlSi₃O₈]

	Colour: colourless Pleochroism: non pleochroic Habit: Tabular crystals, massive habit Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
	Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey Twinning: simple, Carlsbad twins (as seen on the left)

Microcline (triclinic) K[AlSi₃O₈]

	Colour: colourless Pleochroism: non pleochroic Habit: Tabular crystals, massive habit Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
	Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey Twinning: Tartan twinning (as seen on the left)

Plagioclase (triclinic) (Na,Ca)[Al(Al,Si)Si₂O₈]

Polysynthetic twinned Plagioclase

	Colour: colourless Pleochroism: non pleochroic Habit: Tabular, lath-shaped crystals Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
	Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey Twinning: polysynthetic twins (as seen on the left)

Zoned Plagioclase

	Colour: colourless Pleochroism: non pleochroic Habit: zoned crystals Relief: low positive Cleavage, fracture: perfect, one direction cleavage.
	Isotropic or anisotropic: anisotropic Extinction: inclined Interference colours: 1st order, grey Distinguishing Features: oscillatory zoning (as seen on the left)

Perthites

	Intergrowths of sodium-rich and potassium-rich feldspars are called Perthites. They both exsolved from an originally homogeneous high temperature of intermediate composition.

Feldspathoids

Nepheline (hexagonal) KNa₃[AlSiO₄]₄

	Colour: colourless Pleochroism: non pleochroic Habit: stubby prisms, commonly cross sections are 6 sided Relief: low positive Cleavage, fracture: none, irregular fractures
	Isotropic or anisotropic: anisotropic Extinction: straight Interference colours: 1st order, grey

SiO₂ Group

Quartz ß (trigonal) SiO₂

	Colour: colourless Pleochroism: non pleochroic Habit: usually irregular, barley visible grains Relief: low positive Cleavage, fracture: none
	Isotropic or anisotropic: anisotropic Extinction: straight, undulatory extinction Interference colours: 1st order, grey

Chalcedony

	It is a cryptocrystalline form of Quarz Colour: colourless Pleochroism: non pleochroic Habit: cryptocrystalline aggregate of Quarz, rounded pellets Relief: low positive Cleavage, fracture: none
	Isotropic or anisotropic: anisotropic Extinction: straight Interference colours: 1st order, grey Distinguishing Features: spherical aggregates (as seen on the left)

Opal (amorphous) SiO₂ x nH₂O

	Colour: colourless Pleochroism: non pleochroic Habit: amorphous Relief: low negative Cleavage, fracture: none
	Isotrophic. Therefore it doesn't prove crossed polarised light properties.