Minerals In Thin Section


Formula	(K,Na)AlSi₃O₈

Optic class & sign	Biaxial negative

Optical orientation	a near X, b = Z, c near Z

Optical plane	Orthogonal to (010)

Relief	Low negative

Refractive indices	n_x = 1.514 -1.526

	n_y = 1.518 -1.530

	n_z = 1.521 -1.533

	n increases with increasing Na

Birefringence (max.)	0.005 - 0.008

	Δn increases with increasing Na

Optic Angle	2V_x = 35 - 50°, higher for cryptoperthitic varieties above 20 mol% Ab

	2V_z

Sign of elongation	Length-fast, l (-), for grains elongate in a

Interference figure	Diffuse isogyres on a white low-Δn background

Colour / pleochroism	Colourless; may be clouded by alteration minerals

Zoning


Form	Habit	Granular to roughly tabular, elongate ∥ c or a

	Surface	Commonly anhedral; adularia may be euhedral

Cleavage		{001} perfect, {010} distinct, intersecting at about 90°

Twinning		Simple {010} Carlsbad twins (twin axis [001]) are most common

Extinction		Inclined, aɅx = 6 - 14° (a = trace of {001} cleavage) in sections parallel to (010); extinction is composition-dependent, with higher angles relating to higher Na content. Compositional zoning is commonly not developed in orthoclase.


Occurence	Ign	Felsic plutonic rocks; compositions intermediate between Or and Ab occur in silica-poor rocks like syenite.

	Met	High-grade gneisses and granulites

	Sed	Detrital in sediments and sedimentary rocks, particularly in immature clastic deposits; authigenic in diagenetic environments

	Hyd

	Other


Distinctive properties	Low negative n, low Δn, simple twins if present, evidence for unmixing where microscopically identifiable. Tendency to be turbid due to alteration.

Additional comments	Unmixing of albite from potassic alkalifeldspar is common. Albitic Plg may form stringers or blebs inside the host crystal (‘perthite’). Most orthoclase in igneous rocks is perthitic, even if only sub-microscopically. The diagram below shows the relationship between 2V_X, composition and Al-Si order/disorder for alkalifeldspars.

Orthoclase