Minerals In Thin Section


Formula	(K,Na)Ca₄Si₈O₂₀(F,OH) • 8H₂O

Optic class & sign	Uniaxial positive or negative

Relief	Low

Refractive indices	n_o = 1.530 -1.545

	n_e = 1.533 -1.544

	n increases with increasing substitution of OH for F

Birefringence (max.)	0.000 - 0.003

	Anomalous interference colours

Sign of elongation	Length-fast, l (-) in sections ∥ c if crystals are platy; length-slow, l (+) if crystals are elongate in c; reverse relationships apply to optically negative varieties

Interference figure	Low first-order colours in interference figures, broad isogyre cross, no isochromes; anomalous 2V up to 60°

Colour / pleochroism	Colourless

Zoning


Form	Habit	Granular or platy

	Surface	Euhedral to anhedral

Cleavage		{001}

Twinning		{111}, uncommon

Extinction		Straight to {001} and prism faces in sections parallel to c


Reaction textures

Alteration / decomposition	Clay minerals, chert, opal, calcite; apophyllite can be an alteration product of wollastonite


Occurence	Ign

	Met	Marble, calcsilicate rocks; natroapophyllite in skarns

	Sed

	Hyd	Amygdales and veins in basaltic volcanics; also in cavities in granites and aureoles of granititic intrusions; veins in nepheline syenite

	Other


Distinctive properties	Perfect basal cleavage, low relief, very low Δn

Additional comments	Most apophyllite is F-rich and optically positive; changes from positive to negative may occur within single grains. Complete solid solution exists between fluorapophyllite and hydroxyapophyllite, and the few data sets indicate that the refractive indices increase from the most F-rich apophyllite to higher values for hydroxyapophyllite, while delta-n decreases to 0 and then increases again to 0.001, as the optic sign changes from positive to negative. Natroapophyllite is orthorhombic, biaxial positive, with Δn = 0.008.

Apophyllite