Minerals In Thin Section


Formula	(Ca_0.5,Na,K)₄Al₄Si₈O₂₄ • 12H₂O

Optic class & sign	Uniaxial positive or negative

Relief	Moderate-negative

Refractive indices	n_o = 1.460 -1.499

	n_e = 1.462 -1.501

	n decreases with increasing Na

Birefringence (max.)	0.002 - 0.010



Sign of elongation	Not applicable

Interference figure	Indistinct due to zoning, twinning and low birefringence

Colour / pleochroism	Colourless

Zoning	Compositional zoning is common, with variable n


Form	Habit	Rhombohedral (pseudo-cubic), granular

	Surface	Euhedral to anhedral

Cleavage		{1010}, 3 sets

Twinning		Penetration twins on {0001}; {1010} contact twins; segmented basal sections with six sectors of different orientation

Extinction		Symmetrical to {1010} faces and cleavage in sections parallel c


Reaction textures

Alteration / decomposition	Clay minerals, other zeolites; chabazite is itself an alteration product of volcanic glass and Ca-silicates


Occurence	Ign

	Met	Very-low-grade metamorphic mafic rocks of the ‘chabazite subzone’ (lowest-temperature part of the zeolite facies)

	Sed	Lacustrine and marine sediments of pyroclastic origin; authigenic

	Hyd	Vesicle and fracture fillings in basalt; hydrothermally altered rocks

	Other


Distinctive properties	Habit, twinning; common zeolite; intergrowth with other zeolites can make identification problematic Note: Reliable identification of zeolite species may require chemical analysis, X-ray diffraction, and/or DTA

Additional comments	May be optically biaxial with small 2V (up to 40°). Chabazite is also listed as triclinic-pseudotrigonal. The variability of n_O, n_E and the related Δn is not easily presented as a simple binary range in a Δn-n plot, due to the compositional complexity of chabazite. Sr-rich chabazite has refractive indices slightly above the ranges shown here. n-Δn chart: Anl – analcime, Cbz – chabazite, Fau – faujasite, Gis - gismondine, Gme – gmelinite, Hrm – harmotome, Php – phillipsite, Wrk - wairakite

Chabazite