Minerals In Thin Section


Formula	Ca₂Al₄Si₄O₁₆ • 9H₂O

Optic class & sign	Biaxial negative

Optical orientation	X at a high angle to a, Y = b, Z at a high angle to c

Optical plane	(010)

Relief	Low-negative to low-positive

Refractive indices	n_x = 1.525 -1.540

	n_y = 1.531 -1.544

	n_z = 1.541 -1.550

	n decreases with substitution of Ca by Na and K

Birefringence (max.)	0.010 - 0.016

	Δn decreases with substitution of Ca by Na and K

Optic Angle	2V_x = 82 - 86°

	2V_z

Sign of elongation

Interference figure

Colour / pleochroism	Colourless

Zoning	-


Form	Habit	Dipyramidal crystals, radiating aggregates; twinning results in octahedra-like pseudo-tetragonal or pseudo-orthorhombic forms; cross-sections showing two pairs of sectors with different optical-orientations

	Surface	Euhedral to subhedral

Cleavage		{232}

Twinning		Penetration twins; “fourlings” with twin planes approximately {100} and {001}

Extinction		In principle, symmetrical to crystal faces in sections orthogonal to c or a. Not readily evaluated due to complex twinning and patchy extinction.


Occurence	Ign

	Met

	Sed	Vesicle fillings in silica-undersaturated volcanics, such as nepheline basalt, leucitic basalt, olivine basalt and leucite tephrite; miarolitic and other cavities in altered granite and nepheline syentite; hydrothermal veins

	Hyd

	Other


Distinctive properties	Habit, comparatively high n and Δn in relation to other blocky zeolites; uncommon zeolite Note: Reliable identification of zeolite species may require chemical analysis, X-ray diffraction, and/or DTA

Additional comments	Crystal-optical relationships shown here are based on Nawaz (1980), Min. Mag. 43, 841-844. n-Δn chart: Anl – analcime, Cbz – chabazite, Fau – faujasite, Gis - gismondine, Gme – gmelinite, Hrm – harmotome, Php – phillipsite, Wrk - wairakite

Gismondine