Mineral	Formula			Crystal System	Optics			Other Properties
Quartz	SiO2			Hexagonal/rhombohedral	Uniaxial positive			Very common in all rocks
Varieties:	Chalcedony (fine,fibrous crystals) Agate Jasper Chert (Microcrystalline; varieties: flint) Opal* (with interstructural water: a *mineraloid) Color varieties: Amethyst (purple), citrine (yellow) rose (pink); others
Cristobalite	SiO2		Tetragonal			Uniaxial negative	Found in Volcanic rocks
Coesite	SiO2		Monoclinic			Biaxial positive	High P polymorph: meteor impact related
Scapolite	(Na,Ca)4(Al,Si(AlSi2O8)3(Cl,CO3,SO4)		Tetragonal			Uniaxial negative, moderate relief	A metamorphic mineral, formed during high P-T alteration of plagioclase.
Cordierite	(Mg,Fe)2Al4Si5O18		Orthorhombic			Biaxial postive or negative; Moderate relief	Common metamorphic mineral in pelitic rocks
Sodalite	Na3Al3Si3O12•NaCl		Isometric			Isotropic	Distinctive blue color - often found with nepheline and cancrinite (compositionally between the feldspathoids and zeolites)
Zeolite Group Minerals (all hydrated tectosilicates found in sedimentary and low-grade metamorphic rocks)
Natrolite	Na2Al2Si3O10•2H2O	Orthorhombic				Biaxial positive; low relief	Needle-like, radiating crystals; low grade metamorphism of basalts
Stilbite	NaCa2Al5Si13O36•14H2O	Monoclinic				Biaxial negative; low relief	Platy, diamond-shaped crystals; commonly found with natrolite
Chabazite	Ca2Al2Si4O12•6H2O	Hexagona/rhombohedral				Uniaxial negative: low relief	Pseudocubic crystals; reddish to white in color. Found with natrolite and stilbite