6) VEINS, LOW T HYDROTHERMAL, HOT SPRINGS, ZEOLITES

6) Veins, low T hydrothermal, hot springs, zeolites

Halides
fluorite	CaF₂	1) hydrothermal veins, 2) secondary in carbonates with ore minerals, 3) accessory mineral in limestone, 4) accessory mineral in igneous and metamorphic rocks			calcite, galena, barite, pyrrhotite, pyrite, quartz
Zeolite group (tectosilicates)
natrolite	hydrous Na, Al zeolite		1) forms in cracks or on the walls of cavities in mafic igneous rocks	calcite, epidote, chlorite, and other zeolites
Chabazite (a hydrous Ca, Al zeolite) and stilbite (a hydrous Na, Ca, Al zeolite) are two other common zeolites.
Mineraloids
(opal)	SiO₂-nH₂O (amorphous)	1) hot springs and low-temperature fluid deposits, 2) replacement of organic material, 3) accumulations of diatoms			low temperature minerals usually associated with meteoric water and hydrothermal solutions
Opal is amorphous, but composed of ordered piles of silica spheres. There are many varieties of opal ranging from precious opal (brilliant internal play of colors in red, orange, green, or blue) and fire opal (intense orange to red), to less attractive varieties including common opal (commonly pale colored without internal reflections), hyalite (colorless, translucent opal with a globular or botryoidal surface), geyserite or “siliceous sinter” (white to gray with a dull luster, deposited by hot springs), wood opal (fossil wood with opal as the petrifying substance), and diatomite (resembles chalk, but composed of diatoms whose shells are opal).