Tridymite may occur as a primary magmatic phase (i.e., as a direct result of crystallization from a silicate melt) in siliceous rocks but is most abundant in voids in volcanic rocks where it probably was deposited metastably from hydrous gases.
Diopside, common silicate mineral in the pyroxene family that occurs in metamorphosed siliceous limestones and dolomites and in skarns (contact-metamorphic rocks rich in iron); it is also found in small amounts in many chondrite meteorites.
Diopside results from the thermal metamorphism of siliceous limestones or dolomites according to the following decarbonation reaction: In calc-silicate skarns produced by contact metamorphism, diopside is associated with wollastonite, vesuvianite, grossular, and tremolite.
The deposition of siliceous sinter is largely due to the action of algae and other forms of vegetation in the thermal waters; near fumaroles and in the deeper channels of hot springs, it may be formed by alteration of wall rocks.
The oozes are subdivided first into calcareous oozes (containing skeletons made of calcium carbonate) and siliceous oozes (containing skeletons made of silica) and then are divided again according to the predominant skeleton type.
The siliceous glass sponges of the class Hexactinellida are found in deep water and include such spectacular forms as the glass rope sponge (Hyalonema) and the Venus's flower basket (Euplectella), whose beautiful lattice-like skeleton was highly prized as a collector's item in Victorian times.
It occurs in quantity in firebricks and other siliceous refractories.
Rapid precipitation of fine-grained silica results in formation of siliceous sinter or geyserite, as at Mammoth Hot Springs in Yellowstone National Park in the western United States.
In the equatorial zone, an area of high oceanic productivity, calcareous and siliceous oozes are abundant.
Below 4,000 m, calcium carbonate tends to dissolve, and the principal sediment constituents are brown clays and the siliceous remains of radiolarian zooplankton and such phytoplankton as diatoms.
