Tridymite


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Related to Tridymite: cristobalite

tridymite

[′trid·ə‚mīt]
(mineralogy)
SiO2 A white or colorless crystal occurring in minute, thin, tabular crystals or scales; a high-temperature polymorph of quartz.

Tridymite

 

a mineral that is one of the crystalline polymorphs of SiO2. Such trace elements as Fe, Al, and Na are commonly found in tridymite. It crystallizes in the form of colorless or gray tabular crystals, spherical aggregates, spear-shaped twins, and, often, trillings. It is a rare mineral and is usually encountered in cavities of juvenile extrusive acidic rocks and in tuffs; it is also found in meteorites and moon rocks. As a constituent of Dinas brick, tridymite determines the refractory properties of the brick.

References in periodicals archive ?
CheMin analyzed drilled material from a target called "Big Sky" in bedrock away from a fracture and from a fracture-zone target called "Greenhorn." Greenhorn indeed has much more silica, but not any in the form of tridymite. Much of it is in the form of noncrystalline opal, which can form in many types of environments, including hot springs, acid leaching and other wet settings.
The presence of calcite, dolomite, silica (quartz, tridymite, cristobalite), apatite, clay minerals (smectite and smectite-illite) and rare sulfides in the matrix of natural OS, as well as in the IR of natural oil shale and burning test ash is interpreted from the results of XRD and SEM-EDS analyses.
In the mudstone, the < 5 m[micro] calcite fines are in different proportions mixed with < 5 m[micro] silica fines, usually with quartz, but in layers C and D partly also with tridymite and cristobalite, locally with more clay minerals, and, possibly, with roentgen-amorphous silica fines.
Although very effective in iron castings, the tridymite phase change and resulting increase in volume occur at too low of a temperature to prevent veining in steel castings.
Sand additives that reduce veining defects provide liquid on the surface of the sand grain and favor formation of tridymite or cristobalite and greater expansion of the sand.
Typical high temperature non magnetic minerals found in pyrometamorphic rocks associated with coal combustion appear to include several Fe and Al rich clinopyroxene, melilita, cristobalite, tridymite, mullite, cordierite and fayalite solid solutions, as well as glass (Foit et al., 1987; Cosca et al., 1989; Radan & Radan, 1998).
Associated minerals are: hematite, cassiterite, quartz, tridymite, cristobalite, opal, calcite, "zeolites," mimetite and unidentified clay minerals.
The major minerals present in the as received sample are calcite, quartz, tridymite, apatite and traces of some clay minerals (see Fig.
From Figure 1A it can be seen that raw oil shale sample contains calcite, quartz, apatite and tridymite (Si[O.sub.2]) as major phases.
Hold times are generally applied at 1600-2000F (870-1093C) for 2 hr to accommodate the large volume expansion (12%) during the transformation from beta quartz to tridymite. This volume expansion of the refractory reduces the open porosity, thus reducing the potential for molten metal penetration.
This final hold allows for the transformation from tridymite to cristobalite.
The crystallization process of opals goes through stages: first, open-volume crystals of tridymites and crystoballites are formed which are later replaced by more compact quartz.