TRIDYMITE
Tridymite – SiO2 – is a rare and difficult to identify tectosilicate, typical of acid volcanic rocks. It has no economic importance.
Tridymite is one of the polymorphs of SiO2, with quartz, β-quartz, cristobalite, coesite and stishovite. It is a high temperature, low pressure polymorph. It has 7 crystalline forms: 2 hexagonal, 3 orthorhombic and 2 monoclinic. β-tridymite is hexagonal at T>1470ºC and α-tridymite is monoclinic at T<1470ºC. The literature designates tridymite as triclinic below 100ºC and assigns it hexagonal Miller indices due to its pseudo-hexagonal shape.
It usually occurs as very small, white to colorless, pseudohexagonal tabular crystals. It is an indicator of rock formation temperature: below 870ºC beta quartz is stable and crystallizes. From 870ºC to 1470ºC, tridymite is formed. Above 1470ºC, cristobalite is formed. As an igneous rock goes through all these temperatures throughout its cooling process, all 3 phases can be present.
1. Characteristics
Crystal system: See above
Color: Colorless, white, yellowish white or gray.
Habit: Tabular, pseudohexagonal plates.
Cleavage: {0001} poor, {1010} imperfect.
Tenacity: Brittle.
Twinning: Single, multiple or “trillings”, contact or interpenetration.
Fracture: Conchoidal.
Mohs Hardness: 6.5 – 7
Parting: No.
Streak: White.
Lustre: Vitreous, pearly.
Diaphaneity: Transparent.
Density (g/cm³): 2.25 – 2.28
2. Geology and Deposits
Tridymite normally occurs in acid volcanic rocks, in cavities, in lythophysae (cavities filled with concentric layers of various minerals) and as phenocrysts. It is difficult to observe and can rarely be seen on a thin section.
It can occur in basalts.
It also occurs in sandstones that have undergone contact metamorphism, in cornubianites and in meteorites.
3. Mineral Associations
It occurs with cristobalite, quartz (including amethyst variety), potassic feldspar (sanidine), micas (phlogopite), calcite, pyroxenes (enstatite, augite), hematite, pseudobrookite, osumilite, fayalite (olivine), “hornblende” and troilite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.468 – 1.482 nβ: 1.470 – 1.484 nγ: 1.474 – 1.486
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Very low.
Cleavage: {0001} poor, {1010} imperfect.
Habits: Thin tabular, thin lamellar, in cavities form clusters of hexagonal lamellae. Radiated aggregates, in the form of rosettes or stacked like tiles. Common and diagnostic twins (see below). Also as anhedral grains. Crystals of no more than 1 cm, always pseudohexagonal.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence from 0.002 to 0.006, corresponding to first-order interference colors: dark gray to white.
Extinction: No information available.
Elongation sign: No information available.
Twins: Paramorphs commonly have contact twins in {1016}, single, trillings (3 lamellae side by side, hence the mineral’s name) or multiple. Contact or interpenetration twins according to {3034} also occur.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 40 – 86º
Alterations: does not alters, it is an important diagnostic aspect to differentiate it from plagioclases (albite).
May be confused with: shows dark red fluorescence under shortwave UV light.
Quartz, albite and cristobalite are very similar. Quartz is U(+), has higher interference colors, and has no twins. Cristobalite is uniaxial.
Albite and plagioclases in general may be altered (clay minerals, sericite). The tiny size of the tridymite crystals makes these findings difficult.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of tridymite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with tridymite