Transmitted Light
Tetragonal, pseudo-cubic rock-forming tectosilicate (feldspathoid).
PPL: Relief very low, colorless, cleavage not visible. Crystals roundish or with 8 sides, 2 generations are possible. Many inclusions are typical.
XPL: Very low birefringence (very dark). Always polysynthetic twinning (looks similar to microcline, but is much darker and more irregular.). U(+), may be B(+).
Images: Leucite in PPL and in XPL.
Cubic tectosilicate (feldspatoid), rather rare, of sodic volcanic rocks.
PPL: Relief very low. Colorless to blue. Visible cleavage is possible. Cubic, euhedral grains, octahedrons, sections with 6, 8 or 10 sides.
XPL: Completely isotropic (black). Black borders or zonation of black inclusions is possible. Alters to zeolites and clay minerals. Similar to sodalite, nosean, leucite and analcime.
Images: Haüyne in PPL and in XPL.
Cubic tectosilicate (feldspathoid), rather rare, of Si-low igneous rocks.
PPL: Relief low,. Colorless, rarely gray. Cubic. Cleavage usually not visible. Sections with 6, 8 or 10 sides, but normally as anhedral interstitital masses. Pseudohexagonal twins possible. Altered borders are possible.
XPL: Isotropic (completely black), may even be confused with a hole in the thin section!
Images: Sodalite (center) surrounded by feldspars, in PPL and in XPL.
Tetragonal, pseudo-cubic very rare tectosilicate, of volcanic rocks.
PPL: Relief very low. Colorless. Easily goes unnoticed. No cleavage. Never alters. Dendritic, skeletal or fibrous spherulites.
XPL: U(-), almost isotropic (black to very dark gray), may show anomalous 2V of up to 25º.
Images: not yet available.
Cubic tectosilicate (feldspatoid) of alkaline volcanic rocks.
PPL: Relief very low. Colorless, gray or pale bluish. Euhedral grains, pseudo-hexagonal ou rounded. Cleavage not visible. “Haze” of inclusions in sectors, in the center or at the edges.
XPL: Isotropic (completely black), inclusions may simulate weak birefringence. Easy to confuse with analcime, leucite, sodalite and haüyne.
Images: Nosean in PPL and in XPL.
Symmetry-complex uncommon tectosilicate of the Zeolite Group.
PPL: Relief very low. Colorless. Cleavage not visible. Anhedral or 8-sided sections. As phenocrysts or in cavities, may show concentric or sectorized structure. Twins rare.
XPL: Isotropic or very dark gray color. May be anomalously B(-). Somewhat similar to leucite.
Images: Analcime with twinned calcite in PPL and in XPL.
Amorphous (or not so!) very common tectosilicate (Dana, 8th ed.).
PPL: Relief very low. Colorless, yellow or red. Fills vesicles, fractures and interstitial spaces. Massive, reniform or botrioidal, perfect spheres are possible. No cleavage, but contraction fractures may occur.
XPL: Isotropic, but incipient recrystallization to chalcedony is possible. May be "dirty", brownish, due to included clay minerals.
Images: Opal in PPL and in XPL.
Very common amorphous rock-matrix forming material of lavas and sub-volcanic rocks.
PPL: Relief low. Colorless or yellow or brown or red. No cleavage. May show perlitic (concentric) fractures, devitrification crystallites and/or spherulites, pseudo-flow structures and shard textures.
XPL: Completely isotropic if without incipient recrystallization.
Images: Red volcanic glass with a plagioclase crystal in PPL and XPL.
