Transmitted Light
Relief very low, colorless on PPL, cleavage not visible. Tetragonal, pseudo-cubic. Crystals roundish or with 8 sides, 2 generations are possible. Many inclusions are typical. In XPL, very low birefringence (very dark). Always polysynthetic twinning (looks similar to microcline, but is much darker and more irregular.). U(+), may be B(+).
Relief very low. In PPL colorless to blue. Visible cleavage is possible. Cubic, euhedral grains, octahedrons, sections with 6, 8 or 10 sides. In CPL completely isotropic. Black borders or zonation of black inclusions is possible. Alters to zeolites and clay minerals. Similar to sodalite, nosean, leucite and analcime.
Relief low, in PPL 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. In XPL, may be confused with a hole in the thin section! In the image, sodalite (center) surrounded by feldspars.
Relief very low. Rare mineral, typical of cavities of acid and intermediate volcanic rocks. Easily goes unnoticed. Tetragonal, pseudo-cubic. In PPL, colorless. No cleavage. Never alters. Dendritic, skeletal or fibrous spherulites. In XPL, almost isotropic, may show anomalous 2V of up to 25º.
Relief very low. Cubic, in PPL 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. Typical of alcaline volcanic rocks, associated with nepheline, leucite and sanidine. Easy to confuse with analcime, leucite, sodalite and haüyne.
Relief very low, in PPL colorless. Cleavage not visible. Several crystal systems. Anhedral or 8-sided sections. As phenocrysts or in cavities, may show concentric or sectorized structure. Twins rare. In XPL, isotropic or very dark gray color. May be anomalously B(-). Somewhat similar to leucite. In the image, analcime with twinned calcite.
Relief very low, in PPL colorless, yellow or red. Fills vesicles, fractures and interstitial spaces. Amorphous. Massive, reniform or botrioidal, perfect spheres are possible. No cleavage, but contraction fractures may occur. In XPL, isotropic, but incipient recrystallization to chalcedony is possible. May be "dirty", brownish, due to included clay minerals.
Obsidian. Relief low. In PPL colorless, yellow, brown or red. No cleavage. Amorphous, forms the matrix of the rock. May show perlitic (concentric) fractures, devitrification crystallites and/or spherulites, pseudo-flow structures and shard textures. In XPL, completely isotropic if without incipient recrystallization. Only in volcanic and subvolcanic rocks. In the image, red volcanic glass with a plagioclase crystal.
