Transmitted Light
is B(+/-), 2V very variable. Triclinic. Long tabular crystals to anhedral grains. Cleavage usually not visible. Polysynthetic twins (looks like a barcode) are typical. Oblique extinction (3-39º). Alters easily. Without twins, it look similar to quartz, nepheline and alkali feldspars.
is B(-), 2V>60º. Monoclinic, in tabular to anhedral crystals. Cleavages rarely visible. Pertithes, Carlsbad twins and others are typical. Almost always altered to clay minerals and sericite, it becomes cloudy. Extinction and ES are not diagnostic. Very common, important rock forming mineral.
is B(-), 2V = 18-63º. Monoclinic. Cleavage usually not visible. Extinction parallel or oblique (5-9º). ES does not apply. Typical are single Carlsbad twins, may have zonation, may be altered. A feldspar typical of acidic volcanic rocks, forming tabular phenocrysts that can be much elongated.
is B(-), 2V = 66-103º. Triclinic. Cleavage usually not visible. Extinction oblique, difficult to obtain due to twinning. ES(-), usually impossible to determine. Typical twins in two perpendicular directions, forming a twinning in a pattern of “parquet” or “chess”, which are intersections of albite and pericline twins.May be zoned. It is the lowest teperature potassic feldspar, abundant in acidic plutonic igneous rocks.
is B(-) with 2V from 42-52º. Triclinic. Occurs as a phenocryst in volcanic and sodic hypabissal rocks. Show complex polysynthetic twins (resembling those of microcline). Antiperthite and zonation are common. Oblique extinction (5o). Easy to confuse with other feldspars.
is B(+/-), 2V = 35-106º. Orthorhombic, pseudo-hexagonal. Usually in anhedral grains; poikiloblastic textures may occur. Cleavage is not visible. Extinction tends to parallel. Polysynthetic and pseudohexagonal twins (triplets) are common. It easily goes unnoticed. Alters to sericita and chlorite and biotite (=pinitization).
is B(+/-), forms lamellae, fibers or masses. It is common as metamorphic mineral or alteration product. Monoclinic. In PPL, it is pleochroic in pale green but may be colorless. In XPL shows anomalous blue (berlin blue), brown (leather brown) or bluish-gray colors. Extinction 0-9º, unmottled(!). ES(+/-). Very small crystals, convergent light usually not possible. The image shows a phenocryst in volcanic rock that has undergone chloritization.
is B(+), 2V = 58º. Perfect {010} cleavage, distinct {100} and {011} cleavages. It occurs in prismatic, tabular to acicular crystals, also anhedral grains. In XPL, gray to white colors. Extinction and ES are not diagnostic. Monoclinic. Typical of sedimentary rocks. Occurs with anhydrite (very colorful in XPL).
is B(-) with 2V from 36-60º. Rare mineral. Triclinic, pseudo-Monoclinic. Columnar, lamellar, acicular. Brush aggregates common. 3 good cleavages. Oblique extinction of 30-44º or almost //, upper 1st order colors, ES(+/-), twins possible, often zoned. Occurs in high grade marbles.
is B(-) with 2V from 71-86º. Orthorhombic. Granular, prisms with square basal section. Perfect {110} cleavage, in basal sections they intersect at 89º. Parallel extinction, ES(-) (important!), gray interference colors. No twinning/zoning. Black inclusions common. Typical of pelitic schists.
Quartz (2V up to 20º), Nepheline (2V up to 6º), Apophyllite (2V small), Cristobalite (2V up to 25º) (very rare!), Beryl (2V up to 6º)(very rare!).
They usually occur as a filling of cavities in volcanic or hypabyssal rocks, some as cement in sedimentary rocks (e.g., sandstones). In volcanic rocks, care must be taken not to confuse feldspathoids (generally isotropic or with very low birefringence (very dark)) with zeolites.
is B(+/-), usually B(+) with 2V from 0-32º; can simulate to be uniaxial. Rare zeolite. Triclinic, pseudo-hexagonal, has rhombic (pseudo-cubic) forms (twinning). Typical of cavities and fractures. Interference colors very dark. Twins are common. Grains are subdivided into sectors.
is B(-), with 2V from 30-49º. Common zeolite, in tabular forms that constitute aggregates (bow tie!). Monoclinic, extinction of 3-12º. ES non-diagnostic. It may have twins, but zonation does not. Typical of cavities and fractures. Also in metamorphic and sedimentary rocks.
is B(-) with 2V from 36-56º. Common zeolite, typical of cavities and fractures in basaltic rocks. Low relief. Monoclinic. Typically long prismatic habit, forms radiated aggregates. Oblique extinction, ES(-), generalized twins, no zonation. Very similar to other zeolites.
is B(+), with 2V from 0-55º (can simulate being uniaxial). Medium relief. Common zeolite. Perfect {010} cleavage, in tabular crystals. Monoclinic. In XPL is gray-white, with extinction of 0-32º. Twins are possible twins. ES(-), without zonation. Typical of cavities in basaltic rocks; also in rhyolites.
is B(+/-), with 2V from 76-104º. Zeolite with typical fibrous habit, from cavities and fractures in acidic and basic volcanic rocks. Orthorhombic. Perfect {100} cleavage, medium relief. In XPL, black and gray colors (diagnostic!). Extinction tends to parallel, non-diagnostic ES, no twins and zonation.
