Transmitted Light
is B(+/-), 2V very variable. Triclinic.
Very common rock-forming tectosilicate.
PPL: Colorless, cleavage usually not visible.
XPL: 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.
Images: Plagioclase in PPL and in XPL.
is B(-), 2V>60º. Monoclinic.
Rock-forming tectosilicate, very common.
PPL: Colorless, cleavages rarely visible.Tabular to anhedral crystals.
XPL: Pertithes, Carlsbad twins and others are typical. Almost always altered to clay minerals and sericite, it becomes cloudy. Extinction and ES are not diagnostic.
Images: Orthoclase in PPL and XPL.
is B(-), 2V = 18-63º. Monoclinic.
Common tectosilicate, high-T K-feldspar of volcanic and hypabissal rocks.
PPL: Colorless, cleavage usually not visible. Tabular phenocrysts.
XPL: Extinction parallel or oblique (5-9º). ES does not apply. Typical are single Carlsbad twins, may have zonation, may be altered.
Images: Sanidine in PPL and in XPL.
is B(-), 2V = 66-103º. Triclinic.
Common rock-forming tectosilicate of granitoid rocks.
PPL: Colorless, cleavage usually not visible.
XPL: 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.
Images: Microcline in PPL and in XPL.
is B(-) with 2V from 42-52º. Triclinic.
Less common rock-forming tectosilicate, a K-feldspar of volcanic alkaline rocks.
PPL: Colorless, cleavage not visible.
XPL: Show complex polysynthetic twins (resembling those of microcline). Antiperthite and zonation are common. Oblique extinction (5 degrees).
Images: Anorthoclase in PPL and in XPL.
is B(+/-), 2V = 35-106º. Orthorhombic, pseudo-hexagonal.
Rather rare cyclosilicate, of contact metamorphic rocks.
PPL: Colorless, usually in anhedral grains; poikiloblastic textures may occur. Cleavage is not visible.
XPL: Extinction tends to parallel. Polysynthetic and pseudohexagonal twins (triplets) are common. It easily goes unnoticed, confused with other feldspars and even quartz. Alters to sericite, chlorite and biotite (=pinitization).
Images: Cordierite in PPL and XPL.
is B(+/-). Monoclinic.
Very common as metamorphic mineral or alteration product.
PPL: Pleochroic in pale green but may be colorless. Lamellae, fibers or masses.
XPL: Anomalous blue (berlin blue), brown (leather brown) or bluish-gray colors. Extinction 0-9º, unmottled(!). ES(+/-). Very small crystals, convergent light usually not possible.
Images: phenocryst in volcanic rock that has undergone chloritization.
is B(+), 2V = 58º. Monoclinic.
Most common sulphate, typical of sedimentary rocks.
PPL: Colorless, perfect {010} cleavage, distinct {100} and {011} cleavages. It occurs in prismatic, tabular to acicular crystals, also anhedral grains.
XPL: Gray to white colors. Extinction and ES are not diagnostic. Occurs with anhydrite (very colorful in XPL).
Images: In PPL and XPL, bundle of gypsum crystals (gray) with anhydrite (colorful).
is B(-) with 2V from 36-60º. Triclinic, pseudo-Monoclinic.
Rare mineral of high-grade marbles and some igneous rocks.
PPL: Colorless, columnar, lamellar, acicular. Brush aggregates common. 3 good cleavages.
XPL: Oblique extinction of 30-44º or almost paralell, upper 1st order colors, ES(+/-), twins possible, often zoned.
Images: Wollastonite in PPL and XPL.
is B(-) with 2V from 71-86º. Orthorhombic.
Less common nesosilicate of the Al2SiO5 Group, from metamorphic rocks.
PPL: Colorless, perfect {110} cleavage, in basal sections they intersect at 89º. Granular, prisms with square basal section.
XPL: Parallel extinction, ES(-) (important!), gray interference colors. No twinning/zoning. Black carbon inclusions common. Typical of pelitic schists.
Images: Andalusite in PPL and in XPL.
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 (vesicles and fractures) 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. Triclinic, pseudo-hexagonal, has rhombic (pseudo-cubic) forms (twinning).
Uncommon tectosilicate, occurs as secondary mineral in volcanic rocks.
PPL: Colorless, without cleavage.
XPL: Very dark interference colors very dark. Twinning and sectorizing is common.
Images: Chabazite in PPL and in XPL.
is B(-), with 2V from 30-49º. Monoclinic.
Common tectosilicate, a secondary mineral typical of cavities of volcanic rocks.
PPL: Colorless, tabular forms that constitute aggregates (bow tie!).
XPL: Extinction of 3-12º. ES non-diagnostic. It may have twins, but zonation does not. Easily confused with other zeolites, care must be taken.
Images: Stilbite in PPL and XPL.
is B(-) with 2V from 36-56º. Monoclinic.
Rather common tectosilicate, typical a secondary mineral in basic igneous rocks.
PPL: Colorless, low relief. Typically long prismatic habit, forms radiated aggregates.
XPL: Oblique extinction, ES(-), generalized twins, no zonation. Very similar to other zeolites.
Images: Scolecite in PPL and XPL.
is B(+), with 2V from 0-55º (can simulate being uniaxial). Monoclinic.
Very common tectosilicate, may be present in several geological situations.
PPL: Colorless, medium relief. Perfect {010} cleavage, in tabular crystals.
XPL: Gray-white, with extinction of 0-32º. Twins are possible. ES(-), without zonation. Easy to confuse with other common zeolites.
Images: Heulandite in PPL and in XPL.
is B(+/-), with 2V from 76-104º. Orthorhombic.
Less common tectosilicate, usually of cavities in volcanic rocks.
PPL: Colorless, typical fibrous habit, Perfect {100} cleavage (not visible due to fibrous habit), medium relief.
XPL: Black and gray colors (diagnostic!). Extinction tends to parallel, ES is not diagnostic, no twins and zonation.
Images: Mordenite in PPL and in XPL.
