Transmitted Light
it may be a mica (biotite, muscovite, phlogopite, lepidolite, vermiculite) or talc or pirophyllite or brucite.
It is a trigonal carbonate with typical “relief pleochroism” or “chagrin change". May be calcite, dolomite, siderite, magnesite or rhodocrosite. If with paralell extinction, it is aragonite.
It is NOT possible to differentiate these carbonates from each other under the petrographic microscope.
Orthorhombic carbonate: relief that changes from low to very high during the rotation of the stage (“relief pleochroism” or “chagrin change”).
PPL: Colorless, Acicular crystals in radial aggregates or prisms with only 1 cleavage.
XPL: 9th order cream colors. Diagnostic parallel extinction! SE(-), B(-) (2V of 18º). Occurs in basic rocks.
Images: Aragonite in PPL and in XPL.
Very common. In volcanic rocks, it is hexagonal, octagonal or skeletal. In igneous/metamorphic rocks, anhedral. Orthorhombic.
PPL: Colorless, no cleavage. Many fractures with alteration products (gray = serpentine; red = iddingsite).
XPL: Strong and vivid colors up to 3rd order. Paralell extinction. ES(+/-). B(+/-), 2V = 46-98º.
Images: Olivine in PPL and in XPL (with blue as one of the possible colors!).
Common orthorhombic sulphate typical of evaporitic sequences.
PPL: Colorless. may be pleochroic in pale colors. Low relief, 3 perfect cleavages. Anhedral, granular, massive to fibrous.
XPL: Extinction parallel to cleavages. Intense interference colors up to 3rd order green, can show twins, ES(+/-), B(+), 2V = 36-45º. Occurs associated with gypsum (XPL=gray).
Images: Anhydrite in PPL and in XPL.
Rare orthorhombic amphibole exclusive of metamorphic rocks.
PPL: Colorless, may be pale green or pale yellow. Basal sections with 2 cleavages at 54.5º and 125.5º. Asbestiform, fibrous to lamellar.
XPL: Extinction paralell (diagnostic!), in basal sections symmetrical. 1st and 2nd order colors. ES(+), no twins or zoning, B(+) or B(-).
Images: Anthophyllite in PPL and in XPL.
Rare hexagonal feldspathoid with low relief, usually occurs as small interstitial anhedral grains in alkaline igneous rocks.
PPL: Colorless, one perfect cleavage.
XPL: Colors from upper 1st order to mid 2nd order. ES(-) by cleavage, no twins or zoning, U(-) or anomalous B(-).
Images: Cancrinite in PPL and in XPL.
Very rare monoclinic pyroxene, typical of high pressure, medium temperature metamorphic rocks.
PPL: Colorless, basal sections with 2 cleavages intersecting at 56-124º. Anhedral, subhedral, fibrous.
XPL: Colors up to middle of 2nd order. Oblique extinction (33-40º), simple twins possible. B(+), 2V=67-80º.
Image: Jadeite in XPL.
Rare monoclinic or triclinic phyllosilicate, of Al-rich metamorphic rocks.
PPL: Very small tabular subhedral flakes or spherulitic aggregates of acicular crystals. One perfect cleavage.
XPL: Intense 3rd and 4th order colors. Extinction parallel and mottled, ES(+), no twins, no zoning, B(-), 2V = 53-62º. Very similar to muscovite!
Images: Pyrophyllite in PPL and in XPL.
Rare orthorhombic inosilicate (Strunz) or phyllosilicate (Dana) typical of cavities in basic and intermediate volcanic rocks.
PPL: Tabular, prismatic or radiated, can be in bow-tie patterns. Medium relief. Good {001} cleavage.
XPL: 2nd order or anomalous colors (blue or brown). Extinction parallel, undulating or in chess pattern. ES(+/-). Twins rare, zoning possible. B(+), 2V from 64-71°.
Images: Prehnite in PPL and in XPL (in yellow, many other colors are possible).
