Optical properties of uniaxial minerals

Extremely high relief Colorless, but tends to be grainy in appearance. The extremely high relief means even the tiniest irregularities stand out. Perfect rhombohedral cleavage Twin bands are common (A). These often have moderate but highly mottled color.

Calcite: rhombohedral cleavage Twin bands

Calcite High birefringence = Change in relief

Marble

Calcite vein

dolomite A B C D

Dolomite CaMg(CO3)2 Many of the optical properties of dolomite are similar to those of calcite, including the high birefringence that produces distinctive changes in relief with rotation of the stage (note the differences between A and B, rotated 90 degrees with respect to one another).  The creamy high order white interference colours (C and D) are also identical to those of calcite.  Dolomite occurs  in marbles and carbonatites, but its most common occurrence is in sedimentary carbonate rocks, in which it is of secondary (diagenetic) origin, replacing calcite.  In such situations, dolomite is commonly euhedral to subhedral, occurring as masses of rhombs (A-D) or euhedral crystals projecting into pore spaces.  These textures are not commonly exhibited by calcite, which is typically anhedral.  Samples of dolostones are from the Lockport Formation, Beamsville, Ontario (A, B and C) and the Mid-Ordovician “Basal Beds”, Manitoulin Island, Ontario.  All views are 2.2 mm across.  A, B ppl, C, D x-nicols.

Method for distinguishing calcite from dolomite in thin section Cleavage rhombs Relations between lamellar twinning and cleavage planes in dolomite and calcite. This difference can be discerned best when thin sections of the minerals are viewed under a microscope. Twins in dolomite are parallel to the rhomb diagonal. Twins in calcite are parallel to the rhomb edge. See examples on the next page

A B C D

Calcite and Dolomite Twinning Twin lamellae occur in both calcite and dolomite, especially in rocks that have been deformed, such as marbles.  In calcite, such lamellae can occur both parallel to the rhomb edges (A and B) and parallel to the long diagonal of the rhomb, but not the short diagonal.  In dolomite, however, twin lamellae can occur parallel to both the long and short diagonals of the rhomb.  Twin lamellae parallelling the rhomb are illustrated for calcite in A and B and those parallel to the short diagonal of the dolomite rhombs are illustrated in C and D.  And B are from a phlogopite-bearing calcite marble from Dutchess County, New York and C and D are from a dolomite carbonatite from an unknown location.  All images are 2.2 mm across. A, C ppl, B, D x-nicols.

Nepheline (Na,K)AlSiO4 Plane polarized: Colourless low relief Similar to quartz, K-Feldpar and cordierite

Nepheline Crossed Polarizers: Grey to yellow first order No twinning No cleavage Uniaxial negative Plane Polarized Light

Nepheline It never occurs with quartz, unlike the other minerals shown here. It is most often found in silica poor igneous rocks that are rich in alkali feldspar. Nepheline is uniaxial, but in contrast to quartz is optically negative. It lacks cleavage, distinguishing it from the feldspars.

Tourmaline, (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn6)(BO3)3[Si6O18](OH)4 Hexagonal, R3m stubby prisms (A) hexagonal or triangular cross sections (B) zoned in shades of blue, green, or brown no cleavage negative elongation

zoning along elongation Zoning in Cross section unusual among common elongate minerals = strongest absorption when the plane of polarization is perpendicular to the crystal length Zoning in Cross section

Apatite crystals Ca5(PO4)3(OH,F,Cl) colorless commonly elongated hexagonal end sections  very low 1st order birefringence is obvious. negative elongation extremely common in small amounts in igneous rocks

vesuvianite (idocrase) Ca19(Al,Fe3+)10(Mg,Fe2+)3(Si2O7)4(SiO4)10(O,OH,F)10 Plane polarized light Crossed polars – anomalous colours

corundum Al2O3

Corundum is restricted to silica-undersaturated rocks high in aluminum and high-grade pelitic metamorphic assemblages.  It has very high positive relief relative to feldspars, and is never found coexisting in equilibrium with quartz.  In A, corundum has a thin rim of nepheline, and stands out in positive relief relative to the nepheline and associated feldspars (which are first order grey to white in image B).  Images C and D illustrate subhedral crystals of corundum in a mantle eclogite xenolith (dominated by garnet and altered clinopyroxene).  The typical high positive relief of corundum is not evident in C due to the high refractive index of surrounding garnet and altered clinopyroxene. Twinning, a common feature of corundum, is present in most of the grains shown in C and D.  Corundum is uniaxial and elongate grains have parallel extinction.  The interference colours of first order orange in B and first order yellow in D are just above the maximum of first order white typically expected.  Note that due to the extreme hardness of corundum (9 on Mohs scale), corundum in many thin sections is not ground down to the expected 30 microns of a standard thin section.  A and B are from a corundum syenite from an unknown location and C and D are from a corundum eclogite xenolith the Bobbejaan kimberlite, South Africa.  All views are 5.5 mm across. A and C ppl, B and D x-nicols. 

Leucite KAlSi2O6 Leucite occurs as phenocrysts (commonly euhedral) in alkaline mafic volcanic rocks and less commonly in hyabyssal intrusions.  The coarse, euhedral colourless leucite phenocryst in these figures shows negligible relief relative to plagioclase occurring as inclusions and in the basalt groundmass.  The extremely low birefringence of leucite is apparent in B, in which the characteristic complex lamellar twins are also present.  This sample is a leucitite from Aroiccia, Italy.  Both views are 5.5 mm across.  A ppl, B x-nicols.

quartz SiO2 A B D C

Quartz is extremely common in igneous, metamorphic and sedimentary rocks, and very chemically and physically resistant.  Its relief is similar to that of most compositions of plagioclase, and just slightly higher than K-feldspar.  The general similarities of relief among these minerals is evident in A, in which the quartz stands out relative to the feldspars (plagioclase and microcline), as the latter are cloudy in plane light (due to incipient alteration in this rock), whereas the unaltered quartz is clear, also apparent in C.  Quartz has no cleavage.  Maximum interference colour is first order yellow (“straw” yellow) visible in several grains in D.  Quartz deforms readily and, in some rocks, strained grains of quartz do not have uniform extinction.  Instead, sub-domains of individual grains go extinct in slightly different positions, a feature known as “undulose extinction”, seen in the central grain of quartz in B.  Quartz grains are typically anhedral in most rocks and, although quartz is uniaxial and should have parallel extinction, there is usually no hint of crystal form to use as a reference for measurement of extinction angle.  Samples are granites from unknown localities.  All views are 5.5 mm across.  A and C ppl, B and D x-nicols.

Plane-Polarized Light Very high relief zircon ZrSiO4 Plane-Polarized Light Very high relief Usually clear but may be coloured pleochroic haloes due to radiation damage Crossed Polarizers Very high (third- or fourth-order) interference colors

pleochroic haloes due to radiation damage

rutile TiO2 crossed polars plane polarized light

A B C D

Rutile is strongly coloured in plane light, in shades of yellow and brown, and has extreme positive relief, relative to clinopyroxene in A and plagioclase in C.  It is uniaxial and extinction is parallel to {110} cleavage and sections elongated parallel to the c axis.  Maximum interference colours are high-order white, but are typically masked by the yellow-brown absorption colour.  Grains commonly appear the same in plane polarized light and under crossed nicols (compare A and B). The orientation of the radiating acicular bundles in C and D appears to be, in part, crystallographically controlled by the host plagioclase grain.  A and B are  from an eclogite xenolith from the Moses Rock diatreme, Utah and C and D are in a granite from an unknown location.  All views are 2.2 mm across.  A and C ppl, B and D x-nicols. 

Summary of information that can be obtained about a uniaxial mineral Plane polarized light Colour Pleochroism (if present) Habit Cleavage Crossed polars Bifrefringence Optic sign Sign of elongation (if elongated)