Other Biaxial Minerals

B A Gypsum CaSO4 · 2H2O Gypsum is common in sedimentary rocks, especially in marine evaporite deposits.  It is colourless (A) and has low negative relief relative to epoxy and moderate negative relief relative to anhydrite, with which it commonly occurs.  It is typically anhedral, but tabular euhedral grains are elongate parallel to the c axis.  Gypsum has perfect {010} cleavage and good {100} cleavage (A), and has maximum interference colours of first order white to straw yellow, similar to quartz.  It is monoclinic and has inclined extinction.  Sample is from an unknown location.  Both views are 2.2 mm across.  A ppl, B x-nicols.

A B Anhydrite CaSO4  Anhydrite is a common evaporite mineral and is also known from hydrothermal ore deposits.  It is usually colourless in thin section (A) and has moderate positive relief relative to gypsum, for example, with which it is commonly associated.  Anhydrite typically occurs in aggregates of anhedral to blocky or fibrous crystals.  It is orthorhombic and  has three mutually perpendicular cleavage directions, which range from good to perfect (A and B).  Extinction is parallel to these cleavages.  Maximum interference colours are third order green (B) and simple or multiple twins on {011} are common (B).  Sample is from the Faraday Mine near Bancroft, Ontario.  Both views are 5.5 mm across.  A ppl, B x-nicols.

Barite BaSO4 Barite is a common accessory mineral in hydrothermal veins associated with sulphide ore deposits, and as a precipitate in sedimentary rocks such as limestones.  It usually occurs as tabular crystals, forming radial aggregates in some cases, that are colourless in thin section (A).  The moderately high positive relief of barite is evident relative to three fluorite crystals and the epoxy-filled void in A.  The “dusty” appearance of barite in this example is due to an abundance of tiny fluid inclusions.  Barite has four cleavage directions, including perfect {001} cleavage.  The maximum interference colour is first order yellow, and barite goes extinct parallel to {001} (the long dimensions in these images).  Sample is from a fluorite - barite vein from the Noyes fluorite mine near Madoc, Ontario.  Both images are 2.2 mm across.  A ppl, B x-nicols.

Olivine The name ‘olivine’ refers to a range of compositions between Mg2SiO4 (forsterite) and Fe2SiO4 (fayalite), which form a solid-solution.

Olivine compositions No compositions exist here

Plane-Polarized Light Moderately high relief Clear, occasionally very light yellowish or greenish No cleavage Commonly rimmed with greenish/brown alteration products (A) Internal fracturing of grains common (B) Never occurs with quartz Crossed Polarizers Bright second-and third-order interference colors. Alteration products tend to have lower interference colors.

Olivine in dunite

Olivine, phenocryst in a basalt.  fractures concentric with the crystal margin birefringence generally in the second and third order

The aluminium silicates : kyanite, andalusite and sillimanite Al2SiO5 Al2SiO5 ???? Why this formula for a silicate with isolated tetrahedra? Could be written Al.AlO(SiO4) because there are two different Al structural sites as well as isolated [SiO4] tetrahedra. Kyanite, andalusite and sillimanite are polymorphs, because they have the same chemical composition but can exist with different crystal structures.

Kyanite first-order interference colours prismatic habit of crystals

Kyanite Colorless to pale blue high relief strong cleavage parallel to their length.  Relief higher than muscovite, which surrounds the kyanite. Interference colors up to upper 1st order, much lower than muscovite Most sections: slightly inclined extinction; max. 30° Positive elongation

Sillimanite coarse sillimanite prisms with diamond-shaped cross sections “robust sillimanite” lower second order blue birefringence in longitudinal sections higher than kyanite and much higher than andalusite

Silimanite Colourless Sillimanite crystals   occurs as fibers in medium-grade rocks “fibrolite”

andalusite Al2SiO5 A B C D

Andalusite is the low pressure aluminosilicate polymorph that occurs in some contact metamorphic rocks and low-pressure regional metamorphic terranes.  It commonly occurs as elongate square or cross-shaped prisms, and has two good prismatic {110} cleavages, visible in A and B.  It is typically colourless but some examples have a pale pink to colourless pleochroism, seen in the sector-zoned crystal in C.  Andalusite grains with trails of inclusions in a cruciform pattern are termed “chiastolite” (A - C).  Andalusite has moderate positive relief relative to quartz, feldspar and muscovite, which is apparent in D, in which the margin of the andalusite grain has been replaced by muscovite + quartz + biotite.  The relief of the grain in A is not obvious due to the surrounding opaque minerals.  Maximum interference colours are usually first order grey to white.  Extinction is parallel to traces of {110} cleavage in elongate sections and parallel to the arms of the chiastolite cross in sections cut perpendicular to the c axis.  Samples from unknown locations.  All views are 5.5 mm across.  A, C and D ppl, B x-nicols.

chloritoid (Fe,Mg)2Al4O2(SiO4)2(OH)4 B C D

Chloritoid forms in Fe-rich metapelites of low to medium metamorphic grade. It has high positive relief relative to quartz and feldspars and is pleochroic in shades of blue-green (A and B, rotated 90 degrees relative to one another).  It commonly forms platy porphyroblasts that are rectangular in random section, and can have an hour glass arrangement of inclusions.  Interference colours typically range to a maximum of pale first order yellow, and simple and lamellar {001}twinning is common (C, D).  Interference colours may be anomlaous due to strong absorption.  Sample is from an uknown location. All views are 5.5 mm across.  A, B, C ppl, D x-nicols.

Titanite (sphene) CaTiSiO5 x-nicol wedge-shaped crystals extremely high relief extremely high interference colors ppl

double wedge or diamond shape At high magnification, magenta bands indicate interference order.  Here: 6th order

Plane-Polarized Light Very high relief Often brownish or pinkish Wedge- or rhomb-shaped crystals common Crossed Polarizers Extremely high birefringence - ultra-high-order whites result in little change in appearance.

Epidote high relief high-order interference colors

Epidote Mica in igneous rocks has rather high Fe3+ content, and is therefore pistacite The higher the Fe3+ content = the higher the birefringence.  birefringence is 2nd and 3rd order can resemble olivine, but epidote occurs with quartz Epidote Mica Ca2(Al,Fe3+)Al2O(SiO4)(Si2O7)(OH)

Cordierite Plane-Polarized Light Commonly shows feathery alteration veinlets (A). Pleochroic yellow halos around inclusions of zircon are common (B). Iron-hydroxide alterations along cracks (C). Often has numerous spinel inclusions (D). Poor cleavage Crossed Polarizers In relief and interference color, looks much like quartz and K-feldspar. Biaxial with large 2V angles (can be positive or negative)

staurolite Fe2Al9O6(SiO4)4(O,OH)2 B D C

Staurolite is a common mineral in pelites of medium metamorphic grade Staurolite is a common mineral in pelites of medium metamorphic grade.  Views A and B (at right angles to each other) illustrate the pale yellow to yellow-orange pleochroism of staurolite, which has high positive relief relative to the quartz-muscovite matrix and quartz inclusions.  The two adjacent subhedral poikiloblastic grains in C with different orientations show different shades of yellow, and in D illustrate first order yellow interference colours, near the maximum for this mineral.  A and B are from an unknown location and C and D are from the Odjick Formation, Wopmay Orogen, N.W.T., Canada.  All views are 5.5 mm across.  A, B, C ppl, D x-nicols.