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12 Opaque Oxides
12.1 Ilmenite
FeTiO3
Occurrence—Ilmenite is a common accessory mineral that is found in many different kinds of igneous rocks, including pegmatites. It is also a common accessory mineral in diverse metamorphic rocks.
Distinguishing Features—Distinguishing opaque oxides from sulfides in a thin section is challenging, and distinguishing the different oxides is even more challenging. Sometimes it is possible to reflect ceiling lights from a thin section to see if opague minerals are black/silver (oxides) or yellow/gold (sulfides).
Ilmenite is normally opaque (like other oxides and sulfides) and violet to black in reflected light (more violet or purple than hematite). It can be dark-brown to purplish in thinned edges or flakes.
Elongate ilmenite grain altering to leucoxene on its edges (PP)
Ilmenite typically forms disseminated tabular or flaky crystals, often longer in one dimension than the other. It also forms skeletal crystals, irregular grains, and patchy masses. It is sometimes associated with magnetite. Ilmenite alters to a cottony opaque white substance called leucoxene. It has no good cleavage but is often fractured. Twinning is common.
Similar Minerals—Ilmenite is more violet or purple than red hematite (reflected light). It may be difficult to distinguish from magnetite, but ilmenite tends to be platy (elongate in cross-section) whereas magnetite is more equant.
ilmenite
Optical Properties ■■ Rhombohedral; opaque
Fig 12.1.1 Graphitic Schist
This slightly graphitic (gray) schist from Great Smoky Mountains, North Carolina, contains tabs of black ilmenite peppering a matrix of folded gray muscovite and colorless low-relief, quartz and feldspar (PP). In XP, muscovite shows 2nd-order colors, quartz and feldspar show 1st-order gray to white, and ilmenite remains deep black. FOV = 1.6 mm.
This is a close-up image of a garnet interior. Opaque plates of ilmenite inclusions dominate the view. Vein-like inclusions of low-relief colorless quartz are also present (PP). The section is cut perpendicular to the ilmenite plates, so they appear elongate. In XP, quartz has 1st-order white interference colors; garnet should be black, but light leakage makes is very slightly gray, such that the opaque ilmenite inclusions are still visible. The sample comes from southern Vermont. FOV = 1.6 mm.
This amphibolite from the Tauern Window, Austria, contains opaque ilmenite crystals included in green, moderate-relief hornblende that also contains low-relief, colorless quartz inclusions. Brown biotite is also present (PP). In XP, ilmenite remains black. Hornblende shows low 1st-order colors, quartz is 1st-order gray to white, and biotite shows mottled extinction.. FOV = 3 mm.
In this amphibolite from southern Chile, opaque ilmenite forms elongate grains around a single, high-relief pinkish garnet. Other minerals include moderate-relief green to slightly blueish hornblende, low-relief colorless quartz and plagioclase, brown biotite, and moderate-relief, colorless apatite (PP). In XP, the ilmenite and garnet are black, the hornblende shows 1st-order gray to orange interference colors, quartz and feldspar display 1st-order gray to white colors, and the apatite is nearly black to dark gray. FOV = 3 mm.
Occurrence—Magnetite is very common and widespread in igneous, metamorphic, and sedimentary rocks.
Distinguishing Features—Distinguishing opaque oxides from sulfides in a thin section is challenging, and distinguishing the different oxides is even more challenging. Sometimes it is possible to reflect ceiling lights from a thin section to see if opague minerals are black/silver (oxides) or yellow/gold (sulfides).
Magnetite is opaque in transmitted light, gray or steel blue-black in reflected light.
Massive or granular aggregates are common. Euhedral crystals are rhombohedral or truncated square cross sections of octahedra. Some show rhombohedral cross sections. Magnetite has no good cleavages, but fractures are common. Contact or lamellar twins are common.
Magnetite is strongly magnetic.
Similar Minerals—Ilmenite tends to be more platy (elongate instead of equant). Magnetite appears silvery in reflected light; ilmenite is purple or violet in reflected light; chromite is more iron-black or brown-black.
magnetite
Optical Properties ■■ Cubic, opaque
Fig 12.2.1 Schist
This schist from northwest India contains several coarse, equant, opaque grains of magnetite in a bright green chlorite-rich layer, in part formed from alteration of amphiboles (PP). Low-relief, colorless quartz and high-relief pinkish, altered garnet make up most of the rest of the rock. In XP, opaque magnetite is black, garnet is black (isotropic), chlorite is anomalous purple and muddy green, and quartz is white to pale-yellow (indicating the section is a little too thick). FOV=3 mm.
This schist from New Mexico contains a few coarse equant grains of opaque magnetite with low-relief colorless quartz, brown biotite, moderate-relief colorless muscovite, and mats of tan fibrous sillimanite (fibrolite; PP). In XP, the opaque magnetite remains black, muscovite and biotite show high-2nd order colors, quartz is 1st-order gray and white, and sillimanite displays 1st-order yellow to orange colors. FOV
In this magnetite ore from New Mexico, magnetite is black (opaque), garnet has high relief and is pink. Staurolite has high relief and is yellow, and quartz has low relief and is colorless (PP). In XP, magnetite appears black, garnet is also black, staurolite shows low 1st-order gray interference colors, and quartz is 1st-order gray to white. Several orange grains (XP) of either kyanite or sillimanite are also present. FOV=3 mm.
This aluminum-rich schist (meta-paleosol?) from the Austrian Alps contains abundant clumps of opaque magnetite. Other minerals include low-relief colorless quartz, moderate-relief colorless muscovite, moderate-relief pale-green chlorite, high-relief colorless to light-yellow staurolite, and high-relief blue-green chloritoid (PP). We know the opaque is magnetite, partly because a magnet sticks to the rock, but also because of the distinctive square protuberances and embayments (see arrows; PP). In XP, magnetite remains opaque, quartz shows 1st-order gray to white interference colors, muscovite has high 2nd-order colors, staurolite has low 1st-order colors, chlorite is anomalous muddy green, and chloritoid is anomalous blue with twinning. FOV = 3.5 mm.
Fig 12.2.5 Calc-silicate This calc-silicate from Vermont contains a complex assemblage of opaque magnetite, low-relief colorless quartz, high-relief colorless to tan calcite, moderate-relief greenish hornblende, high-relief colorless to yellow-brown epidote, and high-relief brown titanite (PP). In XP, magnetite remains opaque, quartz is 1st-order gray to white, calcite is pastel to pearly, hornblende has low 1st-order colors, epidote has intense 1st-order and anomalous colors, and titanite is pearly. FOV = 3 mm.■■ Larger photos: PP XP
Fig 12.2.6 Magnetite in Schist
This schist from northeastern Georgia contains large crystals of opaque magnetite and high-relief slightly pink garnet. They are swimming in a sea of fine-grained, low-relief, colorless quartz and plagioclase, moderate-relief colorless muscovite, and pale green chlorite (PP). In XP, magnetite remains opaque, and garnet is isotopic. Optical properties of the other fine-grained minerals are difficult to see at this magnification, but quartz and plagioclase are 1st-order gray to white, muscovite shows 2nd-order colors, and chlorite is anomalous gray-green (XP). Plagioclase does not show twinning but some grains are optically zoned. FOV = 3 mm.
Occurrence—Chromite is primarily found in ultramafic rocks, typically associated with olivine and often in cumulate layers.
Distinguishing Features—Distinguishing opaque oxides from sulfides in a thin section is challenging, and distinguishing the different oxides is even more challenging. Chromite’s occurrence in ultramafic rocks, however, is a key to identification.
Chromite is opaque but may appear brown at thin edges in transmitted light. It is light-gray or iron-black to brownish-black in reflected light.
Euhedral crystals are rare. Chromite is most commonly in the form of granular aggregates or masses. It has no cleavage
Similar Minerals—Chromite may be mistaken for graphite, hematite, ilmenite, or magnetite, but chromite normally has a more steel-like color in reflected light.
chromite
Optical Properties ■■ Cubic; isotropic, opaque to nearly opaque ■■ n = 2.05-2.16
Fig 12.3.1 Olivine Chromitite
Colorless, high-relief, equant olivine grains and much smaller opaque chromite crystals appear to be set in a colorless, low-relief matrix of plagioclase. In XP, the plagioclase grains show their polysynthetic twin lamellae and the olivine grains show a range of interference colors. This rock comes from Montana’s Stillwater Complex. FOV = 10 mm.
Opaque chromite fills the spaces between large grains of colorless plagioclase and pale-green, higher-relief augite. This rock occurs near the top (~6780 m) of the much-studied Bushveld Igneous Complex in South Africa. FOV = 7 mm
This colorless olivine phenocryst from a Hawaiian basalt nearly fills the field of view. It contains tiny black specks of chromite (PP). Three of the larger inclusions are circled. Fractures radiate from the chromite because its volume changes less than olivine during cooling, inducing stresses that fracture the host crystal. The glue on the cover slip for this sample is starting to degrade, leading to some false color across the right side of the image (PP). In XP, olivine’s orientation yields only upper 1st-order colors, chromite remains black, and the groundmass in the upper-right and lower-left can be seen to contain tiny crystallites of gray to white plagioclase. FOV = 1.6 mm. Sample courtesy of Craig White, Boise State University.
This basalt from the Galapagos Islands contains phenocrysts of colorless, moderately high-relief olivine containing tiny black specks of chromite – two are circled. The matrix is mostly dark glass with many “hairy” plates of low-relief colorless plagioclase flowing around the olivine (PP). In XP, the opaque chromite is black, olivine’s unusual orientation yields only lower 1st-order colors, the glass is isotropic, and plagioclase shows its typical 1st-order gray to white interference colors. FOV = 1.6 mm. Sample courtesy of Dr. Dorsey Wanless, Boise State University.
