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4 Pyroxenes and Pyroxenoids
4.1 Augite
(Ca,Na)(Mg,Fe,Al)(Si,Al)2O6
Occurrence—Augite, the most abundant kind of clinopyroxene, is common in mafic and ultramafic igneous rocks and their high-grade metamorphic equivalents.
Distinguishing Features—Augite has high relief and is normally colorless or pale-green or pale-brown. It can display a weak pale-green to (a different) pale-green pleochroism. Augite typically shows middle 2nd-order interference colors.
“Stop sign” shaped augite crystal (PP). Photo from www.alexstrekeisen.it.
Augite may appear as short prismatic crystals with four- or eight-sided “stop sign” cross sections. Elongated crystals have a large extinction angle. Basal sections show two good cleavages that intersect at 87° and 93° (nearly orthogonal); longitudinal sections show one good cleavage.
Simple and polysynthetic twins are common; a combination of the two produces a “herringbone” appearance.
Augite may contain exsolution lamellae of orthopyroxene, and may show compositional zoning.
Similar Minerals—Diopside, a clinopyroxene end-member composition, may appear similar to augite, but diopside has slightly greater birefringence, is less deeply colored, and has different rock associations (especially metamorphic calc-silicates). Orthopyroxene has parallel extinction, much lower birefringence, a higher 2V than augite, and may be biaxial (-); it can also show faint pink-green pleochroism. Aegirine and aegirine-augite are related, Na-rich clinopyroxenes with strong green, yellow, or brown colors and (often) strong pleochroism.
augite
Optical Properties ■■ Monoclinic; biaxial (+) ■■ 2V = 25° to 60° ■■ α = 1.670-1.743, β = 1.676-1.750, γ = 1.694-1.772 ■■ δ = 0.024-0.029 ■■ Cleavage flakes typically give good optic axis figure ■■ Symmetrical extinction in basal sections ■■ Maximum extinction angle in longitudinal sections is between 36° and 45° depending on composition; sections that have maximum extinction angles also have maximum birefringence
Fig 4.1.1 Spinel Peridotite
This spinel peridotite xenolith from Kilbourne Hole, NM, shows mostly clinopyroxene (augite) with unusually good cleavage. A large grain of highly-fractured orthopyroxene with slightly greater relief is on the left. A dark greenish-brown spinel is seen in the upper right (PP). In XP, the clinopyroxene has up to high 2nd-order interference colors; the orthopyroxene has only 1st-order gray interference colors; the spinel is isotropic (black). FOV = 4.5 mm.
In this wehrlite (olivine-clinopyroxene peridotite), colorless clinopyroxene (augite) and olivine are hard to distinguish in PP. Brown spinel is distinct. In XP, however, the olivine shows upper 2nd-order colors, whereas the augite displays only 1st-order colors. Two large elongate augite grains in the top-center part of the photo contain simple twins (XP). The spinel is isotropic. FOV = 9 mm. Photos from www.alexstrekeisen.it.
This basalt from India contains mostly clear laths of plagioclase and higher-relief gray augite. In XP, the plagioclase shows twinning and 1st-order gray and white interference colors. The augite has up to mid 2nd-order interference colors. The reddish material present is iddinsite, an alteration product of olivine. The augite displays cleavage, with the angle between cleavages close to 90o in some grains. FOV ~1.5 mm.
This gabbro from the Skaergaard intrusion, Greenland, contains coarse moderately high-relief, tan augite; high-relief colorless rounded olivine; and low-relief colorless plagioclase (PP). In XP, augite shows 1st-order interference colors, as well as a simple twin (white arrows) and exsolution of lamellae (thin bands) of orthopyroxene (yellow arrows). Olivine shows 2nd-order colors, and plagioclase shows typical 1st-order gray to white colors and polysynthetic twinning. FOV = 3 mm.
In PP, a large augite crystal on the left shows two cleavages intersecting at 90°. The large augite crystal in the middle shows only one of the cleavages. The large augite grain on the right doesn’t show any good cleavage. Orientation matters! High-relief olivine grains are near the bottom of the image, and a few opaque magnetite grains are visible. The rest of the minerals are colorless plagioclase, with slight alteration that is clearly visible in XP. This rock comes from Pigeon Point, Minnesota. FOV = 5 mm
In this granulite from the North China Craton, high-relief green clinopyroxene (augite) shows extensive exsolution of fine lamellae (of orthopyroxene), but high-relief pink orthopyroxene does not show obvious exsolution (PP). Moderate-relief hornblende is brown and shows amphibole cleavage. Plagioclase has low relief and is colorless (PP). In XP, clinopyroxene displays characteristic 2nd-order interference colors and inclined extinction. (Thin white lines have been drawn parallel to the hard-to-see cleavage in one grain of clinopyroxene to confirm that the grain has inclined extinction.) Orthopyroxene displays 1st-order orange interference colors; plagioclase is 1st-order gray to white with many twins; hornblende’s intense brown hue muddies its 2nd-order colors. FOV = 5 mm.
Occurrence—Diopside-hedenbergite is a Fe-Mg solid solution series of Ca-rich clinopyroxenes. Minerals of this series are similar to augite but contain Ca:(Fe+Mg) in a ratio closer to 1:1.
The name diopside is given to compositions with Mg>Fe. If Fe>Mg, the pyroxene is called hedenbergite. Members of this series are common in medium- to high-grade marbles and calc-silicate rocks. Hedenbergite is especially common in some Fe-rich skarns. Diopside- and hedenbergite-rich pyroxenes are also found in a variety of igneous rocks, but the pyroxenes in those rocks generally contain significantly more iron and magnesium than calcium, and significant amounts of Al and Na (so are classed as augite).
Distinguishing Features—A key identifier for diopside-hedenbergite pyroxenes is their occurrence in marbles and related rocks. Short prismatic subhedral crystals are common; well-formed cross sections may be four- or eight-sided.
These pyroxenes have high relief and generally good cleavage. Basal sections show two cleavages at 87° and 93°; longitudinal sections show one cleavage.
Diopside typically appears colorless (if little Fe is present). With increasing Fe-content, color becomes more greenish and slightly pleochroic.
Maximum interference colors may be upper-2nd to lower-3rd order. Simple and polysynthetic twins are common.
Similar Minerals—Orthopyroxene has parallel extinction, lower birefringence, and higher 2V than diopside, and may be biaxial (-); it also occurs in very different rocks. Augite is more deeply colored.
diopside
Optical Properties ■■ Monoclinic; biaxial (+) ■■ 2V = 56° to 63° ■■ diopside: α = 1.665, β = 1.672, γ = 1.696, δ = 0.031 ■■ hedenbergite: α = 1.732, β = 1.739, γ = 1.757, δ = 0.025 ■■ Extinction angle varies up to 44° in longitudinal section depending on orientation and composition ■■ Extinction is symmetrical in basal sections
Fig 4.2.1 Diopside Marble
This marble from the Adirondack Mountains, NY, shows high-relief diopside with nearly perpendicular cleavages (PP). In XP, the large grain in the center of the view shows 1st-order interference colors, but the grain to the left is 2nd-order (XP). Calcite surrounds the diopside and is stained pinkish (using alizarin red stain) to distinguish it from dolomite. In XP, the calcite shows very high pastel interference colors and many obvious twins. FOV = 2 mm.
This is a two-mineral photomicrograph: rounded, pale green diopside grains surrounded by calcite crystals. Different crystallographic orientations of the diopside grains explain their different appearances. The grain on the left shows two cleavages intersecting at nearly 90°. The middle grain shows one cleavage. The other grains do not exhibit cleavage. In XP, the middle diopside grain displays lamellae twins. Diopside’s interference colors range up to middle 2nd-order but grains that show two cleavages have lower order colors because they have nearly vertical optic axes. This marble is from the Ruby Range, Montana. FOV = 2.5 mm.
In this rock from Acton, Massachusetts, two colorless moderate-relief diopside (Di) crystals show cleavages intersecting at 90°. Colorless tremolite (Tr) is also present, but has lower relief. Most of the low-relief colorless grains are microcline (Mc). Several calcite grains exhibit polysynthetic twin lamellae in both the PP and XP views. The small brownish high-relief grains are titanite (Ttn). The titanite crystals have black ilmenite overgrowths. In the XP view, diopside shows only 1st-order interference colors, but some tremolite shows bright 2nd-order colors. FOV = 2.5 mm.
The rock consists almost entirely of hedenbergite with a few interstitial calcite grains. In PP, the hedenbergite is pale green, with red-brown iron oxide alteration darkening the elongated grains. In XP, the hedenbergite displays up to low 2nd-order interference colors, again masked in many places by the reddish iron oxide. The calcite has the expected high-order pale pinkish-beige color, except for one prominent crystal in the lower right that is a 1st-order gray. That gray means that the optic axis is vertical, which was confirmed by a centered uniaxial(-) figure in a conoscopic view. FOV = 5 mm.
This calc-silicate from South Carolina contains moderately high-relief pale-green clinopyroxene that is a solid solution between diopside and hedenbergite. The clinopyroxene is overgrown by high-relief colorless to gray epidote and pink-gray garnet (PP). The rest of the rock is low-relief colorless plagioclase and high-relief, slightly reddish-brown, titanite (PP). In XP, clinopyroxene shows upper 1st- to lower 2nd-order interference colors. Epidote is anomalous yellow and blue, garnet is isotropic, plagioclase is 1st-order gray to white, with polysynthetic twins, and titanite shows high-order pastels. FOV = 4 mm.
Orthopyroxene showing classic pink to green pleochroism. Photo from rockptx.com.
Distinguishing Features—Orthopyroxene’s optical properties depend on composition, but it is normally pale, with moderate to high relief. Low interference colors and light-green to pink pleochroism, when present, are key identifiers. It may show compositional zoning.
Well developed crystals are prismatic with rare square or “stop sign” shaped basal sections.
Basal sections show two moderately good cleavages at right angles; longitudinal sections show one good cleavage. Orthopyroxene displays parallel extinction and is length slow.
Orthopyroxene generally has low birefringence. Maximum interference color is mid- to upper-1st order (typically yellow-orange or less). Unusual compositions can display 2nd-order colors.
Similar Minerals—Clinopyroxenes have greater birefringence than orthopyroxene, inclined extinction and lower 2V. Zoisite is typically length fast and has lower birefringence and higher relief; it also occurs in very different rock typess. Kyanite has oblique extinction, higher interference colors, and occurs in different rock types. Andalusite is length fast, has lower relief and occurs in different rock types. Sillimanite can occur with orthopyroxene, but has higher interference colors, lower relief, and different cleavage patterns. Amphibole has different cleavage (60o and 120o angles) and lower relief; common amphiboles also have higher interference colors and inclined extinction.
orthopyroxene
Optical Properties ■■ Orthopyroxene is orthorhombic ■■ biaxial (+) or (-) ■■ 2V = 53° to 90° ■■ α = 1.650-1.715, β = 1.653-1.728, γ = 1.658-1.731 ■■ δ = 0.008-0.022 ■■ Basal sections reveal a biaxial figure with a moderate to large 2V
Fig 4.3.1 Mafic Granulite
This classic mafic granulite from Hartmannsdorf, Germany has unusually dark pink , high relief orthopyroxene and bright green, high relief clinopyroxene, with colorless, low relief plagioclase and opaque magnetite (PP). In XP, interference colors range up to unusually high 1st-order red for orthopyroxene, wnd order blue and yellow for clinopyroxene interference colors, but only 1st order gray and white for plagioclase. Many plagioclase grains show twinning. FOV = 3.5 mm. Photos from www.alexstrekeisen.it.
Orthopyroxene and colorless olivine dominate this ultramafic rock. The pyroxene displays pink to green pleochroism. In XP, interference colors range up to only 1st order yellow for the orthopyroxene, but are 2nd-order blues, yellows, and greens for olivine. FOV = 2.5 mm.
This granulite contains high-relief (pleochroic) pink and green orthopyroxene, high-relief green clinopyroxene, low-relief colorless plagioclase, and opaque magnetite (PP). The orthopyroxene shows primarily 1st-order interference colors up to yellow-orange; the clinopyroxene shows higher-order colors, up to 2nd-order blue-green (XP). Most plagioclase is untwinned. FOV = 5 mm. The photos come from the Oxford Earth Sciences Image Store.
This granulite from Otter Lake, Quebec, contains high-relief pleochroic pink to green orthopyroxene, low-relief colorless cordierite, and light to dark brown biotite. The large cordierite grains in the center and upper right contain yellow-orange pleochroic halos around high-relief accessory minerals, likely either zircon or monazite. In XP, interference colors are low 1st-order yellow for orthopyroxene, 1st-order gray and white for cordierite, and 3rd-order for biotite, which also shows bird’s eye extinction. Cordierite grains are twinned; the twinning appears similar to twinning of plagioclase, but pleochroic halos are distinctive. FOV = 2.5 mm.
In this orthopyroxenite xenolith from the Cima Volcanic field in California’s Mojave National Preserve, a large, high-relief, colorless grain of orthopyroxene is surrounded by finer-grained, low-relief, colorless plagioclase, smaller pyroxene grains, and dark glass (PP). The orthopyroxene shows good cleavage and 1st-order yellow interference colors (XP). FOV = 3.5 mm.
In this websterite (olivine-absent ultramafic rock) xenolith from the Cima Volcanic Field, California, high-relief orthopyroxene and clinopyroxene appear similar, although the orthopyroxene is slightly less colored and displays more visible cleavage . In XP, they are distinguishable by orthopyroxene’s 1st-order gray interference colors contrasting with clinopyroxene’s low 2nd-order blue and reddish-purple colors. FOV = 15 mm.
This is a view of a garnet amphiblite from the Tobacco Root Mountains, Montana. The complex image shows a symplectite texture reaction zone between a garnet porphyroblast on the left and green hornblende on the right. The reaction products are principally high-relief fibers of orthopyroxene intergrown with low-relief plagioclase, both colorless. Magnetite is the opaque mineral and some brown biotite grains are also visible. The orthopyroxene is not easily identified by its optical properties when grains are so small. However, the reaction Grt + Hbl = Opx + Pl +H2O is one that might be predicted by a petrologist who plots those minerals on an ACF diagram. FOV = 1.25 mm
This granulite from the North China Craton contains high-relief pink orthopyroxene, high-relief green clinopyroxene, reddish-brown biotite, and low-relief colorless plagioclase (PP). In XP, orthopyroxene has much lower interference colors – maximum of 1st-order orange – than clinopyroxene, which has bright upper-1st to 2nd-order colors. Plagioclase shows 1st-order gray to white colors and pervasive twinning. FOV = 5 mm.
Occurrence—Pigeonite is a rare clinopyroxene with composition similar to orthopyroxene, but with extra Ca substituting for Fe, and Mg. It is stable only at high temperature, so most commonly occurs in volcanic rocks and shallow intrusive igneous rocks that cooled quickly. Pigeonite may also be found as exsolution lamellae in augite, and in lunar basalts.
Distinguishing Features and Similar Minerals—Pigeonite has typical pyroxene shape and two cleavages intersecting near 90o, moderate birefringence, and small 2V. It is colorless to pale brown-green in thin section and normally does not show much pleochroism. Stubby euhedral to anhedral prismatic crystals are typical.
Maximum interference colors are normally 1st-order red or yellow but may range up to lower 2nd-order.
Simple or polysynthetic twins are common.
“Inverted pigeonite” (XP). Photo from Kurt Hollocher.
Pigeonite is a high-temperature pyroxene, unstable at lower temperature. So it commonly “inverts” to lower-temperature orthopyroxene with cooling. The orthopyroxene may exsolve as it cools further, producing exsolution lamellae of augite within orthopyroxene. The texture produced is called inverted pigeonite. Sometimes this results in a “herringbone” structure. The exsolution of augite may be mistaken for orthopyroxene twinning.
Pigeonite may be difficult to distinguished from augite. But pigeonite has a lower 2V of 0 to 30o (augite 2V is typically 56 to 63o). Additionally, augite can occur in plutonic rocks, but pigeonite cannot (it inverts during slow cooling). Olivine has higher birefringence and lacks cleavage compared with pigeonite. Orthopyroxene has lower birefringence, larger 2V, can be pleochroic, and has parallel extinction.
pigeonite
Optical Properties ■■ Monoclinic; biaxial (+) ■■ 2V = 0° to 32° ■■ α = 1.682-1.722, β = 1.684-1.722, γ = 1.705-1.751 ■■ δ = 0.023-0.029 ■■ Cleavage flakes give off-center interference figures ■■ Pigeonite has symmetrical extinction in basal sections showing both cleavages ■■ In longitudinal sections, extinction may be parallel or inclined depending on orientation ■■ The maximum extinction angle is about 45° depending on composition ■■ Sections that have a maximum extinction angle also have maximum birefringence
Fig 4.4.1 Inverted Pigeonite
In PP, this gabbro from the Skaergaard intrusion looks deceptively simple, with high-relief light-colored pyroxenes, high-relief colorless olivine, and low-relief colorless plagioclase. Cross the polars, however, and one of those pyroxenes reveals a highly speckled interior. This is inverted pigeonite. At very high temperature, orthopyroxene can contain substantial Ca and is called pigeonite. Upon cooling, the Ca-component exsolves (separates), producing highly birefringent clinopyroxene (augite) blebs in a host of normal orthopyroxene. The other minerals are typical – augite and olivine display moderate to high-order interference colors, and plagioclase has 1st order gray to white colors and pronounced polysynthetic twins. FOV = 3.5 mm. Sample courtesy of Dr. Craig White, Boise State University.
This olivine gabbro from Greenland’s Skaergaard Complex contains two different pyroxenes – colorless to pale-gray inverted pigeonite and brownish augite. Other minerals are colorless low-relief plagioclase and high-relief colorless, fractured olivine (PP). The original high-temperature pigeonite inverted to orthopyroxene during cooling. In the process it gained many exsolution lamellae of augite, giving it a herringbone texture. The large labeled augite crystal has a single twin down its center. Some of the smaller augite crystals are interstitial – they filled spaces between plagioclase crystals. In XP, the plagioclase shows its typical gray to white interference colors with numerous fine twins. Olivine shows 2nd-order blue inteference colors. Photos from Kurt Hollocher. FOV = 6 mm.
Both augite and pigeonite are visible in this thin section of a Mesozoic rift valley basalt from Mt. Holyoke, Massachusetts. Both clinopyroxenes can be colorless in PP, although some of the augite grains in this rock appear pale green – reflecting relatively high iron content. Three of the pigeonite grains on the left are circled. In XP, the pigeonite tends to have slightly lower birefringence than the augite. However, identification of pigeonite can be problematic and should be confirmed by looking for its low 2V (<30°). Elongate plagioclase grains show lamellar twinning, but many are clouded by alteration. The opaque mineral is magnetite, identified by the attraction of a magnet to the rock. FOV = 5 mm
Occurrence—Jadeite (NaAlSi2O6) and omphacite (mostly a solid solution between jadeite and Ca(Fe2+,Mg)Si2O6) are high-pressure pyroxenes found in metamorphic rocks of the blueschist and eclogite facies. Other sodic pyroxenes, especially aegirine (mostly NaFe3+Si2O6) and aegirine-augite (solid solutions between NaFe3+Si2O6 and Ca(Fe2+,Mg)Si2O6), are found in alkaline igneous rocks.
Distinguishing Features—Pure jadeite is colorless but rather uncommon; most blueschists and eclogites contain omphacite, which ranges from colorless to light-green and is pleochroic. A key to identifying jadeite is the association with other blueschist and eclogite facies minerals, especially glaucophane, muscovite, garnet, and rutile. In contrast with jadeite and omphacite, aegirine’s strong colors range from tans to vivid green; it is markedly pleochroic.
For jadeite, maximum interference colors are 1st-order gray and white, and anomalous blue interference colors are common. For omphacite, interference colors commonly range up to 2nd-order. The intense color of aegirine masks interference colors, which are also 2nd-order.
Like all other pyroxenes, sodic pyroxenes have two cleavages that intersect at nearly 90°, although fine grain size can make these difficult to see.
Similar Minerals—Diopside and augite can appear very similar to sodic pyroxenes. But jadeite and omphacite occur in blueschists and eclogites, and have very low birefringence or anomalous interference colors to distinguish. Aegirine can occur as overgrowths on augite, which is far less intensely colored and pleochroic. Actinolite has a smaller extinction angle, lower relief, and usually higher-order interference colors.
jadeite
Optical Properties ■■ Monoclinic biaxial (+) ■■ 2V = 68° to 72° ■■ jadeite: α = 1.654, β = 1.657, γ = 1.665, δ = 0.011-0.016 ■■ diopside: α = 1.665, β = 1.672, γ = 1.696, δ = 0.031 ■■ The maximum extinction angle in longitudinal sections is 30° to 45°, depending on composition.
Fig 4.5.1 Blueschist
This blueschist from Panoche Pass, California, contains abundant colorless jadeite, blue glaucophane with diamond-shaped cross-sections and amphibole cleavage, and high-relief epidote (PP). In XP, the jadeite displays low-order interference colors, anomalous in some grains. Glaucophane shows 1st-order yellow-to orange colors (XP); and epidote gets lost in XP. FOV ~ 2.5 mm.
This renowned eclogite from the Dabie mountains, China, contains moderately high-relief pale-green omphacite, high-relief pinkish garnet, dark-red (almost opaque) rutile, and low-relief colorless quartz (PP). The colorless inclusions with radial fractures around them (circled) are characteristic of former coesite, an ultrahigh-pressure polymorph of quartz. The former coesite has inverted completely to fine-grained quartz. Quartz has greater volume than coesite, thus creating the cracks. In XP, omphacite shows low 1st-order colors, garnet is opaque, rutile is too dark to make out its interference colors, and quartz is characteristically 1st-order gray to white. FOV = 5 mm
This eclogite from Sonoma County, CA, has a sea of relatively low-relief, pale-green omphacite, which is slightly altered to dark hornblende along edges. The rock also contains high-relief brown titanite and low-relief colorless muscovite (PP). A single large high-relief garnet, slightly altered to chlorite along its edges, abuts the omphacite. In XP, omphacite’s interference colors range up to 1st-order red. Garnet is isotropic, muscovite displays mottled 2nd-order colors, and titanite shows high-order pastels (difficult to see at this scale). FOV ~ 1.5 mm.
Ijolites , intrusive rocks that contain nepheline and clinopyroxene, are rare. This one comes from south-central Colorado. Most of the field of view is one large twinned aegirine crystal surrounded by colorless nepheline. The aegirine has a deeper green color than the omphacite in Fig. 4.5.3 and has been replaced in places by brown biotite. In XP, the aegirine displays 2nd-order interference colors, the nepheline is gray; the biotite has 2nd-order colors, but its brown color masks this. FOV ~ 3.5 mm.
This eclogite consists mostly of pale-green and pink, moderate-relief omphacite and pinkish high-relief isotropic garnet. The tiny dark grains are probably rutile. The omphacite, like most clinopyroxenes, displays lower 2nd-order interference colors (XP). Photos from Dr. Kurt Hollocher. FOV = 1.2 mm.
This alkaline granite contains large crystals of moderate-relief, yellowy-green aegirine. The aegirine is surrounded by, and includes, low-relief colorless crystals of quartz and feldspar. Aegirine’s green pleochroic colors are similar to some hornblendes, but are yellower. Aegirine also has near right-angle cleavage intersections. In XP, aegirine’s interference colors range to upper 2nd- or lower 3rd-order, a bit higher than amphiboles and most other clinopyroxenes. Aegerine also has a smaller extinction angle than most hornblende, and is length fast. Photos from Dr. Kurt Hollocher. FOV = 3 mm.
Occurrence—Wollastonite is found in high-grade siliceous marbles and related calcareous metamorphic rocks.
Distinguishing Features—Wollastonite has high relief in thin section, is colorless, and commonly forms columnar or fibrous aggregates. Cross sections of crystals may be nearly square.
Wollastonite is tenacious and tends to fracture during thin sectioning (or the glass slide splinters). This fracturing aids identification.
Like all pyroxenes/pyroxenoids, wollastonite has two good cleavages that intersect at near 90o. It has near parallel extinction.
Maximum interference colors for wollastonite are about 1st-order orange.
Similar Minerals—Tremolite resembles wollastonite but has lower relief, oblique extinction, and amphibole cleavage. Sillimanite resembles wollastonite, but is biaxial (-), has higher order interference colors, and does not occur in marbles. There are several other calcsilicate minerals that can occur in the same rocks as wollastonite. Distinguishing them may be difficult without mineral analysis. Spodumene (LiAlSi2O6) has slightly higher relief and interference colors, but is otherwise optically similar to wollastonite. However, spodumene does not occur in calcic metamorphic rocks, while wollastonite does not occur in Li pegmatites (the source of most spodumene).
wollastonite
Optical Properties ■■ Triclinic; biaxial (-) ■■ 2V = 36° to 42° ■■ α = 1.620, β = 1.632, γ = 1.634 ■■ δ = 0.014 ■■ Parallel or near-parallel extinction in longitudinal section ■■ Length slow or length fast depending on orientation
Fig 4.6.1 Wollastonite Skarn
This view of a skarn rock from the Adirondacks, New York contains mostly abundant moderate-relief colorless wollastonite. A conspicuous grain of light-green moderate-relief clinopyroxene (a diopside-hedenbergite solid solution), and a grain of high-relief slightly pink garnet are also seen (PP). An end section of wollastonite shows nearly 90° intersecting cleavages. In XP, wollastonite’s interference colors range up to lower 2nd-order. The clinopyroxene is 1st-order yellow, and the garnet is isotropic. FOV = 1.4 mm.
This low-magnification image of a wollastonite-bearing rock from Willsboro, New York, is dominated by colorless wollastonite. It also contains higher-relief very pale-green diopside and even higher-relief, pinkish highly fractured garnet (PP). Many wollastonite and diopside grains show one or two faint cleavages. In XP, the wollastonite shows 1st-order interference colors (up to yellow). Diopside displays bright 2nd-order colors and garnet is isotropic. Several holes are present where wollastonite plucked during thin sectioning. They appear colorless with low relief, and are isotropic. Photos from rockptx.com. FOV = 20 mm.
This rock from New York’s Adirondack Mountains, contains mostly colorless wollastonite (Wo) and diopside (PP). The two are difficult to distinguish, but the wollastonite has slightly lower relief, and more irregular shapes than the diopside (that has relatively equant grains). Microcline and a few grains of calcite and titanite are also visible. In XP, wollastonite shows 1st-order colors. Diopside has higher-order colors and shows some lamellar twinning. Microcline displays its characteristic cross-hatch twinning. FOV = 5 mm
A single, large colorless wollastonite crystal with numerous cleavage traces dominates this image. It is accompanied by pale green diopside. Wollastonite often cleaves or fractures during thin-section making, and many cleavages are visible in this view; the near 90o cleavage angle is easily seen. In XP, wollastonite’s interference colors are up to 1st-order violet, about as high as wollastonite goes. The photos come from the Oxford Earth Sciences Image Store. FOV = 3.5 mm.
Occurrence—Spodumene is a mineral restricted to granitic pegmatites. Because spodumene is found in pegmatites, crystals may be huge.
Distinguishing Features—Occurrence in pegmatites is a key to identification. Spodumene has high relief in thin section, is generally colorless, and most commonly forms long prismatic crystals with octagonal cross sections. It also occurs as lath-like acicular crystals, rectangular or tabular crystals, and in cleavable masses.
Like all pyroxenes/pyroxenoids, spodumene has two good cleavages that intersect at near 90o. It has near parallel extinction.
Maximum interference colors for spodumene are upper-2nd to 3rd-order.
Similar Minerals—Kunzite is a rare pink variety of spodumene, hiddenite is a rare pale-green variety. Spodumene (LiAlSi2O6) has slightly higher relief and interference colors, but is otherwise optically similar to wollastonite. However, spodumene does not occur in calcic metamorphic rocks, while wollastonite does not occur in Li pegmatites (the source of most spodumene). Tremolite resembles spodumene but has lower relief, oblique extinction, and amphibole cleavage.
Quartz and feldspar surround coarse colorless high-relief grains of spodumene in this pegmatite from Southern Tibet (PP). In XP, spodumene displays 1st-order gray to orange interference colors; quartz and feldspar are 1st-order gray to white. Sample courtesy of Dr. Xiaochi Liu, Chinese Academy of Sciences. FOV = 5 mm.
This lithium pegmatite from Southern Tibet, contains coarse grains of high-relief colorless spodumene with well- developed cleavage. The spodumene is surrounded by coronas of low-relief colorless feldspar with slight reddish alteration, and low-relief colorless quartz (PP). In XP, spodumene shows characteristic, 1st-order, yellow- orange interference colors and simple axial twinning. Quartz and feldspar are first order gray and white. Sample courtesy of Dr. Xiaochi Liu, Chinese Academy of Sciences. FOV = 5 mm.
■ Triclinic; biaxial (+)
