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2 Quartz, Feldspars, and other Framework Silicates
2.1 Quartz
SiO2
Occurrence—Quartz is found in a wide range of igneous, metamorphic, and sedimentary rocks.
Quartz showing undulatory extinction (XP)
Distinguishing Features—Quartz is colorless in thin section, displays 1st-order gray to white (or weak-yellow if the section is a bit too thick) interference colors, and lacks cleavage. It does not show alteration or twinning, and rarely contains inclusions of other minerals. It commonly exhibits undulatory (wavy) extinction. Highly strained grains may show fracture.
Quartz is typically anhedral but may be euhedral prismatic. In some rocks quartz intergrows with orthoclase or microcline in a geometric pattern called graphic granite, or with plagioclase in a wormy texture called myrmekite (Figure 2.2.4). Quartz twinning is common but does not show in thin section.
quartz
Similar Minerals—Quartz can be confused with untwinned feldspars, apatite, cordierite, beryl, or scapolite. All of these display 1st-order interference colors, but none are uniaxial positive and most show cleavage (unlike quartz).
Optical Properties ■■ Hexagonal – uniaxial (+) ■■ ω = 1.5442, ε = 1.5533 ■■ δ = 0.0091, interference figures show no isochromes ■■ Ideally prismatic crystals show parallel extinction; basal sections are always extinct ■■ Many quartz crystals show wavy (undulatory) extinction due to strain induced during cooling or during rock deformation
Fig 2.1.1 Rhyolite
Large phenocrysts (primary igneous crystals) of quartz and plagioclase in this rhyolite are in a fine-grained matrix of quartz and K-feldspar. The largest grain is quartz and has two large domains (XP). Many smaller quartz grains are also present. Zebra-stripe twins distinguish plagioclase (XP). The large quartz and feldspar phenocrysts appear to be much less altered than the finer grained groundmass. FOV = 1.5 mm. ■■ Larger photos: PP XP
Fig 2.1.2 Aplite
Colorless, unaltered quartz dominates this aplite (type of granite) from Boulder County, Colorado, with lesser microcline, plagioclase and a few very small grains of brown biotite. Feldspar grains appear turbid (brown or gray, PP) due to incipient alteration to clay, unlike the clear quartz grains. In XP, quartz shows undulatory extinction and mostly gray and white interference colors. The light yellow interference color for one large grain in the lower right suggests this thin section is a bit too thick. Microcline grains show cross-hatch (tartan) twinning. The single large plagioclase grain in the upper right displays albite twinning. FOV = 2.5 mm. ■■ Larger photos: PPXP
Fig 2.1.3 Sillimanite Gneiss
Colorless quartz grains (PP) showing undulatory extinction (XP) dominate the central part of this image of a pelitic gneiss from western Massachusetts. Small brown flakes of biotite and some colorless, blocky, high-relief sillimanite (PP) occur on both the left- and right-side. Some of the sillimanite appears as square end-sections with a single diagonal cleavage -a diagnostic property. Isotropic garnet and a few high-relief titanite crystals are also present. Quartz shows only low 1st-order interference colors. Biotite shows up to 2nd-order red interference colors (XP). Sillimanite can display high 2nd-order interference colors but colors are lower in this view because of grain orientation. FOV = 1.5 mm.
This view is almost all colorless quartz, but one large colorless grain in the lower right is plagioclase. The quartz grains are packed together in a typical mosaic texture. In the PP view, wispy grains of biotite, altering to chlorite, can be seen – mostly on the right side of the thin section. Two tourmaline crystals are in the lower left; they are zoned with bluish cores and greener rims. Several elongate ilmenite grains are associated with one of the tourmalines. In XP, the quartz and plagioclase show their usual gray and white interference colors; the plagioclase is twinned. The biotite shows mottled pastel colors. Tourmaline’s interference colors are masked by the dark color of the mineral. This specimen comes from near Poughkeepsie, New York. FOV = 3.5 mm.
This quartz vein comes from the Santa Catalina subduction complex, California, contains only low-relief colorless quartz, shot through with a network of fractures (PP). We can see these fractures because they are decorated with tiny fluid inclusions; the inclusions have huge relief contrast with quartz. The XP image reveals the underlying deformation structure of quartz, which consists of large cores of quartz with undulose (wavy) or domainal extinction. Core margins have been recrystallized into a network of tiny quartz grains (arrows). This type of deformation is called bulging-recrystallization. FOV = 8 mm.
This meta-chert from the Santa Catalina subduction complex, California, consists almost entirely of colorless low-relief quartz, spotted with slightly pink high-relief garnets and a few grains of mica. In XP, the labeled quartz grain shows classic chessboard deformation texture, with extinction domains intersecting at about 90° angles. The garnet is isotropic, and the micas show high 1st- to 2nd-order colors. FOV = 8 mm.
This meta-chert from the Santa Catalina subduction complex, California, contains essentially only 2 minerals – a sea of low-relief colorless quartz spotted with tiny grains of high-relief slightly pink garnet. In XP, quartz shows 1st-order gray to white interference colors, and has highly irregular boundaries (amoeboid texture). These textures are characteristic of intermediate-temperature grain-boundary migration recrystallization. The garnet is isotropic. FOV = 8 mm.
Occurrence—Minerals of the K-feldspar group (orthoclase, microcline, and sanidine) are common in many kinds of silicic igneous rocks and their metamorphosed equivalents. Microcline and orthoclase also occur in high-grade metamorphic rocks and in arkoses.
The different K-feldspars are stable under different conditions. Sanidine (Fig 2.2.3), the high temperature polymorph of orthoclase and microcline, occurs typically as phenocrysts in silicic volcanic rocks including rhyolite and trachyte (Na- and K-rich rhyolite). Sanidine may change into orthoclase or even microcline if lava cools slowly enough. Orthoclase and microcline form in lower temperature and/or slower cooled igneous and metamorphic rocks. Just about all microcline forms by recrystallization of sanidine or orthoclase. Both orthoclase and microcline are common in sedimentary rocks, such as arkoses (Fig 2.2.5).
Distinguishing Features—K-feldspars are colorless in thin section but can be cloudy or pale brown due to inclusions or alteration. They display 1st-order gray and white interference colors, low relief, and may show one or two cleavages.
Orthoclase forms subhedral to anhedral crystals, phenocrysts, and spherulites. Sanidine generally occurs as euhedral to subhedral phenocrysts. Microcline occurs as subhedral to anhedral crystals.
Microcline twinning (XP)
Simple (Carlsbad) twins are common in orthoclase and sanidine, separating crystals into 2 or sometimes 3 large domains. Several other types of simple twins also may be present. Microcline almost always has albite and pericline twins, giving wavy or spindly appearance in some orientations; intersection of twins leads to a characteristic “Scotch plaid” or tartan twinning (see Fig 2.2.1).
Orthoclase has one excellent, one good, and one imperfect cleavage. Sanidine and microcline have one perfect and one less perfect cleavage.
K-feldspars generally contain some Na-feldspar (albite) in solid solution. Microcline and orthoclase crystals may contain blobby or lamellar exsolution lamellae (called perthite; see Fig 2.2.2) if the sodic component exsolves from the potassic component.
orthoclase
Similar Minerals—If no cleavage or twinning show, K-feldspar (especially sanidine) may be confused with quartz, but quartz is commonly clearer, is uniaxial, lacks cleavage, and is slightly higher relief. Nepheline is also similar but is uniaxial (-). Alkali feldspar may occasionally be confused with plagioclase, but most plagioclase grains display albite twinning or chemical zoning that manifests as concentric changes in extinction angle. Albite and pericline twins in plagioclase are sharply bounded, and do not show the “distorted” tartan twinning pattern seen in microcline.
Distinguishing between orthoclase, microcline, and sanidine can be challenging or impossible if twins are not evident. Sanidine may be confused with orthoclase, but cleavage, twinning, and especially low 2V may distinguish it. Untwinned microcline may be confused with other feldspars, but tartan twinning generally serves to distinguish it.
Optical Properties ■■ Orthoclase – monoclinic; biaxial (-); 2V = 33° to 65°; α = 1.520, β = 1.525, γ = 1.527, δ = 0.005-0.007. ■■ Microcline – triclinic; biaxial (-); 2V = 77° to 84°; α =1.519, β = 1.523, γ = 1.525, δ = 0.006. ■■ Sanidine – monoclinic; biaxial (-); 2V = 18° to 54°; α= 1.521, β = 1.526, γ = 1.527, δ = 0.005-0.007. ■■ Although biaxial (-), sanidine 2V may be so small that the mineral appears uniaxial ■■ Good interference figures are hard to obtain for microcline, due to twinning ■■ Orthoclase’s extinction angle varies from 0 to about 12° depending on orientation and composition; its principal cleavage is inclined at a small angle to the fast ray. Sanidine’s extinction angle varies from about 0 to 5° depending on orientation. Microcline’s extinction angle varies up to about 15° depending on orientation.
Fig 2.2.1 Granite
This is a granite from near Barre, Vermont. It contains mostly colorless quartz, microcline, and plagioclase. Minor flakes of brown biotite are also present. One grain of quartz shows yellow interference color, suggesting that this section is a bit too thick. The microcline near the center of the view shows very well developed cross-hatched twinning in the XP view. This kind of twinning is also sometimes called “tartan” or “Scotch plaid” twinning. One large grain of subhedral plagioclase in the lower right shows faint compositional zoning. FOV = 2.5 mm.
This orthoclase grain is brown with clay alteration and intergrown with tiny blebs and stringers of colorless albite (PP). A single simple Carlsbad twin (XP) divides two large domains. The wormy patches (blebs and stringers) are perthite exsolution. Surrounding colorless quartz (PP) with simple gray to white interference colors (XP) is characteristically unaltered. FOV = 7 mm. Photos from www.alexstrekeisen.it.
This rhyolite from Chaffee County, C0l0rado, is mostly a fine-grained groundmass of quartz and feldspar. But, the view contains a large sanidine phenocryst with a conspicuous Carlsbad twin (XP). The fine-grained groundmass contains quartz, K-feldspar and specks of magnetite and hematite. FOV = 2 mm.
A large, colorless crystal of microcline at the top of the image has been replaced in the image center by myrmeckite (a colorless intergrowth of plagioclase and quartz) (PP); these are not identifiable except in XP, which shows wavy interference for microcline and wormy texture for myrmekite. The orientation of the large microcline grain does not produce a sharp cross-hatched appearance, rather it is more spindly. Minor biotite (dark brown to tan, PP) is present along the edges of the photo. FOV = 1.5 mm.
This arkose contains single grains of slightly altered microcline (lower left) and plagioclase (lower right) and abundant colorless, unaltered quartz, all cemented with brownish calcite (PP). The large microcline crystal displays both tartan twinning and exsolution lamellae (XP). The large plagioclase crystal has zebra-stripe twinning (XP). The calcite has high-order pastel interference colors and shows conspicuous twining (XP view). FOV = 3.5 mm.
This is a view of perthite in a metaluminous biotite granite. The larger, more altered (brownish) microcline domains contrast with less-altered, colorless, albite exsolution lamellae (PP). Interference colors are nearly identical (XP). K-feldspar typically alters more readily than plagioclase, but a Becke line moves into the phase with the higher index of refraction, albite, if you lower the stage. Photos from Kurt Hollocher. FOV = 1.2 mm.
Occurrence—Plagioclase feldspars are found in a wide variety of igneous and metamorphic rocks and, to a lesser extent, in sedimentary rocks.
Distinguishing Features—Plagioclase is generally colorless in thin section unless altered, and shows gray and white interference colors. When altered, plagioclase may appear grayish. Polysynthetic twins, when present, greatly aid identification.
Classic albite twinning (XP)
Normally plagioclase appears as subhedral to anhedral colorless plates or laths, although crystals can be euhedral in volcanic rocks. Most plagioclase exhibits conspicuous albite and/or pericline twins (polysynthetic twins giving a black and white striped “zebra striped” appearance in XP light). This kind of sharp regular twinning is diagnostic. Simple (Carlsbad) twins may also be present. Twins that go in two directions (Fig 2.3.3) in plagioclase are sharply bounded, unlike in microcline.
Plagioclase has one perfect, one less perfect, and one poor cleavage, but cleavage is typically not obvious in thin section.
Albite (Na-rich plagioclase) can occur intergrown with microcline due to exsolution during cooling, creating perthite that appears as a wavy black and white pattern under XP light (see Fig 2.2.2). Albite-rich plagioclase can also intergrow with quartz in a wormy texture called myrmekite, typically as an alteration product after K-feldspar (Fig. 2.2.4)
Plagioclase may alter to sericite (fine-grained white mica) or to saussurite (epidote). Zoning is often present, and produces concentric waves of extinction as the stage is rotated.
Extinction angles relative to albite-twin boundaries may be used to determine composition (the Michel-Levy method).
plagioclase
Similar Minerals—Plagioclase may be confused with quartz or K-feldspar. But quartz has no cleavage, no polysynthetic twinning, and is uniaxial. K-feldspar commonly has twins different from those of plagioclase and lower relief.
Optical Properties ■■ Triclinic; biaxial (+ or -) ■■ 2V = 75-90° ■■ Optical properties vary with composition but refractive index (1.53 to 1.59) and birefringence (0.007 to 0.013) are low ■■ Plagioclase exhibits low relief and maximum interference colors are normally 1st-order gray and white ■■ Overly thick sections may show faint 1st-order yellow
Fig 2.3.1 Diabase
This rock is from the Palisades Sill in New Jersey. It is a diabase and contains mostly labradorite (a relatively Ca-rich plagioclase) and augite (clinopyroxene). The plagioclase is colorless but a bit “dusty” due to incipient alteration. The augite is a darker greenish-gray color. In XP, albite twins give plagioclase its characteristic striped appearance. FOV = 2.5 mm.
Several low-relief, colorless plagioclase grains populate this latite porphyry (compositionally intermediate Na- and K-rich volcanic rock with large crystals) from Wolf Creek, Montana. Besides being twinned, the larger plagioclase shows a hint of concentric compositional zonation (seen most easily in PP light). The fine-grained groundmass is mostly plagioclase but also contains opaque magnetite (black in PP). FOV = about 3.5 mm.
The colorless minerals (PP) are plagioclase (twinned) and quartz (not twinned). The large plagioclase grain in the center shows both albite and pericline twins, nearly perpendicular to each other (XP). These sharp boundaries distinguish it from microcline. Twinning is discontinuous in other plagioclase grains. The green and brown/tan minerals (PP) are hornblende and biotite, mostly hornblende. The biotite is pleochroic, so some grains have a color similar to that of the hornblende. The two can be tough to distinguish, but biotite has a more flaky (micaceous) habit and different interference characteristics. FOV = 2.5 mm.
This dacite from near Helena, Montana, contains half a dozen large colorless plagioclase grains. The plagioclase is surrounded by fine-grained material that is mostly quartz and feldspar, but a few grains of subhedral brown biotite and green hornblende are also present. Some of the plagioclase shows twinning, and other grains show concentric zoning in XP. FOV = 3.5 mm.
This basalt from the Cima Volcanic Field, southern California (Mojave National Preserve) contains abundant clear plagioclase laths and light-green augite (clinopyroxene) surrounded by dark glass (PP). The plagioclase has gray to white interference colors and the augite displays 2nd-order yellow to blue to pink interference colors (XP). Two vesicles are also present (colorless, PP; isotropic, XP) – one left of center on the bottom edge, and the other near the center of the right-hand side. Some plagioclase exhibits twinning, giving a zebra striped appearance (XP), although it is hard to see at this scale. The interstitial glass is isotropic (black). FOV = 2.5 mm.
A large colorless plagioclase crystal in a dacite porphyry appears quite homogeneous in the PP view, but in XP shows fine horizontal twins (probably albite twins) and subtle oscillatory zoning, represented by alternating darker and lighter domains along the length of the crystal – these domains represent cyclic variations between more and less Ca-rich compositions as the crystal grew. Black glass surrounds the plagioclase. Photos from Kurt Hollocher. FOV = 3 mm.
In this metaluminous (moderate Al-content) granite, patches of brown, grungy-looking, fine-grained sericite replace colorless plagioclase (PP). At least some of the sericite is fine-grained white mica, but because it is generally made up of such small crystals, its birefringence is irregular and generally low. Some of the larger crystals do display 1st-order birefringence. Albite twinning in the plagioclase is clearly visible in XP.
This view shows a basalt from the Deccan Plateau, India. The principle minerals are colorless laths of plagioclase and gray higher-relief augite. The red (in both PP and XP) material is mostly hematite. A few grains of an opaque mineral are also present. FOV = 3.5 mm.
Fig 2.3.9 Amphibolite This is a classic amphibolite, a metamorphic rock that contains plagioclase, hornblende, and little else. In the PP view, the plagioclase is clear, and the hornblende is in various shades of brown and shows near 60o-120o cleavage angles. The only other constituent mineral is an opaque which is either magnetite or ilmenite. In XP, the plagioclase shows its usual 1st-order gray and white colors; hornblende shows up to high 2nd-order colors. The plagioclase in this sample is notable for containing discontinuous twins and twins in two perpendicular orientations. They are pericline and albite twins. In some of the other photos above, plagioclase shows only one set of twins and they are quite uniform/continuous. This rock comes from near Soudan, Minnesota. FOV = 3.5 mm.■■ Larger photos: PPXP
Fig 2.3.10 Granite This granite from the Russian Peak pluton, northern California, is dominated by low-relief colorless, but slightly altered, plagioclase. Other major minerals include low-relief colorless and unaltered quartz, and low-relief colorless but pervasively altered K-feldspar (PP). Mafic minerals include highly pleochroic brown biotite and green hornblende (PP). The XP view reveals the complex internal structure of the large plagioclase grains, with axial twins (vertical white arrows), polysynthetic twins (inclined yellow arrow), and oscillatory zoning (horizontal white arrows). Plagioclase, K-feldspar, and quartz all display 1st-order gray to white interference colors. The intense colors of hornblende and biotite mask their interference colors (XP). Biotite shows birds eye texture. FOV = 8 mm.■■ Larger photos: PPXP
2.4 Nepheline
(Na,K)AlSiO4
Occurrence—Nepheline occurs in Si-poor igneous rocks, including syenite (plutonic) and trachyte (volcanic). It is classed as a feldspathoid (or “foid” for short), meaning its chemistry and structure are similar to feldspars, but it is silica-deficient. Nepheline foidolites (plutonic) and foidites (volcanic) contain more then 60% feldspathoid minerals. Nepheline is associated with diverse minerals, but most are Na- or K-rich.
Distinguishing Features—Nepheline is colorless to cloudy (PP), has very low birefringence (gray-white interference colors), very poor cleavage, and is uniaxial (-). It has several fair to poor cleavages, but they are rarely seen in thin sections.
Nepheline is generally anhedral to subhedral in intrusive rocks. But crystals in extrusive rocks can be short prisms or blocks with rectangular or hexagonal outlines; sometimes they are phenocrysts.
nepheline
Similar Minerals—Nepheline is superficially similar to orthoclase, which is biaxial and has better cleavage. It is occasionally confused with scapolite or melilite (a rare paired-tetrahedra silicate). Quartz has higher birefringence and is uniaxial (+). Apatite has greater relief. Topaz is biaxial.
Optical Properties ■■ Hexagonal, uniaxial (-) ■■ ω = 1.532-1.547, ε = 1.529-1.542, low relief ■■ δ = 0.003-0.005, interference colors range up to 1st-order gray although many images are overexposed, making it look white. ■■ Basal sections exhibit a uniaxial (-) figure with no isochromes ■■ Extinction is parallel for rectangular sections; basal sections remain extinct in all orientations
Fig 2.4.1 Nepheline Syenite
This nepheline syenite from Bancroft, Ontario, contains mostly nepheline and albite-rich plagioclase, with lesser amounts of biotite. In the PP view, both the nepheline and plagioclase area clear. But the nepheline is highly fractured and is altering to sericite (fine-grained muscovite). In the XP view, the plagioclase shows twinning; nepheline does not. The view also includes a large grain of brown-green biotite (PP). The biotite’s 2nd-order interference colors barely show in the XP view because of the strong coloration of the mineral. FOV = 3.5 mm.
This nepheline-augite rock comes from Löbauer Berg, Saxony, Germany. Large crystals of nepheline (Nph) are colorless in PP with needle-shaped inclusions of apatite. Augite is strongly colored yellow-brown to violet-brown (PP). It contains zones of varying color and birefringence. Ilmenite (Ilm) is black, opaque. A few plagioclase crystals are also present. FOV = 5mm
This rock from the Kola Peninsula, Russia, contains large, blocky, nearly colorless, nepheline crystals surrounded by a finer matrix of plagioclase and minor aegirine (a Na-rich clinopyroxene). The nepheline contains many inclusions of plagioclase (PP) and shows nearly black interference color (XP) because it is oriented with its c-axis perpendicular to the slide. The small pyroxene crystals display up to 2nd-order interference colors but are hard to see. FOV = 7 mm.
This image of a syenite from Liaoning Province, northeastern China, is centered on a fractured grain of colorless nepheline, surrounded by colorless, low-relief K-feldspar, with additional reddish-brown biotite, and green, high-relief aegirine (sodic pyroxene; PP). In XP, the nepheline and K-feldspar are 1st-order gray and the intense colors of biotite and aegirine mask their intereference colors. FOV = 3.5 mm. Photos courtesy of Dr. Hao Yang, Jilin University.
This nepheline foidite from Cape Verde contains large, colorless blocky nepheline crystals (PP), some showing twinning (XP). Two grains of augite are grayish (PP) and display 3rd-order interference colors (XP). The matrix contains fine colorless nepheline and opaque magnetite. Some areas are isotropic, suggesting that it also contains glass. FOV = 7 mm. Photos from www.alexstrekeisen.it.
Nepheline (Nph), sodalite (Sdl), and microcline perthite are all colorless and slightly clouded by alteration in the PP view. Cancrinite (Ccn) is also colorless but slightly more clear. In XP, the differences among the minerals are striking, with the 2nd-order colors of the cancrinite standing out against the 1st-order gray of the nepheline, the extinct isometric sodalite, and the plaid microcline twinning interrupted by the plagioclase in the perthite. In PP, acmite (black), calcite (high relief), and apatite (needles) are also visible. FOV = 5mm
Occurrence—Leucite is a rare mineral found in Si-poor, K-rich igneous rocks. It is never found with quartz.
Distinguishing Features—Leucite occurs primarily as phenocrysts in lavas. It is colorless in thin section, has extremely low birefringence (often appearing almost isotropic), and may show complex polysynthetic twinning. Cleavage is absent or very poor.
8-sided leucite crystal. Photo from Bereket Haileab, Carleton College.
Euhedral crystals may have a distinctive 8-sided shape. Complex polysynthetic twins may occur in three directions, somewhat similar to twinning in microcline, but with sharper boundaries.
leucite
Similar Minerals—Analcime and sodalite lack twinning, and sodalite is isotropic. Leucite has higher relief than both of these.
Optical Properties ■■ Leucite – tetragonal; uniaxial (+) ■■ ω = 1.508, ε = 1.509, leucite has low relief ■■ δ = 0.001, only very low 1st-order gray interference colors ■■ Small crystals of leucite may appear to be isotropic
Extinction and Orientation—Leucite is pseudocubic and commonly exhibits wavy extinction.
Fig 2.5.1 Leucite Foidolite
This foidolite contains colorless leucite and greenish-gray clinopyroxene (PP). In XP, the leucite appears almost isotropic, whereas the pyroxene shows up to 2nd-order interference colors. FOV = 7 mm. Photos from www.alexstrekeisen.it.
This tephrite (alkaline volcanic rock) from Vulsini Volcano, Italy, contains colorless phenocrysts of leucite surrounded by a fine-grained glassy groundmass. The leucite crystals are almost isotropic, but faint twins are visible in XP. FOV = 7 mm. Photos from www.alexstrekeisen.it.
This view shows a basalt from Mt. Vesuvius. Rounded leucite (Lct) and prismatic plagioclase (Pl) are colorless in PP, as are a few smaller olivine grains. A large, euhedral, color-zoned augite (Aug) crystal displaying one prominent cleavage is present. The groundmass contains all of these minerals as well as magnetite and apatite. The leucite and plagioclase both show lamellar twins in XP. FOV = 5mm
In PP, small greenish augite, brownish hornblende, and black magnetite grains provide the background for larger, colorless round crystals of leucite dotted with many black inclusions along with some colorless nepheline crystals (not round, no black inclusions). In XP, the leucite crystals are very dark with complex multiple twins, whereas the nepheline grains brighter gray. This rock comes from the Capo di Bove lava flow, Rome, Italy. FOV = 1.25 mm
Colorless, low-relief leucite contrasts with high-relief, gray-green clinopyroxene in a foidolite. In XP, leucite shows typical low 1st-order gray interference colors and twinning, whereas augite shows up to 2nd-order blue interference colors. FOV = 2mm. Photos from www.alexstrekeisen.it.
Round colorless crystals of leucite are each surrounded by tiny yellow-green augite grains. Square nepheline crystals and laths of sanidine are also colorless in PP. Opaque magnetite and mesostasis are also present. In XP, many leucite grains show complex lamellar twinning. Augite stands out with its colorful birefringence. Nepheline and sanidine display 1st-order gray interference colors. This rock comes from Schloss Olbrück, Brohltal, Germany. FOV = 5mm
2.6 Zeolites
Natural zeolites include around 40 species. The most common are analcime, chabazite, clinoptilolite, erionite, ferrierite, heulandite, laumontite, mordenite, natrolite and phillipsite. Analcime, although commonly grouped with zeolites, is actually a feldspathoid.
Zeolites have a compositions similar to hydrated plagioclase. They are solid-solution minerals; Na:Ca:K ratios are variable for most species. Al:Si ratios also vary, as does H2O content. The list below contains some example ideal formulas, grouped according to structural similarities.
Occurrence—Zeolites are common in sedimentary rocks and sediments, in altered volcanics, and in low-grade metamorphic rocks. Larger, more spectacular samples are found in vesicles, cracks, or other openings in mafic volcanic rocks.
Distinguishing Features—These minerals are all colorless and have low relief. Most display 1st-order interference colors. If coarse, they may show one good or perfect cleavage. Twinning is common. Occurrence as a replacement mineral or in vugs or veins in mafic volcanics are key identifying characteristics. Any of the zeolites may be very fine-grained, sometimes as a drusy crust projecting from cavity walls. When fine-grained, distinguishing the different species may be impossible.
natrolite
Similar Minerals—All zeolites have similar optical properties, and they may be difficult or impossible to distinguish petrographically.
When coarse, zeolite habit aids identification. Members of the natrolite group are typically fibrous. If euhedral and visible, crystals of other zeolites are more commonly tabular crystals, sheaf-like or radiating bundles, columns, granular masses and, less commonly, fibers. Because zeolites may be quite coarse, distinguishing the different zeolites in thin section can be problematic because typical petrographic microscopes do not allow viewing at low enough magnification. Chemical and sometimes structural analysis may be required for reliable identification.
Optical Properties ■■ Orthorhombic, hexagonal, or monoclinic; may be biaxial or uniaxial ■■ Optical sign and indices of refraction vary with species and composition ■■ Zeolites have low relief ■■ Highest interference colors are 1st-order ■■ Cleavage flakes have extremely low birefringence
Fig 2.6.1. Natrolite Pegmatite
This pegmatite from Tvedalen, Norway, is mostly cloudy natrolite with one elongate grain of colorless, high-relief diaspore on the right (PP). In XP view, the natrolite shows 1st-order interference colors up to pale yellow. The diaspore shows 3rd-order green interference colors . FOV = 9 mm.
In this rock, colorless, blocky, low-relief analcime (analcime group zeolite) and colorless, low-relief, needly thomsonite (natrolite group zeolite) have grown together in an open cavity (PP). In XP, interference colors for analcime are so low the mineral appears almost isotropic, whereas thomsonite shows about the highest interference colors seen for zeolites – 1st-order yellow and orange.
A vesicle in this basalt is mostly filled with colorless, low-relief phillipsite (PP). The dark material around the vesicle is volcanic glass. In XP, the phillipsite shows low 1st-order interference colors and a radiating habit from vesicle walls. FOV = 7 mm. Photos come from www.alexstrekeisen.it.
In this specimen, fine-grained, low-relief, colorless analcime (PP) fills vesicles in an altered plagioclase-rich basalt from central Arizona. Vesicle fillings like this are extremely common in basalts. In XP, the analcime shows its characteristic low, 1st-order gray interference colors. FOV = 8 mm.
This rock from New Jersey contains low-relief, colorless splays of laumontite (analcime group zeolite) and blocky heulandite (heulandite group zeolite). They are intergrown with quartz (PP). In XP, the quartz has slightly higher 1st-order white interference colors and is slightly less cloudy than the light- to dark-gray zeolites. Note the domainal extinction in the heulandite. FOV = 8 mm.
This view shows relatively rare low-relief, colorless gmelinite (chabazite group zeolite) from Northern Ireland. It forms large toothy crystals (PP) on altered basalt. The XP view reveals optically distinct cores. As with most zeolites, interference colors are 1st-order gray. FOV = 8 mm.
Coarse, colorless, low-relief stilbite occurs with finer-grained, light-blue cavansite (PP). Cavansite is a rare vanadium mineral. In XP, stilbite ranges up to very faintly yellow 1st-order interference colors FOV = 4 mm. Photos from rockptx.com.
Coarse, low-relief, colorless clinoptilolite (heulandite group zeolite) occurs as radiating splays (PP) on this plagioclase-rich, altered basalt from coastal Oregon. In XP, we see much twinning and low, 1st-order gray interference colors that are characteristic of zeolites. FOV = 3 mm.
Blocky, low-relief, colorless clinoptilolite (heulandite group zeolite) and low-relief, colorless, needly gonnardite (natrolite group zeolite) fill open space in this altered basalt (PP). In XP, the clinoptilolite shows optical zoning and low, 1st-order gray interference colors, while gonnardite ranges from 1st-order white colors to appearing isotropic, depending on needle orientation. FOV = 8 mm.
This rock from central Nova Scotia, contains moderate-relief colorless barite (BaSO4), moderate-relief slightly brownish dolomite, and low-relief colorless chabazite (chabazite group zeolite), developed as a crust on altered dark brown to black basalt (PP). In XP, the barite shows 1st-order gray to white intereference colors, dolomite is characteristically pearly gray to pastel. Chabazite’s interference colors are so low, the mineral appears almost isotropic. FOV = 8 mm.
Occurrence—Sodalite is commonly associated with other feldspathoids. It occurs with nepheline, leucite, or cancrinite in Si-poor, alkali rich, igneous rocks, as well as with K-feldspar. It is especially common in syenites.
Distinguishing Features—Sodalite is isotropic, colorless to gray (rarely blue) in PP, with low relief. Euhedral crystals appear hexagonal, but most sodalite is anhedral.
Sodalite has poor cleavage. Borders may be darker and show better (cubic) cleavage than cores.
sodalite
Similar Minerals—Sodalite resembles analcime, but analcime typically occurs as a secondary mineral, has very poor cleavage, and can be weakly birefringent. Haüyne, another isotropic mineral, is similar to sodalite but has higher relief and less prominent cleavage. Fluorite, also isotropic, has lower relief and octahedral cleavage.
Optical Properties ■■ Cubic; isotropic ■■ n = 1.485, relief is low ■■ Isotropic character is a key to identification
Fig 2.7.1 Sodalite Monzonite
This monzonite (felsic plutonic rock) from the Jacupiranga Complex, Brazil, contains colorless low-relief sodalite (large grain in lower-center labeled Sdl), colorless plagioclase, and green-brown biotite. The very low (negative) relief of sodalite gives it a rougher texture. In XP, sodalite is black (isotropic), plagioclase shows typical fine twinning, and the biotite displays a bird’s eye texture. Biotite’s interference colors are masked by the color of the mineral. FOV = 3.5 mm.
This syenite (low-Si, felsic, plutonic rock) contains colorless sodalite (labeled Sdl in the PP view), colorless K-feldspar, and green to dark-green biotite (PP). In XP, sodalite is black (isotropic), biotite shows 2nd-order interference colors (mostly green), and the feldspar shows gray and black interference colors with some faint mottling. FOV = 7 mm. Photos from www.alexstrekeisen.it.
This rock from Scotland contains abundant, colorless sodalite and orthoclase, with dark green aegirine-augite (a Na-rich pyroxene) and brownish green biotite (PP). In XP, the sodalite is opaque, giving the thin section an overall dark color. The feldspar laths show 1st-order gray to white interference colors and the mafic minerals somewhat higher (2nd-order) colors. FOV = 15 mm. Photos modified from virtualmicroscope.org.
Although nondescript in PP, this is the famous Azul (Blue) Bahia “granite” from Brazil, one of the world’s most expensive common countertop and tiling stones. This feldspathoidal syenite is nearly bimineralic. It contains low-relief, colorless sodalite and low-relief K-feldspar that is slightly brownish because of clay alteration (PP). It is only in XP that we can definitively identify sodalite because it is isotropic, and can see clearly the large plates of K-feldspar with their low 1st-order gray to white interference colors. FOV = 9 mm.
Occurrence—Most scapolite is a metamorphic mineral found in marbles, mafic gneisses, and amphibolites, but it can also form in alkaline igneous rocks.
Distinguishing Features—Scapolite is generally colorless, but may be yellowish. It commonly forms rectangular, elongated crystals.
Interference colors may be up to 2nd- or 3rd-order blue depending on composition. Scapolite is tetragonal and so has parallel extinction. It is uniaxial (-) and does not twin.
Scapolite has several good to poor cleavages, intersecting at 90° in a cross sections.
scapolite
Similar Minerals—Scapolite may appear similar to plagioclase or quartz, but it lacks twinning and commonly exhibits higher-order interference colors. Additionally, feldspars have oblique extinction, and quartz is uniaxial (+). Cordierite is biaxial, often contains pleochroic halos, and is associated with different minerals (especially sillimanite). Cancrinite is also uniaxial (-), but has lower refractive indices.
Optical Properties ■■ Tetragonal; uniaxial (-) ■■ ω = 1.539-1.596, ε = 1.537-1.557 ■■ δ = 0.002-0.04 ■■ Basal sections give a uniaxial (-) figure, longitudinal sections give a flash figure ■■ Low to moderate interference colors increase with composition from marialite to meionite ■■ Marialite has gray 1st-order interference color, and meionite has up to 2nd-order violet ■■ Scapolite has parallel extinction
Fig 2.8.1 Scapolite Marble
This marble from the Grenville Province, Ontario, contains abundant colorless, low-relief scapolite (PP). Although optically similar to quartz and feldspar in PP, in XP the scapolite shows 1st- to low-2nd-order interference colors. FOV = 20 mm. Photos from rockPTX.com.
This calc-silicate from central Nepal consists almost entirely of three minerals – low- to moderate-relief colorless scapolite, low-relief colorless quartz, and moderate-relief hornblende in varying shades of greens (PP). The substage diaphragm was closed down to emphasize relief and show that scapolite actually has slightly higher relief than quartz (PP), but this introduced anomalous mottling to the quartz. In XP, scapolite’s intense upper 1st to lower-2nd order interference colors stand out against quartz’s low 1st-order gray to white. Hornblende shows 1st-order orange colors. FOV = 3 mm.
This calc-silicate from central Nepal has a diverse assemblage of low-relief colorless scapolite, low-relief colorless quartz, variable-relief colorless calcite with colored twins planes, high-relief, very slightly greenish clinopyroxene, brown biotite, and high-relief titanite (PP). In XP, scapolite has high 1st- to low 2nd-order colors, distinguishing it from quartz with its low 1st-order gray to white interference colors. Calcite shows pastel to pearl interference colors, as well as pronounced twinning (XP). Clinopyroxene has intense, uppermost 1st-order interference colors, biotite shows 2nd-order colors and birds eye texture, and titanite has super-high interference colors, similar to the colors of calcite (XP). FOV = 3 mm.
This calc-silicate (origin unknown) contains mostly low-relief colorless scapolite and quartz. Other minerals are moderately high-relief colorless clinopyroxene (likely a diopside-hedenbergite solid solution), and high-relief colorless to tan titanite (PP). In XP, scapolite is easily distinguished from quartz because of its bright 2nd-order interference colors. Quartz has only 1st-order gray to white colors. Clinopyroxene also displays 2nd-order colors. Titanite’s colors are so high-order, the grains appear pastel to pearly. FOV = 1.4 mm.
Colorless scapolite (Scp) fills most of the upper have of the image with some clear colorless microcline (Mc) crystals. In the lower half, diopside (Di) is pale green in PP, with cleavage fractures and relatively high relief, and surrounds a few darker green amphibole grains. A large grain of titanite and opaque graphite flakes are also visible in PP. The uniaxial (-) property of scapolite is a quick way to confirm its identity. This rock comes from the eastern Adirondack Mountains, New York. FOV = 5 mm
Occurrence—Quartz is found in a wide range of igneous, metamorphic, and sedimentary rocks.
Occurrence—Minerals of the K-feldspar group (orthoclase, microcline, and sanidine) are common in many kinds of silicic igneous rocks and their metamorphosed equivalents. Microcline and orthoclase also occur in high-grade metamorphic rocks and in arkoses.
Occurrence—Plagioclase feldspars are found in a wide variety of igneous and metamorphic rocks and, to a lesser extent, in sedimentary rocks.
Occurrence—Nepheline occurs in Si-poor igneous rocks, including syenite (plutonic) and trachyte (volcanic). It is classed as a feldspathoid (or “foid” for short), meaning its chemistry and structure are similar to feldspars, but it is silica-deficient. Nepheline foidolites (plutonic) and foidites (volcanic) contain more then 60% feldspathoid minerals. Nepheline is associated with diverse minerals, but most are Na- or K-rich.
Occurrence—Leucite is a rare mineral found in Si-poor, K-rich igneous rocks. It is never found with quartz.
Occurrence—Sodalite is commonly associated with other feldspathoids. It occurs with nepheline, leucite, or cancrinite in Si-poor, alkali rich, igneous rocks, as well as with K-feldspar. It is especially common in syenites.
Occurrence—Most scapolite is a metamorphic mineral found in marbles, mafic gneisses, and amphibolites, but it can also form in alkaline igneous rocks.
