A continuous foliation defined by aligned euhedral biotite characterizes this fine-grained sample. Granoblastic quartz and feldspars fill the voids between biotite crystals. Small, equant opaqes are distributed evenly throughout.
This allotriomorphic granite shows evidence of weathering as most feldspars are partially obscured by a dusting of clay minerals. Tartan twinning of microcline is obvious throughout the sample. The curving of biotite cleavage and subhedral grain shapes indicates the biotite is not pristine. It is commonly found occurring with subhedral hornblende as well as large, clear to light brown, anhedral sphene crystals.
This strongly foliated amphibolite shows evidence of mimetic growth of k-feldspar into elongate crystals due to restriction of a preexisting fabric defined by amphibole and micas. The dominant mica phase, chlorite, has anhedral (almost skeletal) grain shapes. Hornblende is the same. Both phases are highly embayed. Several hornblende grains contain euhedral feldspar 'inclusions' and the classic amphibole grain shape is lost along its margins due to encroaching feldspar grains. Chlorite tends to be associated with elongate epidote crystals or granular aggregates. Dusty fracture zones trace across the fabric of this sample and are filled with cataclastic breccia and glass. Euhedral apatite and small, rounded to larger, anhedral titanite grains are dispersed throughout the thin section, typically oriented with the foliation.
This coarse-grained samples is dominated by subhedral quartzofeldspathic phases. The feldspars are altering to clays and the quartz extinction is undulose. The biotite phase is altering to chlorite in some places and is generally subhedral in shape; kink bands are found in some grains. The hornblende crystals are subhedral and generally cluster with the biotite.
This mineral phases in this gabbro are highly fractured and frequently embayed. The pyroxenes are roughly euhedral in shape and several equant grains are twinned. The opaques in this sample are typically equant in shape and contain inclusions of olivine or a fiery orange-red mineral that appears the same in plane- and cross-polarized light.
The layering in this fine-grained amphibolites is defined by bands of amphibole alternating with granoblastic quartzofeldspathic layers. Granular aggregates of epidote are dispersed randomly throughout the rock. Fractures cross-cutting and off-setting the layering are more visible with the naked eye then under the microscope, though one fracture (with no offset) is filled with a seam of epidote. A couple of porphyroclastic and poikiloblastic chlorite grains are clearly truncated by the fractures.
Most crystals in this thin section are highly embayed, including the distinctive riebeckite grains.
A continuous foliation in this sample is defined by aligned hornblende grains. Altered biotites are poikiloblastic and have kink bands causing undulose extinction. The aligned hornblende grains are juxtaposed against equigranular polygonal-interlobate quartz and aggregates of altered and decussate muscovite and clinozoisite.
This amphibolite has a strong, continuous foliation defined by the alignment of hornblende. Elongate, granular aggregates of epidote are present.
Phenocrysts of resorbed plagioclase and k-feldspar give this otherwise glassy rock its porphyritic texture. A couple of resorbed pyroxene phenocrysts are present. Aligned needles of an opaque phase give the glassy groundmass a trachytic texture.
This medium-grained phaneritic rock contains an equal ratio of plagioclase to clinopyroxene grains. Much of the plagioclase is strongly sericitized and several grains are concentrically-zoned. The clinopyroxenes are frequently twinned, heavily-fractured, riddled with inclusions, and show low degrees of alteration to chlorite and serpentine. The pyroxene cleavage is prominent in this sample. Chlorite, serpentine, iddingsite, biotite, and opaques tend to cluster in randomly oriented mats, where chlorite and serpentine growth are often at the expense of biotite.
The groundmass of this thin section contains a combination of glass and indiscernible crystals. It is densely populated by phenocrysts of a wide range of sizes and composition, which due to their angularity, lend a very fragmental texture to the rock. The quartzofeldspathic phases span a broad range of sizes, though the largest phenocrysts are all brittley-fractured and may be strongly embayed. No reaction rims are present in this sample. Biotite grains do not get as large as the quartzofeldspathic phases and display varying degrees of 'freshness.'
Biotite and olivine dominate this sample. The olivine is frequently twinned and highly fractured. In the interstices between olivine grains grows large, anhedral biotite grains. Both phases have a sieve-like, pock-marked appearance. Rutile and opaques, all equant in shape, are plentiful.
This fine-grained granite contains abundant feldspars, many of which display concentric zoning from plagioclase cores to k-feldspar rims. Biotite grains are altering to and interfingered with chlorite. Rounded hornblende crystals are rare in this sample.
The groundmass in this thin section is a salt-n-pepper mixture of fine-grained, amorphous quartzofeldspathic phases. All phenocryts are subhedral in shape, and the largest of these, the k-feldspar and plagioclase are host to abundant hematite-filled fractures. Biotite and hornblende phenocrysts are much smaller. A few spherulites, seen more clearly in plane than cross polarized light, are found in the groundmass.
This fine-grained, hypidiomorphic, inequigranular plutonic rock is intermediate in composition, with plagioclase as the dominant feldspar. K-feldspar is present in much lower abundance, and quartz is scarce. Mafic phases include anhedral biotite and hornblende, the latter of which is strongly chloritized.
This fine-grained dacite has a strongly cloudy appearance throughout, as well as a film on the surface of the slide that prevents clear focusing. Dusty plagioclase, plucked hornblende, and equant augite are the dominant phases. Euhedral apatite crosscuts most samples. Zeolites and calcite fill most voids or interstices.
Augite and olivine are the two crystalline phases in this 'cloudy' vesicular basalt. Iddingsite replacement of olivine is almost comprehensive; commonly, iddingsite nodes are the only indication of olivine's presence. The groundmass is microcrystalline and altered enough as to be unidentifiable.
Two minerals, olivine and augite, comprise the phenocrystic population. Olivine is larger and more abundant. Both are present in the glassy groundmass, along with trace plagioclase laths, and abundant opaques.
Allotriomorphic plagioclase and highly-fractured olivine are the dominant minerals in this thin section. Clinopyroxene, when present, is found adjacent to olivine. Fractured zones in this rock follow olivine clusters and frequently cross-cut plagioclase crystals in swarms between nodes of granular olivine.
But for the anhedral quartz microphenocryss in this vesicular rhyolite, the rest is a clay-altered glass.
The unknown mineral is the prominent mineral in this thin section. It forms a dusty brown, isotropic matrix with a faint dendritic texture in which subhedral clinopyroxene and plagioclase grains are set. Clinopyroxene, where isolated, is strongly altered to iddingsite. The degree of alteration is much lower where plagioclase and clinopyroxene grains are intergrown. The two minerals have an almost graphic intergrowth texture. Western Minerals Inc. as an 'interstitial ferrotholeiite, very highly fractionatedâ€¦textures are similar to those associated with rapidly frozen or highly viscous melts.'
Based on grain size, two populations of pyroxenes and plagioclase exist in this sample; the first are the fine-grained, equant crystals of the glassy groundmass, and the second, the subhedral to anhedral phenocrysts visible in handsample. The inconsistently-shaped plagioclase grains are sometimes zoned and frequently contain inclusions of glass.
Plagioclase, the most abundant mineral in this thin section, grades smoothly in size from groundmass mircrolites to subhedral microphenocrysts. Tiny, equant opaques join the plag microlites in forming the groundmass. Clinopyroxene, the second most abundant mineral, is found as relatively euhedral microphenocrysts. Plucking of the pyroxenes is severe. A fiberous phase is frequently found infilling or perhaps pseudomorphing a rectangular phenocrystic phase.
The groundmass of this porphyritic basalt is predominantly glass with less abundant plagioclase microlites. The most readily-identifiable mineral of the phenocryst population is plagioclase. Of greater abundance are euhedral, opaque pseudomorphs. Rare inclusions of pyroxene are observed within these opaques. Pockets of chalcedony disrupt the otherwise uniform groundmass.