S

S section ◙ S/C fabric ◙ S-foliation ◙ S-plane ◙ S-tectonites ◙ sanidinite facies ◙ schists ◙ schlieren ▪ schlieren ◙◙ Sedimentary Rocks: Rock Index ◘ sedimentary rocks ◊ serpentines ◙ serpentinites ◙ serpentinization ◙ shear sense ◙ shearing ◙ shear, stress, strain ◙ shear zone ◊ sillimanite ▪ sills ◙ slickensides ◙ -somal ◊ spinels ◊ staurolite ▪ strata ◙ subduction zone magmas ◊ sulfates & evaporates

◊◊ silicates ◊◊

◊◊ aluminosilicates ◊ zeolites ◊

◊◊ cyclosilicates [SinO3n]2n- – tourmaline

◊◊ inosilicates [Si4nO11n]6n- ◊ pyroxenes

◊◊ inosilicates [Si4nO11n]6n- ◊ amphiboles

◊◊ nesosilicates [SiO4]4- ◊◊ magnesium iron silicate ◊ olivine ◊

◊◊ phyllosilicates [Si2nO5n]2n- ◊ biotite ◊ - micas and clays

◊◊ sorosilicates [Si2O7]6- ◊ epidote ◊

◊◊ tectosilicates [AlxSiyO2(x+y)]x− ◊ feldspars ◊ quartz ◊ zeolites

alphabetic sections ◊ A ◊ B ◊ C ◊ D ◊ E ◊ F ◊ G ◊ H ◊ I ◊ J ◊ K ◊ L ◊ M ◊ N ◊ O ◊ P ◊ Q ◊ R ◊ S ◊ T ◊ U ◊ V ◊ W ◊ X ◊ Y ◊ Z ◊ elements of periodic table ◊ Mineral Index ◊ Rock Index

sanidinite facies

Rocks of the sanidinite facies, which results from temperatures higher than the pyroxene hornfels facies, are relatively rare in contact metamorphic aureoles, although they are found as xenoliths in igneous rocks. The facies correlates with the granulite facies, and is characterized by the absence of hydrous minerals, particularly micas. Mineral assemblages depend upon the composition of the protolith: ◙ the commonest sanidinites are basic rocks found along the conduit walls of dikes, which include various assemblages: --- ◙ augite, hypersthene, calcic plagioclase, brookite, and the high temperature quartz polymorph, tridymite. --- ◙ olivine, augite, plagioclase, magnetite, and ilmenite (this is similar to an igneous mineral assemblage). --- ◙ hypersthene, plagioclase, magnetite, ilmenite, pseudobrookite. --- ◙ cordierite, plagioclase, magnetite, hematite, pseudobrookite. ◙ pelitic and quartzo-feldspathic rocks yield sanidine, cordierite, anorthite, hypersthene, sillimanite or corundum, unusual phases like mullite (3Al2O3.2SiO2), and tridymite may replace quartz. ◙ calcareous rocks contain various assemblages containing rare minerals. --- ◙ wollastonite, anorthite, and diopside. --- ◙ wollastonite, mellilite, calcite, larnite, and rare minerals brownmillerite and mayenite.

links: webpages: Contact Metamorphism

schlieren

Schlieren (geology) are typically elongate concentrations of mafic material. A schlieren could be, for example, a tabular zone in a granite with either more or less of some of the minerals in the surrounding granite, typically the dark (mafic) minerals.

The origins of schlieren are not always clear; they may be produced by differential magma flow, or disaggregation of xenoliths, or by other mechanisms. Schlieren are usually interpreted as having arisen by one of four mechanisms: ◙ shearing of heterogeneities (enclaves or xenoliths), ◙ crystal sorting during convective flow, ◙ crystal sorting during magmatic flow, or ◙ crystal settling.

In crystal sorting at the time of formation or crystallization of a magma chamber, mafic minerals such as biotite, rare earth elements of the lanthanide and actinide series, allanite, and the phosphate mineral apatite can orient in a preferred manner that creates bands. Schlieren bands vary in geometry ranging from deformed, tubular, planar, and rings, to arachnid (spider-like) formations.

A schlieren arch is an intrusive igneous body with flow layers that occur along its borders, but which are poorly developed or absent in its interior. A schlieren dome is an intrusive body that is almost completely outlined by flow layers that culminate in one central area.

links: images: schlieren in biotite-rich mantle with granodiorite inside and outside, and close-up of the margin of the schlieren; a prominent schlieren that defines a structure rather like the hinge region of an isoclinal fold, and close-up of the upper left side of the schlieren showing dark, biotite-rich prominent part of the schlieren (curving to the right) with thinner, less prominent biotite -rich streaks extending upwards (the K-feldspar phenocrysts are approximately parallel to the schlieren margin); spidery "arocknid", composed of two sprays of thin schlieren; thick portion of schlieren with irregular convex surface, parallel alignment of K-feldspar phenocrysts, and K-feldspar phenocrysts in the host granodiorite that are nearly perpendicular to the convex margin of the schlieren (top center); K-feldspar-rich mass in normal foliated granodiorite; schlieren.

Schlieren (from the German for 'streaks') are optical inhomogeneities in transparent material that are not visible to the human eye. Schlieren, shadowgraph, and interferometric techniques are used to study the distribution of density gradients within a transparent medium.

schists

Schist (pron. shist) is a medium- to coarse-grained, often shiny, mica-laden rock. Medium-grade metamorphism causes recrystallization, rotation, and new growth of micas (predominantly muscovite, biotite, and chlorites) from fine-grained, mica-bearing rocks such as shales and slates, which results in the well-developed planar to wavy foliation (schistosity) characteristic of schists. Schists, such as garnet-biotite schists containing porphyroblasts of garnet and a schistosity dominated by biotite, are named for their assemblage of minerals. (right - click to enlarge : top, schist bed at Corea Ck.; center, surface of garnet schist (left) and biotite-mica schist (right); bottom, photomicrograph of garnet-mica schist).

links: images, roll-over for preview : gallery of rock photomicrographs : amphibolite : garnet-mica schist : mica schist : talc-tremolite schist : muscovite-foliation between quartz grains : garnet-kyonite-quartz schist : garnet-staurolite-muscovite schist : garnet-staurolite schist : kyanite schist : muscovite mica schist with crenulations : tourmaline mica schist : Connemara Schists : Schist Wave : schist landslide, NZ: : simple animation of metamorphism :

alphabetic sections ◊ A ◊ B ◊ C ◊ D ◊ E ◊ F ◊ G ◊ H ◊ I ◊ J ◊ K ◊ L ◊ M ◊ N ◊ O ◊ P ◊ Q ◊ R ◊ S ◊ T ◊ U ◊ V ◊ W ◊ X ◊ Y ◊ Z ◊ elements of periodic table ◊◊◊ Mineral Index ◊◊◊ ◙◙ Rock Index ◙◙

serpentinite

Serpentinites contain serpentine minerals generated by low-grade hydrothermal serpentinization of mantle-derived ultramafic rocks, such as peridotites, dunites, and komatiites. Serpentines are hydrous magnesium-rich silicate minerals that are typically colored gray, green, or white. (left - serpentinized rock with chrysotile) Hydration of olivines to serpentine and magnetite consumes water and produces a significant amount of heat in an exothermic reaction sufficient heat to elevate rock temperatures about 260 ºC, which can generate non-volcanic hydrothermal vents. The Lost City Hydrothermal Field (LCHF) on the Atlantis Fracture Zone near the Mid-Atlantic Ridge is an example of serpentinization-derived hydrothermal activity. Serpentinization may be among the earliest forms of metabolic energy on Earth. Serpentinization causes volume expansion and concomitant decrease in specific gravity (from 3.3 to 2.7 g/cm3) of the hydrothermally altered rocks. Metasomatic alterations convert olivines and pyroxenes to assemblages that depend upon composition of rock and fluids, temperature, and pressure: serpentine, brucite, magnetite, antigorite (at temperatures greater than 600°C), lizardite, and chrysotile, talc, magnesian chlorites, rare awaruite (Ni3Fe), and native iron. Serpentinites are soft, oily looking green to black fine-grained rock that is often highly sheared, breaking into scaly fragments with smooth shiny faces that tend to have translucent edges. Soapstones are rocks composed of the mineral talc and can be mistaken for serpentinite. Soapstone, however, is usually light green to grey in colour and is soft enough to be scratched by a fingernail, whereas serpentinites can only be scratched by a knife. Serpentinites are associated with subduction zones and are formed by the action of high pressure and heat upon hornblende schists or mafic/ultramafic igneous rocks, such as peridotite, gabbro, or basalt. Serpentinites are mostly located in areas where orogeny has sealed off of an ocean basin. The Coast Ranges in California comprise primarily slices of oceanic crust that have been faulted and folded along the coastline. The rock is relatively weak, so mountains containing high levels of serpentinite are prone to erosion and to recurrent landslides.

links: images: diagram: conceptual model for serpentinization of peridotitic source and subsequent hydrothermal dolomitization and exhalative block shale formation; schematic cross section of the Mariana forearc showing generalized structural relationship of serpentinite mud volcanoes to faulting in the outer half of the forearc; hand-samples: polished serpentinite, serpentinite, 2, 3, 4, 5, 6, 7; serpentinite with calcite; serpentinite compared to basalt; serpentinite with antigorite; serpentinite, Staten Island; close-ups: serpentinite with talc veins; serpentinite with asbestos vein; serpentinite; serpentinite, the Lizard, Cornwall; light brown serpentinite with dark brown veins, and light to dark green serpentinite with brown veins (marble-like); fabric in a piece of cut serpentinite, from a quarry in Chiesa di Valmalenco; formations: light-colored steatite (at base) overlain by a thin band of black metasediments, and cut by thick bands of green serpentinite (formed as a closely packed, matrix-free breccia in rocks probably originally laid down as a sequence of komatiite lava flows), Dunrossness Spilitic Group near Cunningsburgh; ophiolites represented as serpentinite with inclusions of high-pressure metamorphics with blocks of metasilicite- schist quartzite, and intercalation of green schist (high pressure metaophiolites are related to a subduction complex); serpentinite boulder, and close-up; S/C fabric in serpentinite, Coast Range, Central California; serpentinite of the Great Valley complex, Oakland Hills, associated with mercury deposits in the San Francisco Bay region, and with gold deposits in the Sierra Nevada foothills; serpentinite altered to asbestos with veins and nodules of talc, and intensely sheared serpentinites, hanzburgite, dunite and pyroxenite in Del Puerto Canyon; serpentinite, Marin county, and serpentinite boulder; Serpentinite Melange Zone, and block of sheeted dyke complex in Povorotninsky serpentinite melange, Late Mesozoic Thrust-Fold Belt of North-East Russia; serpentinized peroditites, France; submarine serpentinized peridotites; the top of the Western Massif has extensive, low-lying exposures of basement rock, which is probably serpentinite, the product of serpentinization reactions of the type that create hydrothermal systems like the Lost City; Staten Island; serpentinite; close-up of zoned serpentinite found SW of the corner of 43rd Street and Sixth Avenue in Manhattan, NY; Rainbow mine in ultramafic (talc-serpentinite) belt of Vermont; Henneke Series mollisol based on serpentinite; thin-sections: serpentinite fsu; serpentinite; olivine; partial alteration of olivine in a dunite to serpentine group minerals, ppl; peridotite with olivine crystal completely altered to serpentine group minerals, poikilitically enclosed by an unlatered clinopyroxene; peridotite with olivine crystal undergoing alteration to serpentine along fractures; relict olivine crystals in a serpentinite, ppl; totally serpentinized peridotite with pseudomorphs of olivuine and asbestiform crystals of the serpentine mineral chrysotile occupy the centre of the fiel of view (red-brown, opaque material is the iron-rich clay mixture iddingsite, and green material may be bowlingite (chlorite-rich)); serpentinization; serpentine minerals; serpentinization-induced microfracturing; webpages: The Lost City 2005; The Lost City: Serpentinization; NOAA Ocean Explorer Gallery; Serpentinite Locations on Cal Poly Land; Le Massif du Chenaillet; Newly Discovered Ophiolite Scrap in the Hartland Formation of Midtown Manhattan.

silicates

The crystal structure of most rock minerals depends upon the variety of arrangements available to tetravalent silicon, which can bond to various anions or be substituted by small cations, such as aluminum. Silicon can adopt triangular, tetrahedral, cubic, octahedral, and dodecahedral configurations upon bonding to other atoms. In the silicates, silica tetrahedra and octahedra can arrange/rearrange as:
● simple tetrahedra – nesosilicates [SiO4]4-	olivine
● double tetrahedra – sorosilicates [Si2O7]6-	epidote
● rings – cyclosilicates [SinO3n]2n-	tourmalines
● chains – inosilicates [SinO3n]2n-	pyroxenes
● double chains – inosilicates [Si4nO11n]6n-	amphiboles
● sheets – phyllosilicates [Si2nO5n]2n-	micas and clays; biotite, chlorites
● 3D frameworks (lattices) – tectosilicates [AlxSiyO2(x+y)]x−	quartz, feldspars, zeolites
● complex intermediates between the above configurations.
[Jmol kaolinite, porphyrillite]
crystallization : crystal structure : cyclosilicates : diagenetic alteration : single chain inosilicates : double chain inosilicates : nesosilicates : phyllosilicates : recrystallization : sorosilicates : tectosilicates : Chemical Formulae: Nesosilicates : Sorosilicates : Cyclosilicates : Inosilicates : Phyllosilicates : Tectosilicates :

◊◊◊ Mineral Index ◊◊◊

◙◙ Rock Index ◙◙

sillimanite

mineral / chemical formula		properties / significance / occurrence
sillimanite Al2SiO5	images - click to enlarge - top, sillimanite crystal; bottom, sillimanite crystals in biotite gneiss.	Metamorphic white, brown or green aluminosilicate, nesosilicate polymorph of the orthorhombic system. The other aluminosilicate polymorphs are andalusite and kyanite. Sillimanite (crystal or fibrolite varieties) is an index metamorphic mineral resulting from high grade metamorphism (temperatures greater than 550 ºC and all pressures). Fibrolite, named for its characteristic microscopic appearance of bundles of fibers, is the commonest variety of sillimanite. Sillimanite is often associated with garnets, biotite, feldspars, quartz, kyanite and andalusite.
links: images: crystals: sillimanite, embedded; rocks: sillimanite, sillimanite2, sillimanite from Stakholmen, sillimanite from Kölaberget, in gneiss de Kerhornou, 2 ; formations: métatexite à sillimanite, Port Navalo; Série métapélitique de l'Agly, Micaschiste à sillimanite, sillimanite; thin-sections: fibrolite, 2, 3, 4, 5, 6, 7, sillimanite in biotite-garnet-cordierite schist, ppl, sillimanite in biotite-muscovite-andalusite schist, ppl, sillimanite, 2, 3, 4, 5, 6, sillimanites, biotite-sillimanite schist, b, webpages: sillimanite, Metamorphic rocks in thin section; structure: sillimanite, sillimanite and aluminosilicates for comparison andalusite, kyanite, topaz (crystals); other: Stone axes, petrographic analysis and prehistoric exchange

◊◊◊ Mineral Index ◊◊◊

spinels

mineral / chemical formula		properties / significance / occurrence
spinels (X)(Y)2O4		Isometric crystals, usually twinned octohedral habit. In the general formula(X)(Y)2O4, X represents cations occupying tetrahedral sites and Y cations occupying octahedral sites. Divalent, trivalent, and quadrivalent cations can occupy the X and Y sites, and they include Mg, Zn, Fe, Mn, Al, Cr, Ti, and Si. Spinels include spinel – MgAl2O4, magnetite - Fe3O4, chromite - (Fe·Mg)Cr2O4 Spinels are metamorphic minerals or primary minerals in basic rocks. The absence of alkalis in basic magmas prevents the formation of feldspars, resulting in combination of aluminum oxide with magnesia to form spinel, or in formation of corundum from aluminium oxide,. Spinel, (Mg,Fe)(Al,Cr)2O4, is common in peridotite in the outer mantle, between the Moho and a depth of about 70 kilometers. At greater depths, any spinel present becomes increasingly rich in Cr, and pyrope-rich garnet becomes the more stable aluminous mineral in peridotite. At depths much shallower than the Moho, calcic plagioclase is the more stable aluminous mineral in peridotite.
[images: crystals: spinel, 2, structure; weblinks: grey spinel with meionite, spinel crystals, 2, spinel gems, spinel structure, spinel structure 2, spinel structure (UC), black spinel, (crystals UC)

◊◊◊ Mineral Index ◊◊◊

staurolite

mineral / chemical formula		properties / significance / occurrence
staurolite (Fe,Mg,Zn)2Al9(Si,Al)4O22OH2		Nesosilicate with monoclinic crystals, colored brown to black, pleochroic, mostly opaque (white streak). The mineral is named from the Greek for 'cross' because 35% of its crystals have grown in a classic penetration twinning mode in which it appears as if two crystals grew into and out of each other (at 60º or 90º). Occassionally, paired twins can form star shapes (image). Macroscopically visible crystals are prismatic, and often larger than the surrounding minerals (porphyroblasts), often with poikiloblastic texture on thin-section. Staurolite is an intermediate to high grade regional metamorphic mineral that often occurs associated with other metamorphic minerals, including almandine garnet, micas, and kyanite.
links: images: crystal 90º, 2, 3, 60º, 2, mixed, 2, 3; rocks: embedded staurolite, embedded crystal surrounded by muscovite, feldspars and small garnets, embedded 60º, staurolite with embedded kyanite; formations: biotite staurolite schist, Barrovian, Highland Boundary Fault, Scotland - staurolite zone; webpages: staurolite, 2, single and double tetrahedron silicates, Fauske, Norway; thin-sections: staurolite quartzite (quartzite), 2, staurolite wp, staurolite, 2, 3, 4, 5, 6, andalusite and staurolite, poikiloblastic porphyroblasts, b, 2, wp, euhedral staurolite (yellow pleochroic) overgrows shear zone between large light coloured plagioclase porphyroblasts, staurolite (St) as relict within poikiloblastic andalusite, staurolite in a muscovite-biotite schist, 2, Metamorphic rocks in thin section; structure: Staurolite (UC), (crystals UC)

◊◊◊ Mineral Index ◊◊◊

subduction zone magmas

Subduction zones give rise to a range of magmas. In subduction zones, a cool slab of oceanic lithosphere sinks beneath an accretionary sedimentary prism, which, in turn, often lies beneath a forearc basin. (image - Cascadia subduction zone, courtesy USGS, larger image) Ta and Nb are present in anomalously low concentrations in magmas associated with subduction zones, a feature that is considered diagnostic of subduction-related volcanism. Depletion of high-field-strength trace elements (mid-ocean ridge basalts (N-MORB) is the most distinctive geochemical fingerprint of subduction magmatism. Computer modeling image (or compare to image to right) of the thermal structure of a subduction zone, with convergence at 6 cm/year, suggests that rocks cool as the slab pass to great depths, only warming gradually. This type of model is consistent with seismic velocity information, and suggests that the ocean crust of the subducted slab would only melt at depths greater than 600 km. Volcanic island arcs lie above the slab where it has sunk to depths of 120-140 km. Some computer modeling indicates that the temperature here is merely 200-300ºC, which would be far too cool for melting of the ocean crust, but would be sufficiently warm to drive fluid into the overlying mantle wedge, hydrating the overlying mantle. Other computer models, however, suggest that the isotherms are somewhat hotter. Hydration of the mantle wedge depresses the melting temperature of the mantle wedge, which can partially melt to produce a basaltic magma, leaving a harzbergite residue. Alternatively, hydrated mantle could rise bouyantly to melt at shallower depths. The complexity of magma is increased by fractional crystallization as magma ascends or ponds in magma chambers. Rising magma can undergo fractional crystallization through partial solidification of more refractory minerals, or can become contaminated from the material through which it ascends. Contamination of basaltic magmas by partial contact-melting of felsic continental crust would increase the Si level of the melt, generating andesitic and rhyolitic magmas of Cordilleran composite volcanoes (compare island arc with Andean). The more recently that the rocks of subducting oceanic crust have formed, then the warmer those rocks are (graph), and the more easily they could melt upon subduction. The greater the rate of subduction of oceanic crust, then the lower the temperature of the mantle rocks that have received the subducted material (graph). The base of the subducting oceanic crust is initially hotter, but the top of the subducting crust eventually becomes hotter due to heat conducted from the mantle wedge (graph). The impact of cooling by the subducting slab is very important, rapidly dropping the temperature of the mantle wedge, in the absence of convection, below temperatures at which magmas could be generated. However, induced convection in the mantle wedge could retain temperatures above 950 °C as the wedge material is dragged down, rendering hydrous melting possible (graph). So, it is likely that arc magmas are derived from the mantle wedge and that conditions for slab melting are very restricted. However, not only pure mantle material undergoes partial melting. The subducting slab and its transported sediments may also melt, and the mantle above the subduction zone can be modified by repeated episodes of partial melting. In general, the magmas generated in subduction zones have higher volatiles and higher silica content (60% +) than the basaltic magmas formed at divergent plate boundaries. Thus, active subduction zones are crowned by an arc of explosive composite volcanoes.
Basaltic	The most primitive island arc lavas could be related to dehydration of the hydrated ocean crust (amphibolite) as it transforms to dense eclogite at depths of ~100km. The released hydrous fluids could then rise up into the peridotite mantle wedge, promoting melting because magmas form at much lower temperatures in the presence of water, which acts as a flux (graph). The resultant magmas could then rise slowly up to the arc volcanoes, crystallizing iron-enriched Mg-rich olivines and pyroxenes as they ascend. The eruption of basalt (tholeiite) is non-violent.
Calc-alkaline, silicic andesitic, and dacitic magmas	In more mature arcs, hydrous melting of eclogite (silica-rich dacitic magmas when if Si-poor garnet stays in the residue) could produce silicious magmas that react with the mantle wedge and ascend as diapirs, erupting as much more violent hydrous magmas.
Boninites: high-Mg andesites	typically formed at early stage of island arcs
Island Arc Tholeiites	normally restricted to primitive island arcs
Calc-alkaline basalts & andesites	occur in mature island arcs and continental margins
Bajaites (Adakites): high-Mg andesites distinct from boninites	occur at ridge subduction occurs or where mafic rocks have been underplated
Shoshonites: high-Ba, Sr magmas	often late-subduction or post-subduction
Archaean TTG suite (resemble adakites)	distinctive, and believed derived from subducted ocean crust
links: images: diagrams: Pacific "ring of fire": making magma in subduction zones; extension vs. compression at subduction zone; Cambrian-Early Ordovician tectonic evolution of the Humber margin and outboard peri-Laurentian terranes → Taconic collision (Dashwoods with Humber margin) and subduction initiation responsible for the Annieopsquotch ophiolite belt; tectonic history of the Penobscot arc/backarc complex → tectonic evolution of the Popelogan/Victoria arc (Canadian Appalachians are a segment of an extensive accretionary-collisional mountain belt that formed in response to Paleozoic closure of the Iapetus and Rheic oceans)

adapted from here