F

◊ feldspars ◊

◊◊ ferrimagnetic (magnetite)

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felsic to ultramafic

Felsic, intermediate, mafic, and ultramafic rocks vary in their proportions of lighter or heavier elements. The diagram at left indicates the texture, geochemistry, and mineralogy of the major igneous rocks: Increasing to the left: SiO2 content; viscosity; felsic: mafic composition; (K, Na) : (Fe, Mg, Ca) ratio; Increasing to the right: darkness; mafic : felsic composition; (Fe, Mg, Ca) : (K, Na) ratio; temperature of melting: ◙ a/p - aphanitic or porphyritic texture, derived from extruded magma (lava), ... ◘ 1. rhyolite ◘ 3. dacite ◘ 5. andesite ◘ 7. basalt ◙ phaneritic texture, emplaced as magma (plutonic) ... ◘ 2. granite ◘ 4. granodiorite ◘ 6. diorite ◘ 8. gabbro to peridotite 0-100: percentage mineral content: ◊ a. quartz ◊ b. K-feldspar ◊ c. Na-feldspar ◊ pl. plagioclase to Ca-rich plagioclase ◊ d. muscovite ◊ e. biotite ◊ f. amphiboles ◊ g. pyroxenes ◊ h. olivine The TAS classification scheme – Total Alkali Silica – is employed to classify common volcanic rocks based upon alkali and silica content (recalculated to exclude CO2 and H2O). The TAS classification employs the fact that relative proportions of alkalis and silica play an important role in determining mineral assemblages. The TAS classification is used only for volcanics rocks for which the mineral mode cannot be determined. Where mineralogy is known, systems such as the QAPF diagram are applied. The QAPF diagram is employed for classification of common igneous rocks based upon proportions of the minerals: Q quartz : A alkali feldspar : P plagioclase feldspar : and F feldspathoid (foid).
felsic (acid) feldspar SiO2 greater than 63% Al K sg less than 3.0	Felsic (feldspar silica) magmas have specific gravities less than 3 because they are relatively enriched in the lighter elements – silica and oxygen, aluminum, and potassium. Felsic rocks are relatively rich in K-feldspars and have a higher percentage of silica than do other igneous rocks. Felsic minerals are usually light in color and include quartz, muscovite, and the orthoclase feldspars. The commonest felsic rock is granite, which represents the purified end product of the earth's internal differentiation process. Dacite and rhyolite are also felsic.
intermediate SiO2 63-55%	Intermediate magmas and rocks have compositions and specific gravities intermediate between felsic and mafic. Intermediate rocks such as diorites, andesites, and latites develop through fractional crystallization of melts formed where suducting plates sink and heat beneath convergent plate boundaries.
mafic (basic) magnesium ferric Ca Na sg greater than 3.0 SiO2 45-55%	Mafic (magnesium ferric) minerals, magmas, and rocks have specific gravities greater than 3 because they have relatively high proportions of the heavier elements – magnesium, iron, calcium, and sodium. Mafic minerals are usually dark in color. Mafic minerals have comparatively high specific gravities (greater than 3.0) and are typically dark in color. Common rock-forming mafic minerals include olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Common mafic rocks are basalt and gabbro. Mafic magmas are usually derived from differentiation of ultramafic upper mantle and are produced at spreading centers.
ultramafic (ultrabasic) less than 45% SiO2 MgO greater than 18% FeO	Ultramafic rocks are very low in SiO2 (less than 45%), have greater than 18% MgO, high FeO, low K. They are typically composed of greater than 90% mafic minerals (dark colored, high Mg and Fe content). Intrusive ultramafic rocks typically occur in large, layered ultramafic intrusions in which differentiated rock types are layered. These cumulate rocks do not replicate the chemistry of the magma from which they crystallized. The majority of ultramafic igneous rocks formed in Archaean and Proterozoic terranes and are are exposed in orogenic belts. There are very few true ultramafic lavas recognized from the Phanerozoic. Many surface exposures of ultramafic rocks occur in ophiolite complexes in which deep mantle-derived rocks were obducted onto continental crust in association with subduction zones. Examples of ultramafic plutonic rocks include, troctolite-gabbro-norite, dunites-peridotite, anorthosites, hornblendite and and, rarely phlogopitite, and pyroxenite. Volcanic ultramafic rocks include, komatiite, picritic basalt, lamprophyres, kimberlites, and lamproite. Metamorphic ultramafic rocks are typically derived from ultramafic igneous protoliths, and examples include serpentinites, and soapstone.
links: images: hand-specimens: felsic: granite, cu, 2, cu2; pumice, cu; rhyolite, cu; volcanic tuff, cu; intermediate: diorite, cu, 2, cu2; obsidian, cu, cu2; greenstone - porphyritic microtonalite; mafic: basalt; gabbro, cu; scoria, 2, cu; ultramafic: dunite, cu, cu2; peridotite; Saxnäs (orthogneiss, mylonitic orthogneiss); formations: felsic: intermingled mafic and felsic intrusions - dark mafic blobs have fine-grained chilled margins and were injected by the felsic magma, indicating that both were fluid at the time of emplacement; felsic pillows in Montana, Square Butte; felsic pumice, near Burfells; composite dike with cuspate boundaries between mafic and felsic components point to emplacement of coeval mafic and felsic magmas across a magnetite breccia at the Kywjibo deposit, Québec; orbicules in the Dinkey Creek granodiorite; intermediate: ; mafic: a Jurassic quartz diorite containing streaked out mafic minerals, stretched mafic enclaves, and younger brittle fractures; ultramafic: ultramafic lamprophyre dikes in Labrador rocks that show a virtually complete continuum between silica-undersaturated alkaline rocks and carbonatites; ultramafic xenoliths, 2; Anthophyllite, Newdale Ultramafic body, NC; mylonitized and ultramylonitized ultramafic rocks; ultramafic boudin in the upper structural levels of one of the Archean gneiss domes outcropping in the Dewar Lakes area, central Baffin Island; fore-arc peridotites; webpages: Igneous Rock Composition Chart; Identify the Texture, Composition and Rock Type

ferromagnesian minerals

Ferromagnesian minerals comprise the group of minerals with chemical compositions that contain both Fe and Mg.
single chain inosilicate pyroxenes Augite	calcium magnesium iron aluminum silicate Augite is an important rock-forming mineral in many igneous rocks, such as gabbros and basalts. Augite also occurs in some hydrothermal metamorphic rocks. Augite lies between the MgCaSi2O6 diopside endmember and the CaFeSi2O6 hedenbergite endmembers. Augite can also contain significant Al, Na, Ti, and other elements.
double chain inosilicates amphiboles Actinolite	calcium magnesium iron silicate hydroxide Actinolite is the intermediate member in the series that includes Mg-rich tremolite and Fe-rich ferro-actinolite. Actinolite is a relatively common mineral in some metamorphic rocks – varieties include fibrous crystals of asbestos (byssolite) and nephrite (a jade mineral).
phyllosilicate Biotite	K(Mg, Fe)3AlSi3O10(F, OH)2 Biotite is present in most igneous rocks, in pegmatite veins, and in both regional and contact metamorphic rocks where the partitioning of Fe and Mg between biotite and garnet is sensitive to temperature.
phyllosilicate Chlorite	(Mg,Fe)3(Si,Al)4O10(OH)2•(Mg,Fe)3(OH)6 Chlorite minerals are a group of greenish phyllosilicates with endmembers based on substitution of Mg, Fe, Ni, and Mn in the silicate lattice. Zinc, lithium and calcium species are also known within the chlorites.
cyclosilicate Cordierite	(Mg,Fe)2Al3(Si5AlO18) Cordierite
nesosilicate Olivine	magnesium iron silicate The composition of olivines vary between the Mg-rich forsterite (Fo) endmember and the Fe-rich fayalite (Fa) endmember. The nesosilicate, olivine is one of the commonest minerals on Earth, olivine and its polymorphs comprise over 50% of the asthenosphere. Olivine occurs in mafic and ultramafic igneous rocks and as a primary mineral in some metamorphic rocks. Mg-rich olivine crystallizes from magma rich in magnesium and low in silica, forming mafic rocks such as gabbro and basalt. Ultramafic rocks such as peridotite and dunite are more enriched in olivine after extraction of partial melts.
cyclosilicate Tourmaline	(Na,Ca)(Al,Li,Mg)3-(Al,Fe,Mn)6(Si6O18)(BO3)3(OH)4 Tourmaline
other	Iron minerals : Magnesium minerals

feldspars

mineral / chemical formula		properties / significance / occurrence
feldspars XAl(1-2) Si(3-2) O8 K-feldspar endmember KAlSi3O8 Albite endmember NaAlSi3O8 Anorthite endmember CaAl2Si2O8	[images: 1, albite, oligoclase, andesine, labradorite, bytownite, microcline, sanidine, orthoclase]	Family of common tectosilicate, monoclinic, rock forming minerals. Nine of the nearly twenty known members are common, and comprise the greatest percentage of crust-forming minerals. Plagioclase feldspars (P)are sodium/calcium aluminum silicates: albite, oligoclase, andesine, labradorite, bytownite, and anorthite. K-feldspars or alkali feldspar polymorphs (A)are potassium/sodium aluminum silicates: microcline, sanidine, and orthoclase. image - click to enlarge, K-feldspar (pink) and albite (white). Feldspars crystallize from magma as intrusive and extrusive rocks, can occur as compact minerals and as veins, and are present in many types of sedimentary and metamorphic rocks. Rock formed entirely of plagioclase feldspar is known as anorthosite.
The feldspars are family of common tectosilicate, monoclinic, rock forming minerals that crystallized from melts in a continuous series. The alkali feldspars (A) form solid solutions between the K-feldspar polymprph endmembers (KAlSi3O8) and albite (NaAlSi3O8). Sanidine (monoclinic), orthoclase, and microcline (triclinic) refer to polymorphs of K-feldspar – of these, sanidine is most refractory (stable at the highest temperatures), and microcline is the most fusible (stable at the lowest temperatures). The plagioclase feldpars (P) form solid solutions between albite and anorthite (CaAl2Si2O8) as the ratio of sodium : calcium substituted in the mineral's crystal lattice structure decreases. In Bowen's Reaction Series, the calcium-rich, alkali feldspars such as anorthite are the most refractory members (freeze at highest temperatures), plagioclases are intermediate, and sodium-rich feldspars such as the polymorph orthoclase are the most fusible members (freeze at lowest temperatures). The feldspathoids (foids, F, image) are an atypical mineral group related by virtue of their relationship to the feldspars. Feldspathoids are low-silica igneous minerals that could have formed feldspars if only more silica (SiO2) had been present in the original magma. Feldspathoid aluminum to silicon ratio is nearly 1:1 in most members, whereas it is closer to 1:3 in most of the feldspars. The low silica content of parent magmas ensures that feldspathoids are not found in company with primary quartz. Foids include cancrinite, leucite, nepheline, and members of the sodalite group. [image: sodalite semi-precious trade stones] Like the zeolites, low-density feldspathoid minerals have large openings in their crystal structure, but these openings are mostly separate, so do not allow for the movement of ions and molecules enabled by zeolite's contiguous structure. However, feldspathoid openings do allow the structure to accommodate large ions such as chlorine, carbonate and sulfate. [images: intergrown sodalite and nepheline, leucite crystals in basalt]

image courtesy USGS

◊◊◊ Mineral Index ◊◊◊

foliation and shearing

Foliation develops in rocks under the influence of stress. Tectonites are rocks that exhibit a strongly developed fabric (textural microstructure) caused by ductile deformation in regions of dynamic metamorphism. Foliation is defined as a pervasive planar structure that results from the nearly parallel alignment of sheet silicate minerals and/or compositional and mineralogical layering in the rock. Foliative microstructure is diagnostic of the deformation history of the tectonite. Most foliation is attributable to the preferred orientation of phyllosilicates, such as clay minerals, micas, and chlorite. Preferred orientation results from accommodations to non-hydrostatic or differential stress acting on the rock (also called deviatoric stress). Lithostatic stress or confining pressure occurs in rocks because of the burden of overlying rock, and exerts equal pressure from all directions on a buried rock, just as seawater exert equal pressure from all directions at depth. Differential stress results from compressional tectonic stresses that are not equal from all directions: tensional stress (stretching), compressional stress (squeezing), or shearing stress (side to side shearing). Compressional stresses act along the direction of maximum principal stress, whereas extensional stresses act along the direction of minimum principal stress. For the mutually orthogonal directions along which stress is applied, the maximum principal stress direction is designated σ1, minimum principal stress direction is σ3, and intermediate principal stress direction is designated σ2. normal stress, directed perpendicular to a given plane is designated, ση shear stress, directed parallel to a given plane is designated, στ vector components of stress can be expressed according to Cartesian coordinate system as 9 components (x,y, z combinations) relative to the 3 mutually perpendicular x-y-z axes The sense of motion in a shear zone is indicated by S-C geometry, and the intersection of C-S planes is assumed to be perpendicular to movement. Rocks respond to stress differently, depending upon pressure, temperature, and mineralogic composition: In elastic deformation, rock changes shape by a very small amount and the deformation is not permanent. Elastic deformation occurs only with small differential stresses, which are less than the rock's yield strength. In brittle deformation close to the Earth's surface, where rocks are comparatively cool, rock behaves in a brittle fashion, fracturing in response to differential stress greater than the rock's yield strength. Rock adjacent to the failed rock springs back to its original shape in the elastic rebound that is responsible for earthquakes. In ductile deformation, rock deeper than 10-20 km is subjected to enormous lithostatic stress, and the high temperatures of burial render the hot rock softer and more malleable. At these depths, in the lower continental crust and mantle, rock undergoes plastic deformation and flows in response to application of a differential stress that is stronger than its yield strength. Rock undergoes ductile deformation by gradual creep along crystal grain boundaries and planes within crystal lattices. Dynamic recrystallization occurs where the temperature is greater than 55% of the melting temperature for that depth (pressure), and diffusion of ions/atoms allows for continuous and large deformation. High stresses at locked points cause the atoms to diffuse to lower stress points. Where the temperature is greater than 85% of the melting temperature for that depth (pressure), diffusion creep occurs through ionic/atomic diffusion and continuous recrystallization. A penetrative planar foliation develops throughout the rock mass at the initiation of shearing. Incipient shear foliation typically orients normal (perpendicular) to the direction of principal shortening as minerals flatten, producing L-tectonites in which the shear foliation is diagnostic of the direction of shortening. Within assymmetric shear zones, minerals are flattened and skewed. Pronounced displacements within shear zones may cause shear foliation at a shallow angle to the gross plane of the shear zone. Such foliation at a shallow angle to the main shear foliation can manifest as a sinusoidal set of foliations that curve into the main shear foliation. Such rocks are known as L-S tectonites. If the rock mass begins to undergo large degrees of lateral movement, then the strain ellipse will lengthen into a cigar shaped volume, and shear foliations distort into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites. (images - click to enlarge - top left, folded metagraywacke and schist; middle left, at typical example of dextral shear foliation (sinusoidal) in an L-S tectonite, with pencil pointing in direction of shear sense; middle right, pronounced assymmetric shearing has stretched the conglomerate pebbles of this L-tectonite into elongate cigar shaped rods, Glengarry Basin, Australia; bottom left, dextral asymmetric shear in basalt, Labouchere mine, Glengarry Basin, WA)
Shear zone	A shear zone (shear) is a wide zone of distributed shearing in rock; typically shearing is a type of fault. However, it may be difficult to locate a distinct fault plane within a shear zone. Shear zones may form zones of much more intense foliation, deformation, and folding.
Brittle conditions	Under brittle rheological conditions (cooler rocks, less confining pressure) or at high rates of strain, rocks tend to fail by brittle failure with breaking of minerals, which become ground up into a breccia with a milled texture.
Brittle-ductile conditions	Shear zones that develop under brittle-ductile conditions respond to deforming stresses by accommodating the compressive stress with less rock fracture. Accommodation occurs within the minerals and the mineral lattices along foliation planes.
Ductile conditions	Shearing under ductile conditions may involve dislocation creep within minerals, by fracturing of minerals and growth of sub-grain boundaries, as well as by lattice glide, particularly on platy minerals, especially micas. Mylonites are an indication of ductile shear conditions. Ductile shear produces distinctive textures: S-planes, C-planes and C' planes.
C-planes	C-planes or cisaillement planes form parallel or oblique to the shear zone boundary, such that the angle between the C and S planes is always acute, and defines the shear sense. Generally, the lower the C-S angle the greater the strain.
C-surfaces	C-surfaces in mylonites are sub-parallel to the shear zone walls and are considered to represent discrete planes of concentrated simple shear. They are spaced zones of finer-grained material, micro shears, in which offsets can be seen.
C'-planes	C' planes are also known as shear bands or secondary shear fabrics. C'-planes are rarely observed except in ultradeformed mylonites, particularly phyllonites, forming at an angle of about 20 º to the S-plane.
S-foliations	S-planes or schistosité planes or sissalement planes are parallel with the shear direction, define the flattened long-axis of the strain ellipse, and generally produce a planar fabric resulting from the alignment of micas or platy minerals. S-tectonites display a dominant planar fabric, usually indicating compressive strain (σ1). S-tectonites can also result from a lack of minerals capable of developing lineation (such as mica-dominated phyllonites). Shear-bands are extensional features with discrete, penetrative surfaces that form at angles less than 35º to the shear zone margin and curve into the C-orientation. Overlap of S-foliation and transposed foliation orientations reflects the fact that S-foliations form at a wide range of scales during heterogeneous deformation. With increasing deformation, S-surfaces rotate into parallelism with C-surfaces to produce a composite S/C fabric. During progressive deformation shear-bands rotate into near parallelism with the shear plane and are subsequently overprinted by younger sets of shear-bands.
S-surfaces	S-surfaces in mylonites are inclined to shear zone walls (consistent with shear sense) and are considered to represent the flattening plane of the finite strain ellipse during simple shear. They are more penetrative than C-surfaces, and are defined by defined by long axes of deformed grains or clasts, which are the preferred orientations of micas and other inequant minerals.
S/C fabric	S/C fabrics result from deformations sufficiently strong for S-surfaces to rotate into parallelism with C-surfaces. Composite shear zone fabrics develop in mylonites formed from originally layered/foliated rocks. The original orientation is termed the dominant slip foliation (DSF). Normal slip crenulations are compensating structures that allow persistence of slip planes, so NSF are spaced and are not parallel to the shear zone walls. Reverse slip crenulations are compensatory thickening structures that develop when the DSF is oriented clockwise from slip plane in dextral slip, and counterclockwise in sinistral slip. Reverse slip crenulations are usually spaced, or isolated fold structures
external links	Strain is the geometrical expression of deformation caused by the action of stress on a physical body. The strain tensor, ε, is a symmetric tensor used to quantify the strain of an object undergoing a small 3-dimensional deformation. links: images: webpages: Shear zones and mélanges (copyright © John Waldron)