C

◙◊ carbonates ◊ cement ◊ chlorites ◊ clast ◊ crystallization ◊

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carbonates

Carbonates of iron, sodium, calcium, and calcium/magnesium:

Composition	Formula	Mineral name	Rock
iron carbonate	FeCO3	siderite	ironstone
sodium bicarbonate	NaHCO3	baking soda	nahcohlite
calcium carbonate	CaCO3	calcite	limestone
calcium, magnesium carbonate	(Ca, Mg)CO3	dolomite	dolostone

charnockite

Charnockites are an assemblage of of foliated metamorphosed igneous rocks that share genesis through differentiation of the similar parent magmas. Charnockites typify the deep continental crust of the Earth, and they are commonly found in granulite-facies metamorphic terranes. Charnockites are characterized by the presence of strongly pleochroic, reddish or green hypersthene. Acid members of the charnockite series includes are rich in quartz and microcline; intermediate members correspond mineralogically to norites (cu), quartz-norites and diorites; and, basic members contain pyroxene and olivine. The rocks have a banded structure and many charnockite minerals are schillerized – containing tiny platy or rod-shaped enclosures that lie parallel to particular crystallographic planes or axes. The surfaces of these enclosures reflect light, giving the contained minerals an atypical appearance – quartz appears blue and opalescent, the feldspar has a milky shimmer, and hypersthene gleams bronzy and metalloidal. The banding results predominantly from fluxion in the viscous crystallizing intrusive magma, accompanied by differentiation or segregation of the mass into bands of different chemical and mineralogical composition. Charnockites are of wide distribution in the southern hemisphere, including India (i, 2), Sri Lanka, Madagascar, and Africa. Very similar rocks described as pyroxene granulites, pyroxene gneisses, and anorthosites occur in Norway, Sweden, France, Germany, Scotland, and North America. Charnockites are generally regarded as being of Archean or Precambrian age, and share a history with the Earth's most primitive gneisses.

links: images: hand-specimens: charnockite c-u, Norway; charnockite, a crystalline granitic rock (hypersthene monzonite - mangerite), composed largely of two feldspars (perthite and andesine), with 5 volume percent orthopyroxene (hypersthene), with minor amounts of quartz, myrmekite, hornblende, biotite, magnetite and clinopyroxene, and with traces of apatite, sulphides and zircon, Pangnirtung region, seBaffin Island; charnockite, Tanzania; formations: in situ charnockite formation in granitoid gneiss of Wanni Complex, N of Kurunegala, Sri Lanka; charnockite patches make a patterned array, quarry at Kottavattom, South India; dark charnockite forms veinlets and patches obliterating the structure of granodioritic Hbl-Bt gneiss in Kabbaldurga quarry, South India; charnockite; marble on the banks of the Hudson River, Warrensburg, coarse marble with xenoliths of charnockite, Hudson River, Warrensburg, Adirondacks; Sjelset charnockite; charnockite; close-ups: charnockite, 2, 3; in situ charnockite formation in garnet-sillimanite gneiss, Kerala Khondalite Belt, Southern Granulite Terrain, southern India; charnockite vein overprinting foliation in Hbl-Bt gneiss, Ponmudi, South India; xenolith of Proterozoic crustal rock (charnockite) in a dike of alkali-ultramafic rock (greenish black), Turiy Mys (Terskiy Coast, Kola, Russia); blue quartz in charnockite, Old Rag Mountain, Shenandoah National Park; charnockite containing large xenocrysts of gray andesine (2, 3) detached or mixed from anorthosite margin, between Minerva and Newcomb, Adirondacks; charnockite on High Peak, 2, Tobacco Row Mountain, Amherst County; charnockite, India; charnockite (on right), at top; lower contact of an aplitic xenolith in the Bitterfontain charnockite, and slight weathering in the Bitterfontein charnockite provides the means to distinguish the mineralogy and coarse texture; thin-sections: charnockite from Blue Ridge province, Grenville province; charnockite pl, xp, Norway; pl, xp, Charnockite d'Ansignan dans le massif de l'Agly, Pyrénées; Sjelset charnockite - albite-twinned plagioclase (black and white, lower left), orthoclase (gray), quartz (cream, white), myrmekite with circular to oval cross-sections of coarse quartz vermicules (white to gray, lower right); diagrams: PT diagram of high grade metamorphic facies; webpages: Charnockites; Petrological features of granulites, charnockites and migmatites; field trip to eastern and central Adirondacks

chlorites

mineral / chemical formula		properties / significance / occurrence
chlorites (Mg,Fe)3(Si,Al)4O10(OH)2·(Mg,Fe)3(OH)6		The 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. ▪ clinochlore: (Mg5Al)(AlSi3)O10(OH)8 ▪ chamosite: (Fe5Al)(AlSi3)O10(OH)8 ▪ nimite: (Ni5Al)(AlSi3)O10(OH)8 ▪ pennantite: (Mn,Al)6(Si,Al)4O10(OH)8 Chlorites are monoclinal and consist of 2:1 layers that are held together by Mg(OH)2 sheets. Clinoclore, pennantite, and chamosite are the commonest chlorite varieties. The wide range of composition results in great variation in physical properties, allowing chlorite group minerals to exist over a wide range of temperature and pressure conditions. Chlorite minerals are ubiquitous within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and deeply buried sediments. In igneous rocks, chlorites are alteration products of mafic minerals such as amphiboles, biotite, and pyroxenes. Chlorite is a common metamorphic mineral, usually indicating low-grade metamorphism, and is the diagnostic species of the zeolite facies and of lower greenschist facies. It occurs in assemblages in pelitic schists that also include quartz, albite, sericite, and garnet. Chlorite is commonly found in hydrothermal ore deposits that result from retrograde metamorphic alteration mineral of existing ferromagnesian minerals or as metasomatic products via addition of Fe or Mg or other compounds into the rock mass. In ores, chlorites are often associated with epidote, sericite, adularia and sulfide minerals. Metamorphism of ultramafic rocks can produce predominantly clinochlore chlorite in association with talc. Chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere carried down by subduction (experimental simulation). Chlorite may even be present in the mantle volume from which island arc magmas are generated.
chlorites:
composition	(Fe, Mg, Al)6(Si, Al)4O10(OH)8
color	usually green; can also be white, yellow, red, lavender, or black
lustre	vitreous, dull or pearly
transparency	translucent to transparent
crystal system	monoclinic; 2/m
crystal habits	usually found as alteration products of iron-magnesium minerals and as inclusions in other minerals; aggregates can be scaly, compact, platy and as crusts; rarely occur as large individual barrel or tabular crystals with an hexagonal outline
cleavage	in crystals - perfect in one direction, basal
fracture	lamellar
hardness	2 - 3
specific gravity	2.6 - 3.4
streak	pale green to gray to brown
other	cleavage flakes are flexible but not elastic; common mineral for the greenschist facies (low grade metamorphism)
associations	numerous, including garnets, biotite, quartz, magnetite, talc, serpentine, danburite, topaz and calcite
occurences	numerous, including Transvaal, South Africa; Zermatt, Switzerland; Guleman, Turkey; Lancaster Co., Pennsylvania, Brewster, New York; San Benito Co., California, USA
indicators	color, cleavage, associations and crystal habits
images: chlorite, 2, chamosite, sphalerite, quartz cluster with chlorite inclusion; rocks: chlorite schist, 2, chlorite, 2, greenish hue due to chlorite, light colored minerals are serasite, a fine grained muscovite, along with K-feldspar, chlorites in diabase; formations: Série métapélitique de l'Agly, Schiste à chlorite; weblinks: chlorite group, chlorites, chlorite, petrographic; thin-section: chlorite, 2, chlorites, sem-Authigenic chlorite, Norphlet sandstone, sem, sem strzegomite, wp, wp2; structure: crosslinked chains, sheets

image courtesy of USGS

◊◊◊ Mineral Index ◊◊◊

chromite

mineral / chemical formula	properties / significance / occurrence
chromite (Fe, Mg)Cr2O4	Chromite is a spinel group mineral, iron magnesium chromium oxide, in which Mg is invariably present and Al and Fe can substitute for Cr. Chromite is commonly associated with olivine, magnetite, serpentine, and corundum. It occurs in peridotite, layered ultramafic intrusive igneous rocks, and metamorphic rocks such as serpentinites. (At top left is a chromite hand-specimen; at bottom left, chromite layers alternate with anorthosite layers in the Bushveld Igneous Complex. Below left is a chromite hand-specimen.) Chromite is the only ore of chromium, and ore deposits of chromite formed as a result of early magmatic differentiates. Chromite is mined in Brazil and Cuba; 80% is mined in India, Iran, Pakistan (Zhob District of Balochistan), Oman, Zimbabwe, Turkey, and Southern Africa (50%). Southern Africa's vast Bushveld Igneous Complex is a huge, layered mafic to ultramafic igneous body in which some layers (image at left) consist of 90% chromite in the rare rock type, chromitite.
links: images: formations: chromite in dunite lenses that are hosted in harzburgite, Cyprus, and chromite vein; chromite beds, Dwars River, sAf ; chromite in Chamrousse Ophiolite; prospect hole drilled in Ophiolite and Franciscan graywacke for magnesite and probably chromite; chromite layer within a massive dunite, Blue Ridge Province, North Carolina; hand-specimens: chromite (spinel); chromite, 2, 3, 4; Merensky Reef = feldspathic pyroxenite, top chromite stringer, pegmatoidal feldspathic pyroxenite with sulphides, basal chromite stringer, mottled anorthosite; chromite ore; chrome spinel; chromite c-u; massive black chromite in serpentine matrix; friable, granular chromitite of the UG-2 Reef from Rustenburg Platinum Mines (Breskop shaft); crystals: chromite, 2; uvarovite garnet on chromite with kammemerite, Urals (prettier than rather dull chromite!); kammemerite with tiny chromite crystals; chromite inclusions in diamond; close-ups: chromite pod in dunite; rhythmic layering represented by olivine-(chromite) and olivine-bronzite-(chromite) cumulate with poikilitic intercumulus augite in the megacyclic unit III at the Keski-Penikka area; chromite vein, 2; chromite veins in yellow to green antigorite with picrolite, chromian antigorite and dolomite, Lancaster County, Pennsylvania; thin-sections: basal chromite adcumulate, (chromite (PPL) is bright grey, dark grey to black is intercumulus plag), chromite anorthosite, alternating chrom and plag cumulates - PPL, bottom chromite seam overlain by sulphide + phlogopite pyroxenite - 3mm - XP, chromite pyroxenite, chrom + opx cumulate - PPL, chromite pyroxenite, chrom + opx orthocumulate with intercumulus plag - XP , chromite pyroxenite, chrom + opx orthocumulate with intercumulus plag - PPL , chromite pyroxenite, chrom + opx adcumulate - XP, chromite cumulate grading up into chrom + opx cumulate - 3mm - PPL, chromite anorthosite, alternating chrom and plag cumulates - PPL, from Rustenberg Layered Suite; Partial cross- polarized light photomicrograph. Low interference colors = orthopyroxene; higher colors = olivine; black = chromite. Image width 14 mm; webpages: L'Ophiolite de Chamrousse (Engl); Chrome and Platinum Ore; 30th Martian meteorite found in Algeria: NWA 2046:

image of Rustenburg Layered Suite, copyright Dr David Waters, University of Oxford; Image courtesy OESIS http://oesis.earth.ox.ac.uk/index.php; chromite image courtesy of US Government.

composition

Chemical composition, together with conditions of temperature and pressure, determines the internal structure of minerals, and hence determines mineral identity (chemistry + structure) and properties. Compositions range from pure elements to complex silicates.

Crystal structures reflect the regular internal atomic or ionic arrangement of component elements, and fit into one of fourteen Bravais lattices and one of six crystal systems.

Based on elemental chemical composition, minerals are divided into anion-based classes according to the Dana classification system: in order of relative abundance: ◊ silicates – most rocks comprise >95% silicates ◊ amphiboles, biotite, chlorites, epidote, feldspars, micas, olivine, pyroxenes, quartz ◊◊ aluminosilicates (kyanite, sillimanite, zeolites) ◊ carbonates ◊ sulfates ◊ halides ◊ oxides ◊ sulfides ◊ phosphates ◊ elements

crenulation

Crenulations develop when an original planar fabric is distorted by a later planar fabric imposed at an angle to the original plane.

links: images: diagrams: diagram of crenulation in metamorphic texture; close-ups: crenulations in shear zone, Kaapelinkulma; crenulations; formations: crenulation lineation, Proterozoic Windermere Supergroup, British Columbia; thin-sections: crenulation cleavage in muscovite-biotite -garnet schist; crenulation cleavage development in the Moretown Formation, Appalachians, western Massachusetts; webpages: Fabrics and strain

crystallization

Crystallization is the process by which minerals form an ordered state (crystal). In chemistry, crystallization occurs from solution, and in geology from molten rock. Crystals have a regularly repeating internal structure in which constituent atoms, molecules, or ions are packed in a regularly ordered, repeating pattern extending in all three spatial dimensions, and hence display external plane faces.
Recrystallization involves an internal reorganization through partial melting (metamorphism), interaction with percolating chemical solutions (metasomatism), or through a diagenetic process by which unstable minerals in buried sediment are transformed into more stable minerals. Thus, recrystallization can involve a reorganization of elements of the original minerals in a rock in response to changes in temperature, and/or pressure, and/or the activity of pore fluids. Recrystallization can result in an increase in the degree of crystallinity, or crystal perfection or both. Dynamic recrystallization involves nucleation and growth of new grains, which occurs during metamorphic deformation, rather than resulting from later high-temperature metamorphism as in static recrystallization.
Diagenetic alterations take place in deposited sediments, during and after lithification, and occur at relatively low temperatures and pressures. There exists a gradational rather than distinct boundary between diagenesis and metamorphism, which occurs under conditions of higher temperature and pressure. Diagenesis alters a rock's original mineralogy and texture, and can also contribute to fossilization of buried skeletal material by replacing the collagen of bones with minerals.
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 :