K

◙ keratophyres ◊ K-feldspars ◙ kimberlites ◊ K-spars ◊ kyanite ◊

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keratophyres

Keratophyres are Na-rich extrusive rocks, containing pure sodic plagioclase with or without quartz (soda rhyolite). As salic extrusive and hypabyssal rocks, keratophyres usually comprise secondary albite or albite-oligoclase and chlorite, epidote, and calcite. Some keratophyres contain sodic orthoclase, sodic amphiboles, and pyroxenes. Keratophyres commonly are associated with spilitic rocks and interbedded with marine sediments. Keratophyre dikes and sills are associated with some ophiolite complexes, and keratophyre lava flows are found in some island arcs.

links: images: hand-specimen: 1: altered quartz-keratophyre, folded, 2, 3, 4, Horneblende in quartz-keratophyre, and blue quartz in quartz-keratophyre, Stekenjokk; close-up: wallpaper, 2, wp; formations: outcrop, thin-section: 1, strongly altered Stekenjokk Quartz, somewhat altered Quartz-keratophyre with newly formed mica minerals (red-green colours) and calcite (coloured, striped), Quartz-keratophyre with quartz (fine grained, grey shades) and feldspar (coarse, striped, grey grains), with metamorphic minerals horneblende (coloured) and garnet (black), wp; webpages: fr; Soda Rhyolite, Massif de Cressy-sur-Somme;

K-feldspars

mineral / chemical formula		properties / significance / occurrence
K-feldspar endmember KAlSi3O8	__________________________ images - click to enlarge - top, orthoclase from Smrekovica, Veľká Fatra, Slovakia; middle, microcline; bottom, green microcline feldspar (amazonite variety).	The K-feldspars or alkali feldspar polymorphs include the polymorphs: microcline, orthoclase, sanidine, and related varieties). Amongst the K-spars, sanidine is the most refractory (stable at the highest temperatures), and microcline is the most fusible (stable at the lowest temperatures). 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, then the sodium-rich feldspars, and finally the polymorph K-spar orthoclase is the most fusible members (freeze at lowest temperatures between biotite and muscovite).
microcline:
composition	KAlSi3 O8 potassium aluminum silicate
color	off-white,yellowish, flesh pink, brown or green
lustre	vitreous to sometimes pearly; dull if weathered
transparency	usually translucent to opaque
crystal system	triclinic; bar 1
crystal habits	blocky, or tabular crystals with a nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations; twinning is common.
cleavage	perfect in one and good in another direction forming nearly right angled prisms
fracture	conchoidal
hardness	6 - 6.5
specific gravity	2.5 (av.)
streak	white
other
associations	quartz, muscovite and plagioclase feldspars
occurences	Microcline can be found as a major rock forming component in granites, syenites and in metamorphic gneisses. Pikes Peak region of Colorado, North Carolina, other sites in the USA; Russia; Norway; and, Madagascar.
indicators	occurence, twinning, color and luster
orthoclase:
composition	KAlSi3O8 potasium aluminum silicate; polymorph of other minerals that share chemistry, but have different crystal structures
color	off-white, yellow, or shades of red, orange to brown
lustre	vitreous; dull if weathered
transparency	usually opaque, may be translucent, or rarely transparent
crystal system	monoclinic; 2/m
crystal habits	blocky or tabular crystals with nearly rectangular or square cross-section with slightly slanted dome and pinacoid terminations; twinning is common.
cleavage	good in 2 directions forming nearly right angled prisms
fracture	named based on the Greek for "straight fracture," because its two cleavages are at right angles to each other
hardness	6
specific gravity	2.53 - 2.56
streak	white
other	orthoclase forms at intermediate temperatures between the stability fields of sanidine and microcline
associations	quartz, plagioclase feldspars, micas, garnets, tourmalines, and topaz
occurences	some crystals may show opalescence (moonstone)
indicators	color, lack of striations, cleavage, twinning if present and occurrence

kimberlite

Kimberlites provide the source of mined diamonds and are named for Kimberley, South Africa. They are ultrapotassic, ultramafic, igneous rock composed mainly of olivine, phlogopite, pyroxene and garnet. Kimberlites contain a variety of indicator minerals with unusual chemical compositions: including, potassic richterite, chromian diopside (a pyroxene), chromium spinels, magnesian ilmenite, and garnets rich in pyrope plus chromium. (classification) Group I (basaltic) kimberlites are CO2-rich ultramafic potassic igneous rocks dominated by a primary mineral assemblage of forsteritic olivine, magnesian ilmenite, chromian pyrope, almandine-pyrope, chromian diopside (sometimes subcalcic), phlogopite, enstatite and of Ti-poor chromite. Group II (micaceous) kimberlites (or orangeites) are ultrapotassic, peralkaline rocks rich in volatiles (dominantly H2O), characterized of by phlogopite macrocrysts and microphenocrysts, sometimes with resorbed olivine macrocrysts and euhedral primary crystals of groundmass olivine. Groundmass micas vary from phlogopite to "tetraferriphlogopite" (anomalously Fe-rich phlogopite). Kimberlites occur in sheeted dyke complexes of tabular, vertically dipping feeder dykes at the root of the volcanic pipes that extend down to the mantle; kimberlite also rarely occurs in maars. Kimberlite pipes result from explosive diatreme volcanism of anomalously enriched exotic deep mantle derived magma.

links: images: formations: hypabassal kimberlite, Iron Mountain, Wyoming; kimberlite pipe; Kolo kimberlite; diamond mine, 2; hand-specimens: The three major kimberlite facies: hypabyssal kimberlite containing numerous olivine macrocrysts, Lac de Gras kimberlite field, NWT; diatreme-facies tuffisitic kimberlite breccia containing abundant xenoliths of sedimentary country-rock, "Triple B" diatreme, Ontario; resedimented volcaniclastic kimberlite with abundant macrocrysts of serpentinized olivine, diopside and pyrope in a clay-rich matrix, Lac de Gras, NWT.kimberlite breccia; diamond bearing: diamond, 2; Liqhobong kimberlite with Fancy Yellow diamond embedded; thin-sections: Hypabyssal-facies kimberlite from the Lac de Gras kimberlite field (NWT) as seen under the petrographic microscope in plane-polarized light and crossed polars. Note the presence of xenoliths (X) and olivine macrocrysts (M). Xenolith CX is of crustal origin, whereas MX is a fragment of harzburgite from the Earth's upper mantle (field of view ~4 mm); kimberlite, 2; severely altered (polygonal shapes are relict crystals of olivine which have completed decomposed to serpentine and other alteration products); maps: Kimberlites in Canada; diagrams: kimberlite bodies (distributed technology, digital signal processing and data over sampling); body; model of sub-cratonic sources; kimberlite pipe, 2; kimberlite formation; pipes to mine; webpages: A4 Kimberlite Pipe : C14 Kimberlite : Diamond Lake Kimberlite : Buffonta Dyke : Kimberlite Indicator Minerals

komatiites

Komatiites are ultramafic, often cumulate, volcanic, subvolcanic, or intrusive igneous rocks derived from mantle melts. They have high to very high MgO, low SiO2, low K2O, and low Al2O3. The mineralogy of komatiites comprises forsteritic olivine (Fo90 and upwards, which can only result from crystallization of olivine from a high Mg melt), Ca- or Cr-pyroxenes, anorthite (An85 an upwards), and chromite. Some cumulate komatiites contain radiating spinifex sprays of olivine crystals with a texture attributable to rapid crystallization of the olivine from a Mg-rich melt. Other mineral species encountered in komatiites include pargasitic amphibole (with greater than 20%MgO), phlogopite, baddeleyite, ilmenite and pyrope garnets. The fluid nature of komatiite lava resulted in very thin layers of komatiites except where lava pooled in lava tubes, lava ponds, or other conduits. A typical komatiite lava flow has six stratigraphically related elements; A1 - pillowed and variolitic chilled flow top, often grading and transitional with sediment A2 - Zone of quickly chilled, feathery acicular olivine-clinopyroxene-glass representing a chilled margin on the top of the flow unit A3 - Olivine spinifex sequence composed of sheaf and book-like olivine spinifex, representing a downward-growing crystal accumulation on the flow top B1 - Olivine mesocumulate to orthocumulate, representing a harrisite grown in flowing liquid melt B2 - Olivine adcumulate composed of >93% interlocking equant olivine crystals B3 - Lower chill margin composed of olivine adcumulate to mesocumulate, with finer grain size. Preserved in some komatiite flows are top breccias and pillow margin zones, which are essentially volcanic glasses that quenched in contact with overlying water or air and so record an anyhdrous MgO content of up to 32-40% MgO for the liquid composition of the komatiite lavas. The Wannaway section of the Widgiemooltha Komatiite has clear preservation of MgO greater than 39%. The two geochemical classes of komatiite are aluminium undepleted komatiite (AUDK) known as Group I komatiites, and aluminium depleted komatiite (ADK) known as Group II komatiites. Melting experiments suggest that Al-depleted komatiites were produced by high degrees of partial melting of hydrous mantle at low pressure, whereas Al-undepleted komatiites are produced by high degree partial melts at greater depth, allowing melting of Al-rich pyroxene (which does not melt at low pressures). Accordingly, komatiites are considered to have arisen through greater than 50% partial melting, and thus have high MgO with low K2O and other incompatible elements. By comparison, kimberlites, which are righ in Mg, K, and other incompatible elements, are believed to have formed through less than 1% partial melting (in magmas fluxed by water and carbon dioxide). Boninite magmatism, which is similar to komatiite magmatism, results from the melting induced by volatile flows above a subduction zone. Boninites with 10-18% MgO tend to have higher LILE (Ba, Rb, Sr) than komatiites. Virtually all known komatiites have suffered metamorphic changes and are therefore 'meta-komatiites'. As a result of metasomatism, the mineralogy of komatiites reflects primary magmatic chemistry and komatiites are usually highly altered and serpentinized or carbonated. Where the partial pressure of carbon dioxide within the magmatic fluid (the XCO2) is above 0.5, the metamorphic reactions favor formation of carbonated komatiites containing talc, magnesite (magnesium carbonate), and tremolite amphibole (talc-carbonation reactions), whereas below XCO2 of 0.5 production of hydrated komatiites containing serpentine-antigorite, brucite is favored (in the presence of water). Under higher metamorphic grades, the komatiites dehydrate, yielding anthophyllite, enstatite, olivine and diopside. True komatiites are very rare and virtually all shield rocks of Archaean age, older than 2Ga, and very few few Proterozoic or Phanerozoic komatiites are known, whereas some high-Mg lamprophyres are known from the Mesozoic. The youngest komatiites are from the island of Gorgona on the Caribbean oceanic plateau. Komatiites occur in association with other ultramafic and high-magnesian mafic volcanic rocks in Archaean greenstone belts. Komatiitic magmas are considered to be a source of spatially associated tholeiite basalts (based on the Karelian greenstone belt of northwest Russia). Magmas of komatiite compositions have a very high melting point, so eruption temperatures of these highly fluid lavas are calculated to have been in excess of 1600 °C. The Archean-age restriction of komatiites is considered due to comparative cooling of the mantle, which could have been 500 °C hotter during the Hadean and Archean than it is currently. (Because radioactive elements with relatively short half-lives, such as U235, have decayed considerably since the Hadean, radioactive heating of the Earth has diminished.)

links: images: hand-specimens: unaltered komatiite; close-up: section through the coarse-bladed olivine spinifex zone in the slowly cooled interior of a komatiite unit (Kapvaal), 2, 3 - long, dark stripes outline the shapes of olivine (altered to serpentine and magnetite) blades that reach 15 cm in length in this portion of the unit, and lighter-colored triangles and quadrilaterals contain tremolite, chlorite and magnetite (regions where spinel and high-Ca pyroxene crystallized after the early olivine blades); thin-section: "spinifex texture" defined by extremely acicular olivine phenocrysts - probably a sign of rapid crystallization from a significantly-undercooled magma; komatiite; V-8 komatiite (Barbeton, South Africa) spinifex texture, ppl; formations: ~2.7 Ga komatiite flow (Pyke Hill, Ontario, Canada) with impressive spinifex texture, and derived from a mantle source with a chondritic 187Os/188Os ratio, and close-up; komatiite lava tube, Spinifex Ck, Barberton Greenstone Belt; komatiite showing spinifex texture; a komatiite pillow, 2 in the (namesake) Komati formation; Komatitic tuff horizon (Koikary structure), individual lava fragments occur in the komatiitic tuff (Koikary structure) Komatiitic tuff bed at a contact between peridotitic and piroxenitic komatiite flows (Palaselga structure), spinifex-zone in komatiite flow (Koikary structure), Komatiitic tuff horizon (Koikary structure) in which pelite beds are observed to alternate with psammitic tuff lenses, variolitic texture in pyroxenitic komatiites from the Koikary structure, pyroxenitic komatiite pillow lavas in the Koikary structure, Komatiitic autobreccia (Sovdozero structure), polygonal jointing in a peridotitic komatiite flow (Plaselga structure), polygonal jointing in a peridotitic komatiite flow (Palaselga structure), tholeiitic pillow lava (Palaselga structure), Vedlozero-Segozero greenstone belt (Central Karelia, Russia); diagram: generalized komatiite flow; komatiite flow; webpages: thin section gallery

kyanite

mineral / chemical formula		properties / significance / occurrence
kyanite		Kyanite, named from the Greek word for 'blue', is a typically blue, anisotropic aluminosilicate series mineral (others are the polymorphs andalusite and sillimanite). Kyanite is commonly found in Al-rich metamorphic pegmatites and/or sedimentary rocks. Kyanite is typically found in association with its polymorphs, as well as with other silicate minerals, including: andalusite, Al2SiO5 sillimanite, Al2SiO5 quartz, SiO2 staurolite, Fe2Al9Si4O22(OH)2 micas, AB2-3(X, Si)4O10(O,F,OH)2 garnets, A3B2(SiO4)3 Kyanite undergoes an irreversible expansion when fired at high temperature, and is a diagnostic mineral of the Blueschist Facies that result when basalts are subjected to high pressures and low temperatures (depth of 15 to 30 kilometers, 200 to ~500 ºC).
kyanite: polymorph with andalusite and sillimanite
composition
color
lustre
transparency
crystal system
crystal habits
cleavage
fracture
hardness
specific gravity
streak
other
associations
occurences
indicators
links: images: crystal, 2, kyanite; rocks: Kyanite-Garnet-Biotite Gneiss, Kyanite-(Staurolite) schist, Kyanite pseudomorph after Chiastolite (Andalusite), blue Kyanite grain in eclogite like rock, Borisovkyi Sopki, Kyanite with Ruby from Chainyt, kyanite, embedded; formations: bladed crystals of kyanite,Willis Mountain, Virginia, Série métapélitique de l'Agly: Micaschiste à andalousite; webpages: kyanite

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