Rocks & Minerals: August 2007

E

◙ eclogites ◊ epidote ◙◊ evaporates & sulphates ◊

◊◊

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

eclogites

Eclogites are dense, coarse-grained mafic metamorphic rocks of basaltic composition that form at depths below that of the crust.

(images - click to enlarge - top left, eclogite containing omphacite (green), pyrope-garnet (red), quartz (milky), kyanite (blue) and some phengite (golden white); bottom left, plane polarized light-photomicrograph of a thin-section of eclogite (Turkey) with green omphacite (+ late chlorite) + pink garnet + blue glaucophane + colorless phengite; below right, hand-specimen of eclogite)

The eclogite facies results from the high temperature (>400–1000 °C) and very high pressure (1.2 GPa) conditions required to metamorphose basaltic rocks to an eclogite assemblage, which includes garnets (almandine-pyrope) plus clinopyroxene (omphacite).

The extremely high pressure, medium to high temperature Franciscan metamorphism reflects burial at depths of 45 km depth. Because eclogites form at depth, their minerals are stable at pressures greater than those typical of the crust of the Earth. The rock's striking appearance is due to red to pink garnets (almandine-pyrope) set against a green matrix of sodium-rich pyroxenes (omphacite).

Eclogites originate in the mantle or lowermost part of thickened continental crust :

◙ from mafic magmas that crystallize and cool within the mantle or within the lower crust of continents.

◙ through high-pressure metamorphism of mafic igneous rocks (usually basalt or gabbro) as tectonic forces drive oceanic crust down into the mantle at subduction zones. Such ultrahigh-pressure (UHP) metamorphism has been defined as metamorphism within the eclogite facies but at pressures greater than those of the quartz-coesite transition. Some UHP rocks appear to record burial at depths greater than 150 km. UHP eclogites are generally formed from precursor mineral assemblages typical of blueschist metamorphism as pressures exceed 1.2 GPa (45 km depth) and temperatures reach >400–1000 °C (usually > 600-650 °C).

Because eclogite is an unusually dense rock, it can play an important role in driving convection. Accessory metamorphic minerals include kyanite, rutile, quartz, lawsonite, coesite, amphiboles, phengite, paragonite, zoisite, dolomite, corundum, and, rarely, diamond. Feldspars are not stable in eclogites, and glaucophane and titanite (sphene) either form during exhumation as pressures on the rock decrease, or they may be earlier formed minerals that have not entirely reacted away.

Eclogites containing the hydrous calcium-aluminium silicate, lawsonite are very rarely exposed at the Earth's surface, although they are experimentally predicted to form during normal subduction of oceanic crust at depths between ~ 45-300 kilometers. However, lawsonite eclogite are exposed - Franciscan Complex of California; xenoliths in Arizona; Guatemala (Motagua fault zone); Corsica; Australia; the Dominican Republic; Canada (British Columbia); and, Turkey.

links: images: hand-specimen: eclogite Norway, eclogites, garnet omphacite disthene, St Philibert de Grand-Lieu; ; close-ups: eclogite Switzerland; interlayered blue schist and eclogite (green layers jadeite +chlorite; dark blue glaucophane; red, equant crystals are garnet); 2; HP and UHP metamorphism is recorded in the eclogites of basaltic protoliths; eclogite with secondary hornblende; lawsonite eclogite and lawsonite blueschist, Sivrhisar, Turkey; formations: eclogite boudins, Norway; eclogite vein, Holsnoy, Norway; eclogite folds, Nordfjord, Norway; granulite rocks (grey) were transformed into eclogite rocks (green) at a depth of 60 kilometers, adjacent to brittle pyroxenite rocks (black), Holsnoy, Norway; Eclogite; eclogite exposed; massive eclogite lens in marble, Norway; vein of garnetite in eclogite, Tromso; partial melting along the boundary between eclogite and marble; garnet-rich zone between leucosome and host eclogite; eclogite Tinashan; light grey quartzo-feldspathic gneisses from the Western Gneiss Region of Norway. The dark pods near the center of the frame are eclogites; thin-sections: eclogite fsu; eclogite garnet; coesite in eclogite; eclogite, 2; garnets and clinopyroxene are the two major minerals in eclogite; amphibole (probably hornblende) is a tip-off that this eclogite has been subjected to retrograde metamorphism (reduced temperature and pressure); webpages: Geology Wallpaper Gallery; Northern Scandinavian Caledonides; NASA; map: UHMP terranes:

image of eclogite hand-specimen courtesy of Siim Sepp

epidote

mineral / chemical formula		properties / significance / occurrence
epidote Ca2(Al, Fe)3(SiO4)3(OH)	images - click to enlarge, epidote crystals.	Single silicate tetrahedrons (SiO4) and double silicate tetrahedrons (Si2O7) in monoclinal, commonly prismatic, often deeply striated, often twinned, pleochroic 'pistachio' green, grey, brown or nearly black crystals of calcium aluminium iron sorosilicate. The species piemontite and allanite same isomorphous group, and are manganese and cerium epidotes respectively. Common secondary mineral in igneous rocks, crystalline limestones and schistose metamorphic rocks. Epidote also results from hydrothermal metamorphism of igneous rock minerals (including amphiboles, feldspars, garnets, micas, pyroxenes).
epidote:
composition	Ca2(Al, Fe)Al2O(SiO4)(Si2O7)(OH) structurally complex mineral with single silicate tetrahedrons, SiO4, and double silicate tetrahedrons, Si2O7
color	"pistachio" green, yellowish or brownish green, brown, or black
lustre	vitreous
transparency	transparent to translucent crystals
crystal system	monoclinic; 2/m
crystal habits	long, somewhat prismatic or tabular crystals with a typically dominant pinacoid against which the crystal is often flattened; terminations are wedge shaped or tappered pyramids; many clusters show grooved slender crystals or acicular sprays; also massive, fiberous or granular.
cleavage	good in one direction lengthwise
fracture	uneven to conchoidal
hardness	6-7
specific gravity	3.3-3.5
streak	white to gray
other	striated to the depth of grooves in some crystals
associations	calcite, biotite, hornblende, actinolite, andradite garnet, other metamorphic minerals
occurences	Untersulzbachtal, Austria; Italy; Baja, Mexico; USA
indicators	only one direction of cleavage, crystal habit, color and hardness
links: images : crystals, cystal, 1, 2, 3, 4, 5, 6, 6a, 6b, 6c, 7, epidote crystal in calcite, epidote et actinote, epidote crystals in quartz; formation: olivine green veins containing epidote, epidote crystals in the Nahant Sill; thin-section: 1, 1p, 2, wp : weblinks: prehnite on epidote, gallery, green monster mine, Epidote Mineral Galleries, wp, wp2; structure: epidote, epidote (UC), (crystals UC)