ECLOGITE (from Gr. EKXoyi, a selection)

This article appears in Volume V08, Page 896 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ECG-EMS

ECLOGITE (from Gr. EKXoyi, a selection) , in petrology, a typical member of a small group of metamorphic rocks of

special

SPECIAL

interest

INTEREST

on account of the variety of minerals they contain and their microscopic structures and geological relationships. Typically they consist of

pale

PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

green or nearly colourless augite (omphacite), green hornblende and

pink

PINK

garnet. Quartz also is usually present in these rocks,. but felspar is rare. The augite is mostly a variety of

diopside

DIOPSIDE

and is only occasionally idiomorphic. The garnet sometimes forms good dodecahedra, but may occur as rounded grains, and encloses quartz, rutile, kyanite, and other minerals very frequently. The hornblende is usually

pale

PALE (through Fr. pal, from Lat. palms, a stake, for paglus, from the stem pag- of pangere, to fix; " pole " is from the same original source)

green and feebly dichroic, but, in some eclogites which are allied to garnet-amphibolites, it is of dark brown colour. Among the commoner accessory minerals are kyanite (of blue or greyish-blue tints), rutile, biotite,

epidote

EPIDOTE

and zoisite, sphene, iron oxides, and
B.C. 2000; obi. =23 55.5' A.D. 1700; obi. =23
1500 =23 52.3 1750 =23
1000 =23 48.9 1800 =23,
500 =23 .45.4 185o =23
0 =23 41.7 1900 =23
A.D. 500 =23 38.0 1950 =23
1000 =23 34.1 2000 =23
1500 =23 30.3 2050 =23
2000 =23 26.4 2100 =23
2500 =23 22.5
pyrites. The rutile is invariably in small brown prisms; the kyanite forms bladed crystals, with perfect cleavage; felspar, if present, belongs to basic varieties rich in lime. Other minerals which have been found in eclogites are bronzite, olivine and glaucophane. The last mentioned is a

bright

BRIGHT, JOHN (1811-188g)
BRIGHT, SIR CHARLES TILSTON

blue variety of hornblende with striking pleochroism. The eclogites in their chemical composition show close affinities to gabbros; they often exhibit relationships in the field which 'show that they were primarily intrusive rocks of igneous origin, and occasionally con-tact alteration can be traced in the adjacent schists. Examples are known in Saxony, Bavaria, Carinthia, Austria, Norway. A few eclogites also occur in the north-west highlands of

Scotland

. Glaucophane-eclogites have been met with in Italy and the Pennine Alps. Specimens of

rock

allied to eclogite have been found in the diamantiferous peridotite breccias of

South

SOUTH
SOUTH, ROBERT (1634–1716)

Africa (the so-called " blue ground "), and this has given rise to the theory that these are the parent masses from which the Kimberley diamonds have come. (J. S. F.)

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