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TONALITE

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Originally appearing in Volume V27, Page 1 of the 1911 Encyclopedia Britannica.
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TONALITE  , in See also:

petrology, a See also:rock of the See also:diorite class, first described from See also:Monte Adamello near Tonale in the Eastern See also:Alps . It may be described as a See also:quartz-diorite containing See also:biotite and See also:hornblende in nearly equal proportions . The See also:principal See also:felspar is See also:plagioclase, but See also:orthoclase occurs also, usually in small amount . Those varieties which are See also:rich in orthoclase, in addition to plagioclase, have been called quartz-monzonites or adamellites, but a better See also:term is grano-diorite, which has been very generally adopted in See also:America for rocks which are intermediate in See also:character between the granites and the diorites . The hornblende of the diorites is See also:green, sometimes with a tinge of See also:brown; the biotite is always brown and strongly pleochroic . Often these two minerals are clustered together irregularly or in parallel growths . They have generally a fairly strong tendency to idiomorphism, but may sometimes enclose plagioclase felspar in ophitic manner . Both of them decompose to See also:chlorite, See also:epidote and See also:carbonates . The plagioclase felspar, which may See also:form more than one-See also:half of the rock, is See also:andesine or See also:oligoclase; See also:simple crystals are rare, the See also:majority being complex growths with centres of felspar rich in See also:lime, while in the See also:external zones the proportion of soda felspar increases greatly . The inner portions have often well-defined, but very irregular, boundaries, and are sometimes sponge-like, with the cayities filled up with a later, more See also:acid, See also:deposit . This seems to indicate that growth has taken See also:place in stages, alternating with periods when the crystallized felspar was eroded or partly dissolved . The orthoclase sometimes forms irregular plates enclosing individuals of plagioclase .

Quartz occurs both in irregular simple grains and as micropegmatite . Occasionally See also:

pale green See also:pyroxene is visible in the centre of crystals of dark green hornblende . The See also:accessory minerals See also:apatite, See also:magnetite and See also:zircon are always See also:present, and very See also:common also are orthite in See also:coffee-coloured zonal prisms practically always encircled by yellow epidote, and reddish-brown crystals of See also:sphene, simple or twinned . In external See also:appearance the tonalites are very like the granites but usually darker in See also:colour . Tonalite-porphyrites often accompany them, having the same See also:composition but with phenocrysts of felspar, quartz, hornblende and biotite in a See also:fine-grained ground-See also:mass . See also:Veins and threads of fine See also:grey rock, mainly composed of quartz and felspar, often intersect tonalite-masses and have been called tonalite-aplites, seeing that they See also:bear the same relations to aplites as the aplites do to the granites . They contain more soda-lime felspar than the normal aplites . Towards their margins the larger alpine masses of tonalite often assume banded or gneissic facies, due apparently to See also:movement during intrusion .

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