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TONALITE , in See also: petrology, a See also: rock of the 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 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 majority being complex growths with centres of felspar rich in 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 acid, deposit
.
This seems to indicate that growth has taken 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 pale greenSee 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 appearance the tonalites are very like the granites but usually darker in colour
.
Tonalite-porphyrites often accompany them, having the same composition but with phenocrysts of felspar, quartz, hornblende and biotite in a See also: fine-grained ground-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 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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