_variety_of_[[nepheline_syenite_from_[[Sweden.html" style="text-decoration: none;"class="mw-redirect" title="nepheline.html" style="text-decoration: none;"class="mw-redirect" title="leucocratic variety of [[nepheline">leucocratic variety of [[nepheline syenite from [[Sweden">nepheline.html" style="text-decoration: none;"class="mw-redirect" title="leucocratic variety of [[nepheline">leucocratic variety of [[nepheline syenite from [[Sweden ([[särnaite)
Syenite is a coarse-grained [[Intrusion|intrusive [[igneous rock with a general composition similar to that of [[granite, but deficient in [[quartz, which, if present at all, occurs in relatively small concentrations (< 5%). Some syenites contain larger proportions of mafic
components and smaller amounts of felsic
material than most granites; those are classed as being of intermediate composition
. The volcanic equivalent of syenite is trachyte
Composition of syenites
component of syenite is predominantly alkaline
in character (usually orthoclase
feldspars may be present in small proportions, less than 10%. Such feldspars often are interleaved as perthitic
components of the rock.
minerals are present in syenite at all, they usually occur in the form of hornblende
is rare, because in a syenite magma the formation of feldspar consumes nearly all the aluminium, however less Al rich phyllosilicates
may be included such as annite
Other common accessory minerals are apatite
Most syenites are either peralkaline
with high proportions of alkali elements relative to aluminum, or peraluminous
with a higher concentration of aluminum relative to alkali
elements (predominantly K, Na, Ca).
Formation of syenites
Syenites are products of alkaline igneous activity, generally formed in thick continental crust
al areas, or in Cordilleran subduction
zones. To produce a syenite, it is necessary to melt a granitic or igneous protolith
to a fairly low degree of partial melting
. This is required because potassium is an incompatible element and tends to enter a melt first, whereas higher degrees of partial melting will liberate more calcium and sodium, which produce plagioclase, and hence a granite
, adamellite or tonalite
At very low degrees of partial melting a silica
undersaturated melt is produced, forming a nepheline syenite
, where orthoclase is replaced by a feldspathoid
such as leucite
Conversely in certain conditions, large volumes of anorthite
crystals may precipitate from thoroughly molten magma in a cumulate
process as it cools. This leaves a drastically reduced concentration of silica in the remainder of the melt. The segregation of the silica from the melt leaves it in a state that may favour syenite formation.
Occurrence of syenites
Syenite is not a common rock. Regions where it occurs in significant quantities include the following.
*In the Kola Peninsula
of Russia two giant nepheline
syenite bodies exists making up the Lovozero Massif
and the Khibiny Mountains
. These syenites are part of the Kola Alkaline Province
* In North America syenite occurs in Arkansas
. Regions in New England
have sizable amounts, and in New York
es occur. The "great syenite dyke" extends from Hanging Rock, South Carolina
through Taxahaw, South Carolina
to the Brewer and Edgeworth mine in Chesterfield, South Carolina
Syenite pebbles, containing fluorescent sodalite
, were moved from Canada to Michigan by glaciers;
these glacial erratic
pebbles have been given the trade name "yooperlite".
In other parts of the world, these types of rocks are known as sodalite-syenite and occur in Canada, India, other US states, Greenland, Malawi, and Russia.
* In Europe syenite may be found in parts of Switzerland
, in Plovdiv
, Bulgaria and in Ditrău
*In Africa there are syenite formations in Aswan, Egypt
, and in Malawi
in the Mulanje Mountain Forest Reserve
. Syenite rock was used to make the Quay with Sphinxes
*In Australia syenite occurs as small intrusive bodies in nearly every state. In New South Wales
, a large syenite intruded during the breakup of Gondwana
in the Cretaceous
*Instead of the usual rock syenite, some of the more important events in New England, Arkansas, Montana, New York (syenite gneisses), Switzerland, Germany, Norway, Plovdiv, Bulgaria, Malawi (Mulanje Mountain Forest Reserve) and Romania (Ditrău). The Malvern Hills
, which is on the border between the counties of Herefordshire and Worcestershire United Kingdom are also formed of syenite.
fjords in southeastern Greenland
, where a bay within the latter ''(Syenitbugt)'' and a headland ''(Syenitnæs)'' are named after the rock.
The term syenite was originally applied to hornblende granite like that of Syene
(now Aswan) in Egypt
, from which the name is derived.
''Episyenite'' (or ''epi-syenite'') is a term used in petrology
to describe the depletion of silicon dioxide
) in rock rich in silicon dioxide.
A process that results in depletion often is termed ''episyenitization''. The term refers only to the macroscopic
effect of relative depletion in a rock; it does not imply anything about the nature of the physical processes leading to the depletion in any particular case. Many different metamorphic
processes can lead to episyenitization. For example:
* chemical components in a stagnant melt can diffuse under the influence of chemical potential gradient
s that cause their segregation from low- SiO2
components when the melt begins to solidify
* a SiO2
-undersaturated fluid may dissolve quartz
from rock and remove it by advection
, thus leaving the parent rock depleted of silica.
* a marginally molten rock mass may retain its unmolten silica-rich components, while the molten, silica-depleted fluid cools to form a syenite.
* on beginning to cool, a fully molten silica-rich melt might precipitate its silica-containing components, leaving the silica-depleted melt to form a syenite afterwards.
* List of rock types
* E. Wm. Heinrich. Microscopic Petrography, McGraw-Hill, 1956