Theralite (from Greek "to pursue") is, in
petrology, the name given to calcic
foidal gabbro
Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...
, a plutonic hylocrystalline rock consisting of
augite,
olivine
The mineral olivine () is a magnesium iron silicate with the chemical formula . It is a type of nesosilicate or orthosilicate. The primary component of the Earth's upper mantle, it is a common mineral in Earth's subsurface, but weathers quickl ...
, calcic
plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
(
labradorite
Labradorite (( Ca, Na)( Al, Si)4 O8) is a calcium-enriched feldspar mineral first identified in Labrador, Canada, which can display an iridescent effect (schiller).
Labradorite is an intermediate to calcic member of the plagioclase series. It ...
), and
nepheline
Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatite ...
, along with accessories including
biotite,
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...
,
ilmenite
Ilmenite is a titanium-iron oxide mineral with the idealized formula . It is a weakly magnetic black or steel-gray solid. Ilmenite is the most important ore of titanium and the main source of titanium dioxide, which is used in paints, printing ...
and
analcime
Analcime (; ) or analcite is a white, gray, or colorless tectosilicate mineral. Analcime consists of hydrated sodium aluminium silicate in cubic crystalline form. Its chemical formula is Na Al Si2 O6· H2O. Minor amounts of potassium and calci ...
.
Theralite is the intrusive equivalent of nepheline
basanite, a foidal
basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
with essential calic plagioclase and essential olivine.
Tephrite
Tephrite is an igneous, volcanic (extrusive) rock, with aphanitic to porphyritic texture. Mineral content is usually abundant feldspathoids (leucite or nepheline), plagioclase, and lesser alkali feldspar. Pyroxenes (clinopyroxenes) are common ...
is foidal basalt with essential calic plagioclase but without essential olivine. It is essentially the volcanic equivalent of
essexite
Essexite (), also called nepheline monzogabbro (), is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.
Modern petrology identifies rock ...
. The discovery of theralite was looked forward to with interest as it was of rare occurrence, and as completing the series of basic rocks containing nepheline as an essential constituent.
With the increase in silica (SiO
2) and the concomitant reduction in nepheline, theralite becomes gabbro. With a decrease in silica and reduction in olivine theralite grades into teschenite and with the addition of sodic feldspar, grades into essexite. With a further reduction in silica such that there is no feldspar these rocks become melteigites. With the addition of alkali feldspar replacing or becoming more dominant than calcic plagioclase theralites grade into foidal syenite, including
shonkinite
Shonkinite is an intrusive igneous rock found in few places in the world. It is unique in having low silica, feldspathoid minerals, and large blocky crystals of black augite. It makes up much of the hard dark grey rock comprising certain mountains ...
.
Pyroxene in these rocks may be of green colour or purplish-brown
and rich in
titanium
Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
.
Properly theralite is only for a specific type of calcic foidal gabbro, those with essential nepheline and without essential analcime or sodic feldspar.
Teschenites
Teschenites were once considered part of the theralite group. They contain essential pyroxene, olivine and analcime rather than nepheline.
Modern petrology would classify the teschenites as an analcime-bearing gabbro. They are petrogenetically unrelated to nepheline gabbros as analcime is not produced by the same genetic processes.
Essexites
Essexite
Essexite (), also called nepheline monzogabbro (), is a dark gray or black holocrystalline plutonic igneous rock. Its name is derived from the type locality in Essex County, Massachusetts, in the United States.
Modern petrology identifies rock ...
is a form of silica-undersaturated pyroxene gabbro, containing nepheline and orthoclase as essential constituents, though often in small proportions. Essexites can be considered to represent the intrusive equivalent of an alkaline basalt.
Essexites are usually associated with other alkaline, silica undersaturated to
monzonitic intrusive rocks and coeval high-alumina basalts.
Essexites are no longer considered part of the theralite group and are more properly known as ''nepheline monzogabbro'' or ''nepheline monzodiorite''. The presence of orthoclase indicates a petrogenetic difference with nepheline gabbros.
References
* {{EB1911, wstitle=Theralite, last=Flett, first=John Smith, authorlink=John Smith Flett, volume=26, pages=791–792 This contains more detailed analytic and location data.
* theralite (mineral)- Britannica Online Encyclopedia
* Heinrich, Microscopic Petrography, McGraw-Hill, 1956
Plutonic rocks
Igneous petrology