Shonkinite
Encyclopedia
Shonkinite is a mafic foidal (feldspathoid
bearing) syenite
, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar
in the form of sanidine
, nepheline
, augite
, biotite
, and olivine
. Shonkinite is also used for mafic nepheline syenite
with aegerine-augite
as the pyroxene, and with the addition of plagioclase
(andesine to labradorite). Nepheline in shonkinite from the type locality at Shonkin Sag
in the Highwood Mountains
of north-central Montana
, from which the rock gets its name, is largely altered to natrolite
and stilbite
. The Shonkin Sag laccolith
is a significant source of shonkinite.
The Adel Mountains Volcanic Field
in north-central Montana
is a significant and abundant source of shonkinite, which is found primarily in Montana in the United States
, Ontario
in Canada
, and the southwest Pacific Ocean
island of Timor
.
Feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks....
bearing) syenite
Syenite
Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or present in relatively small amounts Syenite is a coarse-grained intrusive igneous rock of the same general composition as granite but with the quartz either absent or...
, an intrusive, holocrystalline rock which in the restricted sense is composed of potassic feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
in the form of sanidine
Sanidine
Sanidine is the high temperature form of potassium feldspar 4O8. Sanidine most typically occurs in felsic volcanic rocks such as obsidian, rhyolite and trachyte. Sanidine crystallizes in the monoclinic crystal system. Orthoclase is a monoclinic polymorph stable at lower temperatures...
, nepheline
Nepheline
Nepheline, also called nephelite , is a feldspathoid: a silica-undersaturated aluminosilicate, Na3KAl4Si4O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatites...
, augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
, biotite
Biotite
Biotite is a common phyllosilicate mineral within the mica group, with the approximate chemical formula . More generally, it refers to the dark mica series, primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more aluminous endmembers...
, and olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
. Shonkinite is also used for mafic nepheline syenite
Nepheline syenite
Nephelene syenite is a holocrystalline plutonic rock that consists largely of nepheline and alkali feldspar. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike granites, but dark green varieties are also known...
with aegerine-augite
Augite
Augite is a single chain inosilicate mineral, 2O6. The crystals are monoclinic and prismatic. Augite has two prominent cleavages, meeting at angles near 90 degrees.-Characteristics:Augite is a solid solution in the pyroxene group...
as the pyroxene, and with the addition of plagioclase
Plagioclase
Plagioclase is an important series of tectosilicate minerals within the feldspar family. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a solid solution series, more properly known as the plagioclase feldspar series...
(andesine to labradorite). Nepheline in shonkinite from the type locality at Shonkin Sag
Shonkin Sag
The Shonkin Sag is a prehistoric fluvioglacial landform located along the northern edge of the Highwood Mountains in the state of Montana in the United States. The Sag is a river channel formed by the Missouri River and glacial meltwater pouring from Glacial Lake Great Falls...
in the Highwood Mountains
Highwood Mountains
The Highwood Mountains cover approximately 4,659 km² in north central Montana in the U.S., east of Great Falls and Benton Lake National Wildlife Refuge, at the northern end of the Lewis and Clark National Forest...
of north-central Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
, from which the rock gets its name, is largely altered to natrolite
Natrolite
Natrolite is a tectosilicate mineral species belonging to the zeolite group. It is a hydrated sodium and aluminium silicate with the formula . The type locality is Hohentwiel, Hegau, Germany....
and stilbite
Stilbite
Stilbite is the name of a series of tectosilicate minerals of the zeolite group. Prior to 1997, stilbite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named:*...
. The Shonkin Sag laccolith
Laccolith
A laccolith is a sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively...
is a significant source of shonkinite.
The Adel Mountains Volcanic Field
Adel Mountains Volcanic Field
The Adel Mountains Volcanic Field is an ancient volcanic field of heavily eroded 75 million year old igneous rocks about 40 miles long and 20 miles wide in west-central Montana about 30 miles southwest of the city of Great Falls...
in north-central Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
is a significant and abundant source of shonkinite, which is found primarily in Montana in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, Ontario
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
in Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
, and the southwest Pacific Ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
island of Timor
Timor
Timor is an island at the southern end of Maritime Southeast Asia, north of the Timor Sea. It is divided between the independent state of East Timor, and West Timor, belonging to the Indonesian province of East Nusa Tenggara. The island's surface is 30,777 square kilometres...
.