Zircon mineral

Zircon

Used as a gemstone for over 2000 years. Today it is the primary ore of zirconium.

Zircon Crystal: A small cluster of nicely-formed zircon crystals which clearly demonstrate that zircon is a member of the tetragonal crystal system. The four-sided crystals are prismatic with a square cross-section and terminate with a pyramid. The largest crystal in the cluster is about 1.7 centimeters in length. Specimen from Mt. Malosa, Malawi.

What Is Zircon?

Zircon is a zirconium silicate mineral with a chemical composition of ZrSiO₄. It is common throughout the world as a minor constituent of igneous, metamorphic, and sedimentary rocks.
Zircon is a popular gemstone that has been used for nearly 2000 years. It occurs in a wide range of colors and has a brightness and fire that rivals those of diamond. Colorless zircon is sometimes used as a lower-cost alternative for diamond. Zircon should not be confused with cubic zirconia, which is a man-made material.
Zircon is present in most soils and clastic sediments. Zircon-rich sediments are mined and the recovered zircon is used to produce zirconium metal and zirconium dioxide. These are used in a wide variety of manufactured products and industrial processes.

      

Zircon Gem Rough: Stream-rounded crystals of zircon from an alluvial deposit in Australia. Although brown zircon is not highly marketable, much of it can be altered to a rich blue color by heating in a reducing atmosphere to between 900 and 1000 degrees Celsius. These stones range from about 5 to 10 millimeters in size.

Geologic Occurrence Of Zircon

Zircon is a primary accessory mineral in most granitic rocks. It is also present in gneiss and other rocks derived from the metamorphism of zircon-bearing igneous rocks. Zircon is so common and widely distributed across the rocks of Earth's surface that it could be considered to be a ubiquitous mineral. However, zircon is usually not noticed in rocks and sediments because of its very small particle size. Grains of zircon over a few millimeters in size are rare - they are usually under one millimeter in size. It is one of Earth's most common but most overlooked minerals.
Zircon is highly resistant to chemical alteration and abrasion. When rock units containing zircon are weathered and their sediments are eroded, enormous numbers of tiny zircon crystals are dispersed. These can persist in soils, sediments, and sedimentary rocks for millions - even billions - of years. They can survive several cycles of uplift, weathering, erosion and deposition.

Some of the largest crystals of zircon are formed in pegmatites, carbonate igneous rocks known as carbonatites, and in limestones altered by hydrothermal metamorphism. These large zircons are sometimes of high clarity and suitable for use as gemstones.

Physical Properties of Zircon
Chemical Classification	Silicate
Color	Usually yellow, brown, or red. Also colorless, gray, blue, and green.
Streak	Colorless. Usually harder than the streak plate.
Luster	Vitreous to adamantine, sometimes oily.
Diaphaneity	Translucent to transparent
Cleavage	Imperfect
Mohs Hardness	7.5
Specific Gravity	4.6 to 4.7
Diagnostic Properties	Hardness, luster, specific gravity
Chemical Composition	ZrSiO4
Crystal System	Tetragonal
Uses	Ore of zirconium metal, ore of zirconium dioxide, whitening agents, white pigment, gemstones, radiometric dating.

Zircon as a Gemstone

Zircon has been used as a gemstone for over 2000 years. Its very high dispersion and refractive index give it a brilliance and fire that rival those of diamond. For that reason, colorless faceted zircon has been used as both a popular and fraudulent substitute for diamond.
Gemologists and many knowledgeable jewelers are able to distinguish zircon from diamond with a quick examination. To do this they look into the stone, through the table facet, and focus on the pavilion facet junctions, with a 10x loupe. The pavilion facet junctions should appear as double-images caused by zircon's double-refraction. Diamond is singly refractive and will not show doubling of features within the stone. This same test can be used to distinguish zircon from cubic zirconia.
Zircon is a popular gem because it is available in a variety of pleasing colors. Most natural zircons are yellow, red, or brown. Heating and irradiation can be used to produce colorless, blue, green, and many other zircon colors. Blue is the most popular zircon color. About 80% of the zircons sold today are blue.
Although it is not as durable as diamond, zircon has good physical durability as a gem. It has a hardness of 7.5 and imperfect cleavage. That combination makes it suitable for most gemstone uses that include rings, earrings, pendants, brooches and other jewelry. Some zircon, especially gems that have been heat treated, can be brittle. The facet edges of these gems are susceptible to nicks and chipping.

Zircon Damage in Tanning and Nail Salons

Most blue zircon sold in jewelry today is produced by heat treating brown zircon in a reducing atmosphere to 900 to 1000 degrees Celsius. Some people have damaged these blue zircons by exposing them to ultraviolet radiation in tanning beds or under ultraviolet lamps used to cure acrylic fingernail adhesives. The blue color can degrade to brown with just minutes of exposure. The color of some of these gems have been restored by exposure to low wattage incandescent light. To avoid ruining a nice gem, remove jewelry at tanning and nail salons. [1]

Zircon has a specific gravity of 4.6 to 4.7, which is much higher than the typical detrital sediment grain that is between 2.6 and 2.8. This specific gravity difference allows zircon grains to be recovered from the sediments by mechanical separation. Specific gravity separation methods make it possible to profitably recover zircon and other heavy minerals at an ore grade of just a few percent. Zircon is often a coproduct at mining/processing operations whereilmenite and rutile are being mined for titanium. In the United States, zircon is mined in Virginia, Georgia, North Carolina and Florida.
Industrial-grade zircon is mined from land- and marine-based deposits of alluvial origin in many parts of the world. Australia, South Africa, China, Indonesia, Mozambique, India, Ukraine, Sri Lanka, Madagascar, Brazil, Kenya and several other countries were important producers in 2014. These alluvial deposits contain mainly sub-millimeter grains of zircon derived from the weathering of granitic rocks.
Gem-grade zircon has been produced from alluvial deposits in Sri Lanka, Cambodia, Myanmar, and Vietnam for hundreds of years. More recent gem-grade deposits are in Australia, Nigeria, and Madagascar. Deposits mined for gem-grade zircon must contain crystals at least several millimeters in size with good clarity. They are typically derived from the weathering of carbonate rocks and other rock types associated with hydrothermal activity. Some of the best gem-grade zircon crystals are mined directly from cavities in pegmatite.

Zircon, Zirconium, Zirconia and Cubic Zirconia

There is much public confusion between four materials: zircon, zirconium, zirconia and cubic zirconia. Summary definitions of these terms are provided below.
Zircon is a naturally occurring mineral with a chemical composition of ZrSiO₄.
Zirconium is a silvery white metal and a chemical element. It has an atomic number of 40 and an atomic symbol of Zr.
Zirconia is the white crystalline oxide of zirconium with a chemical composition of ZrO₂. A naturally occurring, but rare, form of ZrO₂ is the mineral baddeleyite.
Cubic Zirconia is a synthetic gemstone with an appearance that is very similar to diamond. It sells for a tiny fraction of the cost of diamond and has historically been the most commonly used diamond simulant.
All of these materials are related. Zirconium, zirconia and cubic zirconia are all produced from industrial-grade zircon.

Industrial Uses of Zircon

Zircon References

[1] Reversible Color Modification of Blue Zircon by Long-Wave Ultraviolet Radiation, by Nathan D. Renfro, Gems & Gemology, Volume 52, Number 3, Fall 2016. [2] Zircon and Hafnium, by George M. Bedinger, 2014 Minerals Yearbook, United States Geological Survey, August 2016. [3] Deposit Model for Heavy-Mineral Sands in Coastal Environments, by Bradley S. Van Gosen, David L. Fey, Anjana K. Shah, Philip L. Verplanck, and Todd M. Hoefen, Mineral Deposit Models for Resource Assessment, Scientific Investigations Report 2010–5070–L, United States Geological Survey, 2014. [4] Mineville, Eastern Adirondacks – Geophysical and Geologic Studies, by Anjana Shah, article on the Mineral Resources Program website of the United States Geological Survey, accessed November 2016.

Zircon sand has a low expansion coefficient and is very stable at high temperatures. It is used as a refractory material in many foundry and casting applications. One of its most common uses is in the production of ceramics.
Zirconium dioxide (zirconia) is produced by heating zircon sand to a high enough temperature to break down the zircon molecule. In powdered form, zirconium dioxide is bright white, highly reflective and thermally stable. It is used as an opacifier, whitening agent, and pigment in glazes and stains used on ceramics and pottery. Yttria-stabilized zirconia is used to manufacture cubic zirconia, fiber optic components, refractory coatings, ceramics, dentures and other dental products.
Zircon serves as the primary ore of zirconium metal. Zirconium is used in a variety of metal products that require a resistance to heat and corrosion. It is used to make high-performance alloys, specialty steel, lamp filaments, explosive primers, computer equipment and many electronics components.

Billion-year-old zircons: These zircon grains were hand-picked from a quartz-albite rock collected in Essex County, New York. This petrographic microscope transmitted light image reveals cracks, inclusions, and age “zones” throughout the grains. The cores and rims of the zircon grain reflect magmatic and tectonic events that occurred within the region about 1-1.15 billion years ago. [4]

Zircon and Radioactive Decay

Many zircon crystals contains trace amounts of uranium and thorium. These radioactive elements were incorporated into the zircon at the time of crystallization. They convert into their decay products at a steady rate. The ratio of parent materials to daughter products can be used to estimate the time of crystallization. Using this method, the oldest mineral grains in the world are zircon crystals found in Australia. They are estimated to be about 4.4 billion years old.
When radioactive elements in zircon crystals or nearby materials decay, radiation is emitted. The zircon crystal can be damaged by this radiation. Some zircon has been so damaged by exposure to this radiation that it no longer retains the clarity and optical properties of an attractive gem material. This is why some zircon is not suitable for use as a gem.

Geology news about Mars planet

NASA's Mars rover Curiosity has begun the steep ascent of an iron-oxide-bearing ridge that's grabbed scientists' attention since before the car-sized rover's 2012 landing.

"We're on the climb now, driving up a route where we can access the layers we've studied from below," said Abigail Fraeman, a Curiosity science-team member at NASA's Jet Propulsion Laboratory in Pasadena, California.

"Vera Rubin Ridge" stands prominently on the northwestern flank of Mount Sharp, resisting erosion better than the less-steep portions of the mountain below and above it. The ridge, also called "Hematite Ridge," was informally named earlier this year in honor of pioneering astrophysicist Vera Rubin.

"As we skirted around the base of the ridge this summer, we had the opportunity to observe the large vertical exposure of rock layers that make up the bottom part of the ridge," said Fraeman, who organized the rover's ridge campaign. "But even though steep cliffs are great for exposing the stratifications, they're not so good for driving up."

The ascent to the top of the ridge from a transition in rock-layer appearance at the bottom of it will gain about 213 feet (65 meters) of elevation -- about 20 stories. The climb requires a series of drives totaling a little more than a third of a mile (570 meters). Before starting this ascent in early September, Curiosity had gained a total of about 980 feet (about 300 meters) in elevation in drives totaling 10.76 miles (17.32 kilometers) from its landing site to the base of the ridge.

Curiosity's telephoto observations of the ridge from just beneath it show finer layering, with extensive bright veins of varying widths cutting through the layers.

"Now we'll have a chance to examine the layers up close as the rover climbs," Fraeman said.

Curiosity Project Scientist Ashwin Vasavada of JPL said, "Using data from orbiters and our own approach imaging, the team has chosen places to pause for more extensive studies on the way up, such as where the rock layers show changes in appearance or composition. But the campaign plan will evolve as we examine the rocks in detail. As always, it's a mix of planning and discovery."

In orbital spectrometer observations, the iron-oxide mineral hematite shows up more strongly at the ridge top than elsewhere on lower Mount Sharp, including locations where Curiosity has already found hematite. Researchers seek to gain better understanding about why the ridge resists erosion, what concentrated its hematite, whether those factors are related, and what the rocks of the ridge can reveal about ancient Martian environmental conditions.

"The team is excited to be exploring Vera Rubin Ridge, as this hematite ridge has been a go-to target for Curiosity ever since Gale Crater was selected as the landing site," said Michael Meyer, lead scientist of NASA's Mars Exploration Program at the agency's Washington headquarters.

During the first year after its landing near the base of Mount Sharp, the Curiosity mission accomplished a major goal by determining that billions of years ago, a Martian lake offered conditions that would have been favorable for microbial life. Curiosity has since traversed through a diversity of environments where both water and wind have left their imprint. Vera Rubin Ridge and layers above it that contain clay and sulfate minerals provide tempting opportunities to learn even more about the history and habitability of ancient Mars.

For more about Curiosity, visit:

https://mars.jpl.nasa.gov/msl

News Media Contact

Guy Webster
Jet Propulsion Laboratory, Pasadena, Calif.
818-354-6278
guy.webster@jpl.nasa.gov 

Laurie Cantillo / Dwayne Brown
NASA Headquarters, Washington
202-358-1077 / 202-358-1726
laura.l.cantillo@nasa.gov / dwayne.c.brown@nasa.gov 

2017-241

North America geology

The geology of North America is a subject of regional geology and covers the North American continent, third-largest in the world. Geologic units and processes are investigated on a large scale to reach a synthesized picture of the geological development of the continent.

The divisions of regional geology are drawn in different ways, but are usually outlined by a common geologic history, geographic vicinity or political boundaries. The regional geology of North America usually encompasses the geographic regions of Alaska, Canada, Greenland, the continental United States, Mexico, Central America, and the Caribbean.The parts of the North American Plate that are not occupied by North American countries are usually not discussed as part of the regional geology. The regions that are not geographically North American but reside on the North American Plate include parts of Siberia (see the Geology of Russia), and Iceland, and Bermuda. A discussion of North American geology can also include other continental plates including the Cocos and Juan de Fuca plates being subducted beneath western North America. A portion of the Pacific Plate underlies Baja California and part of California west of the San Andreas Fault.

North American CratonEdit

Main article: Laurentia

The stable core of the continent is the North American Craton. Much of it was also the core of an earlier supercontinent, Laurentia. The part of the craton where the basement rock is exposed is called the Canadian Shield. Surrounding this is a stable platform where the basement is covered by sediment; and surrounding that are a series of orogenic zones.

Canadian ShieldEdit

Main article: Canadian Shield

On a map showing only metamorphic rocks, the Canadian Shield forms a circular pattern north of the Great Lakes around Hudson Bay.

The Canadian Shield is a large area of Archean through Proterozoic igneous and metamorphic rocks in eastern Canada and north central and northeastern United States.

The earliest part of the shield is metamorphosed Archean rocks, originally volcanic in origin. Numerous terranes were accreted onto this Archean core during the Proterozoic to form the Canadian Shield.The southern Archean province is the Superior Craton, it is formed by the combination of a greenstone-granite and a gneiss terrane.The margins of the Canadian Shield have been covered by sedimentary rocks, such as in Michigan where a series of sediments has filled in the Michigan Basin.The exposed sections are often where glaciers have removed this overlying regolith to reveal the underlying glacially scarred crystalline rock.

Stable platformEdit

The North American craton

The stable platform is an area in which the North American Craton forms a basement and is covered by sediment. This area now forms much of the Interior Plains and the slope of the Appalachians below the mountains proper.This area has been covered by a shallow inland sea, which became the site of deposition for most of the overlying sedimentary rock.^{[citation needed]} The sea receded as the continent rose becoming covered by stream, lake, and wind deposits.Orogenies in the surrounding provinces have had little effect on the craton, making it an epeirogenic region, and, as such, the stable platform is mostly a crystalline basement, covered by sedimentary rocks, interrupted only by occasional domes, such as the Cincinnati Arch, Wisconsin Dome, and Ozark Dome.

Midcontinent rift systemEdit

One billion years ago, the Midcontinent Rift System began to extend along a 2,000 kilometres (1,200 mi) path, across both the Canadian Shield and the Stable Platform. The rift, however, failed, and afterwards crustal movement reversed. A range formed then eroded, forming basins on either side of a horst. These rocks have been buried beneath sediment in many areas, but are exposed in some areas, especially around Lake Superior.

Grenville OrogenEdit

The Grenville Orogen developed during the Proterozoic along eastern and southern margin of the North American Craton.The largest outcrop of Grenville age rocks is an approximately 400 kilometres (250 mi) wide band southeast of the Grenville Front which stretches from the central Labrador coast southwest across southern Quebec and southeastern Ontario to Georgian Bay on Lake Huron. The southeastern boundary of this area is approximately the St. Lawrence River. Rocks of the Grenville outcrop in the Adirondack Mountains of northern New York and throughout the Appalachians. The Llano Uplift of central Texas and the Franklin and Hueco Mountains of west Texas have been correlated with the Grenville as have occurrences in Mexico.^[13]

Appalachian OrogenEdit

Main article: Geology of the Appalachians

Map of Appalachian geological provinces

The fold and thrust belt of the Appalachians is continuously exposed for 2,000 kilometres (1,200 mi) from Pennsylvania to Alabama.^[7] In the south, it extends under the coastal plain, but is covered by Mesozoic sediments.North of this fold and thrust belt, the Acadian Orogen of the middle Devonian is an area where deformation has exposed granite plutons.The center of the range is a pair of provinces running north and south parallel to each other, the eastern Blue Ridge Province and the western Valley and Ridge provinces. These are surrounded by the Appalachian Plateau on the west, and the Piedmont Province to the east.Faulting extends throughout the region and is caused by numerous spatially and temporally varied sources.

Inliers of Late Mesoproterozoic age are present on the west of the core of the Appalachians, and these inliers are associated with the Grenville orogeny.During the Proterozoic terranes were accreted onto the province.During the Taconic orogeny 445 to 435 million years ago, accretion continued, an island arc collided with the North American continent, and mountains were raised. These mountains slowly eroded and deposited sediment into the Catskill delta, stretching from New York to Pennsylvania.

PiedmontEdit

Main article: Geology of the Piedmont

The eastern portion of the orogen is made up of the Piedmont plateau, a 150 to 300 metres (490 to 980 ft) elevation area composed of Paleozoic marine and volcanic sediments deformed into crystalline metamorphic rocks and intruded by granite domes.

During the Proterozoic a series of terranes were accreted onto the North American craton, forming the Piedmont of the central Appalachians. Following the Grenville orogeny, mountains eroded, and the sediments from this erosion were deposited below the mountains. The bedrock of the plateau formed about 470 million years ago during the Taconic orogeny, when a volcanicisland arc collided with the ancestral North American Continent.

Passive MarginEdit

As the Atlantic Ocean opened the Atlantic Coast turned from an active margin into a passive one. Terranes were no longer accreted onto the margin, instead, sediment eroded off the Appalachians began to be deposited on the coast, forming a coastal plain and continental shelf.During the Jurassic and Triassic, marine and other sediment was deposited to form the Atlantic coastline. The sediment has formed a clastic wedge making up most of the coastal plain and continental shelf.

The passive margin of the Gulf of Mexico is a series of sedimentary deposits from upland areas surrounding the margin. The environment of deposition for these sediments has changed, varying spatially and temporally. When the ocean level was high shallow marine deposits occurred; when they were low fluvial and deltaic deposits form the majority of mass. From the Triassic until the early Jurassic, faulting localized as extension faulting and wrench faulting. As the basement subsided, sediment accumulated, during the Mesozoic and Cenozoic, forming the modern wedge, containing salt basins.

The passive margin in eastern Mexico is made up of a series of basins. These basins are mostly igneous or metamorphic rocks covered by sediments, except in the Burgos Basin, where Cenozoic volcanism has occurred.Much of the sediment is from erosion of the thrust belts west of the margin.

The Yucatán Peninsula is a Cretaceous to Oligocene carbonate platform. Uplift started in the Oligocene and lasted till the Pleistocene. Today the platform is exposed and under the influence of karstification.

North American CordilleraEdit

On a map showing only volcanic rocks, the west coast of North America shows a striking continuous north-south structure, the American Cordillera.

The North American Cordillera extends up and down the coast of North America and roughly from the Great Plains westward to the Pacific Ocean, narrowing somewhat from north to south. It includes the Cascades, Sierra Nevada, and Basin and Range province; the Rocky Mountains are sometimes excluded from the cordillera proper, in spite of their tectonic history. The geology of Alaska is typical of that of the cordillera.

A rupture in Rodinia 750 million years ago formed a passive margin in the eastern Pacific Northwest. The breakup of Pangea 200 million years ago began the westward movement of the North American plate, creating an active margin on the western continent. As the continent drifted West, accretion of various terranes onto the west coast occurred.As these accretions occurred, crustal shortening accompanied them during the Sevier orogeny and during the Mesozoic into the early Cenozoic, and was accompanied by faulting.During the Cenozoic, crustal extension began accompanied by magmatism that came to characterize much of the area.

Rocky MountainsEdit

Main article: Geology of the Rocky Mountains

The Rocky Mountains were formed by a series of events, the last of which is the Laramide Orogeny. One of the outstanding features of the Rocky Mountains is the distance of the range from a subducting plate; this has led to the theory that the Laramide Orogeny took place when the Farallon plate subducted at a low angle, causing uplift far from the margin under which the plate subducted.

The lithology of the Rocky Mountains in western Canada includes a thin-skinned fold and thrust belt involving Neoproterozoic through Mississippian series of carbonates, shales, argillites and sandstones.

The Colorado Plateau is a stable region dating back at least 600 million years. As a relative lowland, it had been a site of deposition for sediments eroded from surrounding mountain regions. Then, during the Laramide Orogeny, the entire plateau was uplifted until about six million years ago. Erosion during and following the uplift removed sediment from the plateau. This load removal resulted in isostatic uplift and a second passive rise for the plateau.

Intermontane ProvinceEdit

Main article: Geology of the Basin and Range Province

Cedar Breaks National Monument, Utah.

Between the Rocky Mountains and the coast ranges is an area dominated by extensional forces. The extension of this region has occurred both regionally and locally in events beginning in the Jurassic; however, most extension was localized until the mid Miocene. These local events occurred in the Jurassic, late Cretaceous, and one spanning from the Eocene until the Oligocene. Regional extension occurred during the middle of the Miocene from around 20 million years ago until 10 million years ago.

The Basin and Range Province is a series of linear block fault mountains with adjacent sediment-filled downfaulted valleys, having been caused by crustal extension around 17 million years ago. The valley floors are made up of thick sediment deposits which have eroded off the mountains and filled the valleys, so that the region is a regular series of ridges spaced out by flat sediment valleys.

CoastEdit

Main article: Geology of the Pacific Coast Ranges

On the West coast of North America, the coast ranges and the coastal plain form the margin, which is partially bounded by the San Andreas Fault, a transform boundary of the Pacific Plate. Most of the land is made of terranes that have been accreted onto the margin. In the north, the insular belt is an accreted terrane, forming the margin. This belt extends from the Wrangellia Terrane in Alaska to the Chilliwack group of Canada.

The timing of the accretion of the insular belt is uncertain, although the closure did not occur until at least 115 million years ago.Other Mesozoic terranes that accreted onto the continent include the Klamath Mountains, the Sierra Nevada, and the Guerrero super-terrane of western Mexico.80 to 90 million years ago the subducting Farallon plate split and formed the Kula Plate to the . Many of the major batholiths date from the late Cretaceous.As the Laramide Orogeny ended around 48 million years ago, the accretion of the Siletzia terrane began in the Pacific Northwest. This began the volcanic activity in the Cascadia subduction zone, forming the modern Cascade Range, and lasted into the Miocene. As extension in the Basin and Range Province slowed by a change in North American Plate movement circa 7 to 8 Million years ago, rifting began on the Gulf of California.

Southern CordilleraEdit

The Sierra Madre mountain ranges of Mexico are separated by the Mexican Plateau, and transected by the Trans-Mexican Volcanic Belt. The Southern extent of the American Cordillera makes up Western Mexico and northern Central America.This includes the Sierra Madre Occidental, the Sierra Madre del Sur, and the Trans-Mexican Volcanic Belt.

The Cordillera ends in the south in a belt of miogeoclines, including the Sierra Madre Oriental fold and thrust belt, the Mesa Central, and parts of the Sierra Madre del Sur. This belt also extends into Guatemala and Honduras in Central America.