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56-733: Stratigraphy of the Catoctin Formation is closely related to the Harpers Formation , Weverton Formation , and the Loudon Formation. The Harpers Formation is part of the Chilhowee Group . This formation is found above the Weverton Formation and is around 2000–2750 feet in thickness. This formation contains gray phyllite and slate which can be found with banded quartz throughout the rock. It varies in thickness throughout

112-399: A polarizing microscope . The extinction angle is an optical characteristic and varies with the albite fraction (%Ab). The intermediate members of the plagioclase group are very similar to each other and normally cannot be distinguished except by their optical properties. The specific gravity in each member (albite 2.62) increases 0.02 per 10% increase in anorthite (2.75). Plagioclase is

168-420: A bigger matrix, made up of sericite and chlorite , compared to arkosic rocks, making up 20-50% of the rock. The graywackes contain quartz and feldspar in angular to sub-rounded sediments. The bottom of the Catoctin Formation in other places shows a much larger accumulation of sediments such as the ones found near the head of Hawksbill Creek where the rocks are around 100 feet thick. Sedimentary rocks found here

224-524: A crushed grain mount can be obtained by the Tsuboi method, which yields an accurate measurement of the minimum refractive index that in turn gives an accurate composition. In thin section , the composition can be determined by either the Michel Lévy or Carlsbad-albite methods. The former relies on accurate measure of minimum index of refraction, while the latter relies on measuring the extinction angle under

280-521: A dip between 65° East and 65° West. Purple volcanic slate can be found as a thin layer of purple, red, or brown slate that separates the Chilhowee Group and the Catoctin Formation. The slate has an average thickness of 50 to 100 feet and rests on top of the greenstone which suggests that it is younger than the greenstone, although this is debated. When placed under a microscope the rock shows original plagioclase that has been replaced with sericite and some darker opaque material. Also found throughout

336-400: A more calcium-rich rim on a more sodium-rich core. Plagioclase also sometimes shows oscillatory zoning, with the zones fluctuating between sodium-rich and calcium-rich compositions, though this is usually superimposed on an overall normal zoning trend. Plagioclase is very important for the classification of crystalline igneous rocks. Generally, the more silica is present in the rock, the fewer

392-535: A plagioclase feldspar is typically denoted by its overall fraction of anorthite (%An) or albite (%Ab). There are several named plagioclase feldspars that fall between albite and anorthite in the series. The following table shows their compositions in terms of constituent anorthite and albite percentages. The distinction between these minerals cannot easily be made in the field . The composition can be roughly determined by specific gravity, but accurate measurement requires chemical or optical tests. The composition in

448-645: 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 properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796–1872) in 1826. The series ranges from albite to anorthite endmembers (with respective compositions NaAlSi 3 O 8 to CaAl 2 Si 2 O 8 ), where sodium and calcium atoms can substitute for each other in

504-477: A silica content of 60.7 wt%; and 1,275 °C (2,327 °F) in dacite with a silica content of 69.9 wt%. These values are for dry magma. The liquidus is greatly lowered by the addition of water, and much more for plagioclase than for mafic minerals. The eutectic (minimum melting mixture) for a mixture of anorthite and diopside shifts from 40 wt% anorthite to 78 wt% anorthite as the water vapor pressure goes from 1 bar to 10 kbar. The presence of water also shifts

560-506: A specific stratigraphic formation in West Virginia is a stub . You can help Misplaced Pages by expanding it . This article related to the Cambrian period is a stub . You can help Misplaced Pages by expanding it . Plagioclase Plagioclase ( / ˈ p l æ dʒ ( i ) ə ˌ k l eɪ s , ˈ p l eɪ dʒ -, - ˌ k l eɪ z / PLAJ -(ee)-ə-klayss, PLAYJ -, -⁠klayz ) is

616-502: A useful estimate of composition if measured accurately. The index of refraction likewise varies smoothly from 1.53 to 1.58, and, if measured carefully, this also gives a useful composition estimate. Plagioclase almost universally shows a characteristic polysynthetic twinning that produces twinning striations on [010]. These striations allow plagioclase to be distinguished from alkali feldspar. Plagioclase often also displays Carlsbad, Baveno, and Manebach Law twinning. The composition of

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672-444: Is 6 to 6.5, and cleavage is perfect on [001] and good on [010], with the cleavage planes meeting at an angle of 93 to 94 degrees. It is from this slightly oblique cleavage angle that plagioclase gets its name, Ancient Greek plágios ( πλάγιος 'oblique') + klásis ( κλάσις 'fracture'). The name was introduced by August Breithaupt in 1847. There is also a poor cleavage on [110] rarely seen in hand samples. The luster

728-590: Is an intrusive rock composed of at least 90% plagioclase. Albite is an end member of both the alkali and plagioclase series. However, it is included in the alkali feldspar fraction of the rock in the QAPF classification. Plagioclase is also common in metamorphic rock. Plagioclase tends to be albite in low-grade metamorphic rock, while oligoclase to andesine are more common in medium- to high-grade metamorphic rock. Metacarbonate rock sometimes contains fairly pure anorthite. Feldspar makes up between 10 and 20 percent of

784-550: Is around 100–400 feet in thickness. Unconformities can be found between the Catoctin Formation and the Laudon Formation as well as between the Catoctin Formation and the basement rock. This formation has metabasalts and metarhyolite as result of metamorphism. The metabasalts can also be seen paired with amygdalar layers and quartz, calcite, and epidote. The metarhyolite is seen with breccia and purple slate. Hornblende - calcite schist and greenstone are found folded and altered in

840-481: Is best seen at the top of flows compared to farther down the flow. Dikes of greenstone as wide as 50 feet can be found cutting across the Catoctin Formation. These dikes of greenstone are made up of pyroxene and plagioclase with chlorite and small amounts of calcite , quartz , and epidote . Many of these dikes found in the formation have columnar jointing , faint cleavage , and are fine grained. Most of these dikes are also less durable to weathering and have

896-451: Is found within this formation. Columnar jointing that was originally perpendicular to the field now lies at an angle above the surface. Most columnar jointing is plunging to the south or south-east with some at an angle of 50° from original orientation. There are very low dipping joints which can give the formation a steep look. These low-dipping joints are associated with flow surfaces and can be used to determine attitude. This relationship

952-406: Is graywackes, phyllites, and argillites . The argillite found in this area is finely laminated and interbedded in the other rock layers, it can be seen going upward from a layer of coarse sandstone to a layer of dark argillite. Many places in the Catoctin Formation contain unknown sediments at the base of the formation. Graywackes and metamorphosed gneiss are seen in the sedimentary rocks at

1008-559: Is known as the alkali feldspar series. Thus, almost all feldspar found on Earth is either plagioclase or alkali feldspar, with the two series overlapping for pure albite. When a plagioclase composition is described by its anorthite mol% (such as An40 in the previous example) it is assumed that the remainder is albite, with only a minor component of potassium feldspar. Plagioclase of any composition shares many basic physical characteristics, while other characteristics vary smoothly with composition. The Mohs hardness of all plagioclase species

1064-452: Is known as the plagioclase series. The composition of a particular sample of plagioclase is customarily expressed as the mol% of anorthite in the sample. For example, plagioclase that is 40 mol% anorthite would be described as An40 plagioclase. The ability of albite and anorthite to form solid solutions in any proportions at elevated temperature reflects the ease with which calcium and aluminium can substitute for sodium and silicon in

1120-500: Is not original lava . Altered pyroxene in rocks indicates granular crystals or basaltic glass texture found filling the spaces between plagioclase . The Catoctin Formation has porphyritic flows and sedimentary members that can be found north of Rose River and have a slight dip direction to the south-east. To the west part of the Catoctin Formation is an eroded surface can be found that has up to 150 feet of sedimentary rocks at

1176-428: Is usually white to greyish-white in color, with a slight tendency for more calcium-rich samples to be darker. Impurities can infrequently tint the mineral greenish, yellowish, or flesh-red. Ferric iron (Fe ) gives a pale yellow color in plagioclase feldspar from Lake County, Oregon . The specific gravity increases smoothly with calcium content, from 2.62 for pure albite to 2.76 for pure anorthite, and this can provide

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1232-566: Is vitreous to pearly and the diaphaneity is transparent to translucent. The tenacity is brittle, and the fracture is uneven or conchoidal, but the fracture is rarely observed due to the strong tendency of the mineral to cleave instead. At low temperature, the crystal structure belongs to the triclinic system , space group P 1 Well-formed crystals are rare and are most commonly sodic in composition. Well-shaped samples are instead typically cleavage fragments. Well-formed crystals are typically bladed or tabular parallel to [010]. Plagioclase

1288-402: Is well preserved in the Catoctin Formation but most of it shows harsh columnar jointing. The well-preserved columnar jointing up to 20 feet tall and 1 foot in diameter. The harsh columnar jointing is shorter in height and can have a diameter of 2 to 3 feet. Most columnar jointing is parallel but some have a curved or random orientation. Deformation can also be seen in the columnar jointing that

1344-478: The Earth's crust and the upper mantle , is thought to be the depth where feldspar disappears from the rock. While plagioclase is the most important aluminium-bearing mineral in the crust, it breaks down at the high pressure of the upper mantle, with the aluminium tending to be incorporated into clinopyroxene as Tschermak's molecule ( CaAl 2 SiO 6 ) or in jadeite NaAlSi 2 O 6 . At still higher pressure,

1400-663: The Earth's crust . Part of the feldspar family of minerals, it is abundant in igneous and metamorphic rock , and it is also common as a detrital mineral in sedimentary rock . It is not a single mineral , but is a solid solution of two end members , albite or sodium feldspar ( NaAlSi 3 O 8 ) and anorthite or calcium feldspar ( CaAl 2 Si 2 O 8 ). These can be present in plagioclase in any proportion from pure anorthite to pure albite. The composition of plagioclase can thus be written as Na 1−x Ca x Al 1+x Si 3−x O 8 where x ranges from 0 for pure albite to 1 for pure anorthite. This solid solution series

1456-559: The Catoctin Formation Fine-grained greenstone is found throughout the Catoctin Formation and commonly has lineation in it. The greenstone also has amygdules that are elongated parallel to cleavage planes and thinner perpendicular to cleavage planes. Places with lots of amygdules are inferred to be old flow tops. Lots of minerals that are found in the matrix of Catoctin rocks show evidence of shearing , where greenstones don't show any evidence of shearing indicates that it

1512-464: The Catoctin Formation and is around 150–450 feet in thickness. This formation contains iron oxide , dark, phyllite along thin beds of arkosic quartzite and layers of conglomerate rock. Also contains layers of the matrix that contains blue and green slate as well as a conglomerate with quartz pebbles. The Catoctin Formation found under the Chilhowee Group and above the basement rock(1.2-1.0 Ga) and

1568-621: The Catoctin Formation continuous to happen as it is exposed to different forms of weathering . Sediments found at the base of the formation are poorly sorted rocks that contain arkoses , conglomerates , and graywackes . Arkosic sediments and phyllites are where the sediment is thinner, conglomerates and graywackes are usually in thicker areas. Arkosic sediments have angular to sub-rounded grains that have very little matrix made up of epidote in between grains of quartz and feldspar . Conglomerates contain small amounts of greenstone and phyllite with pebbles of cobbles of quartz. Graywackes have

1624-524: The Catoctin Formation. Large greenstone dikes can be found all through the formation particularly at the Big Meadows area where a 1,800-foot section of greenstone is exposed. Throughout the areas, cleavage strikes to the north and dips to the east and lineation that plunges east of southeast. Four major deformation events took place after the emplacement of the formation in the Precambrian. The burial of

1680-529: The Catoctin Formation. The Basement Rock lays under all of the other rock formations and has an unconformity between it and the Catoctin Formation. The basement rock contains granite , anorthosite , quartz monzonite , syenite , and para-gneiss . Many different types of features can be found throughout the Catoctin Formation. These features include columnar jointing , low-dipping joints, amygdules , sedimentary dikes , flow breccias , dikes of greenstone, and purple volcanic slate . Some columnar jointing

1736-518: The aluminium is incorporated into garnet . At very high temperatures, plagioclase forms a solid solution with potassium feldspar, but this becomes highly unstable on cooling. The plagioclase separates from the potassium feldspar, a process called exsolution . The resulting rock, in which fine streaks of plagioclase ( lamellae ) are present in potassium feldspar, is called perthite . The solid solution between anorthite and albite remains stable to lower temperatures, but ultimately becomes unstable as

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1792-509: The area and has folds such as anticlines that cause the formation to repeat on itself. Lots of cleavages can also be found throughout which causes the beading to break apart easily. The Weverton Formation is part of the Chilhowee Group. This formation is found below the Harpers Formation and above the Loudon Formation and is around 1250 feet in thickness. This formation has lots of beds and layers of different types of rock throughout

1848-503: The base of the formation, to the east of the formation is a steep fault cuts off the formation. Two primary faults can be found from Hawksbill Gap to Cedar Run and through Franklin Cliffs to Rose River. Near Tanners Ridge, a gradual anticline can be seen in the Catoctin Formation. The top of Chapman Mountain contains sedimentary rocks that belong to the Chilhowee Group, just below those rocks are 50–150 feet of volcanic slate that belongs to

1904-527: The base of the formation. These graywackes and gneiss contain angular grains are quartz and feldspar can be found in the gneiss with a matrix of chlorite and sericite. Farther below the metamorphosed gneiss contains more large quartz and feldspar with little matrix. Harpers Formation The Harpers Formation is a geologic formation in Maryland , Pennsylvania , Virginia , and West Virginia , consisting of schist , phyllite , and shale . It dates back to

1960-401: The beautiful play of colors known as chatoyance . In addition to its importance to geologists in classifying igneous rocks, plagioclase finds practical use as construction aggregate , as dimension stone , and in powdered form as a filler in paint, plastics, and rubber. Sodium-rich plagioclase finds use in the manufacture of glass and ceramics. Anorthosite could someday be important as

2016-481: The composition of the crystallizing plagioclase towards anorthite. The eutectic for this wet mixture drops to about 1,010 °C (1,850 °F). Crystallizing plagioclase is always richer in anorthite than the melt from which it crystallizes. This plagioclase effect causes the residual melt to be enriched in sodium and silicon and depleted in aluminium and calcium. However, the simultaneous crystallization of mafic minerals not containing aluminium can partially offset

2072-401: The composition with which plagioclase crystallizes also depends on the other components of the melt, so it is not by itself a reliable thermometer. The liquidus of plagioclase (the temperature at which the plagioclase first begins to crystallize) is about 1,215 °C (2,219 °F) for olivine basalt , with a composition of 50.5 wt% silica; 1,255 °C (2,291 °F) in andesite with

2128-565: The depletion in aluminium. In volcanic rock, the crystallized plagioclase incorporates most of the potassium in the melt as a trace element. New plagioclase crystals nucleate only with difficulty, and diffusion is very slow within the solid crystals. As a result, as a magma cools, increasingly sodium-rich plagioclase is usually crystallized onto the rims of existing plagioclase crystals, which retain their more calcium-rich cores. This results in compositional zoning of plagioclase in igneous rocks. In rare cases, plagioclase shows reverse zoning, with

2184-588: The early Cambrian period . It is considered part of the Chilhowee Group . The type section is in gorges of the Potomac River and the Shenandoah River at Harpers Ferry, West Virginia . The Harpers Formation overlies the Ledger Formation ( dolomite ) due to a thrust fault in small roadside quarry (currently overgrown) on Pottery Hill, southwest of York, Pennsylvania . This article about

2240-413: The epidote-amygdaloid having some slightly round pieces and the mud-lump having sub-angular pieces. Sedimentary dikes are found in the Catoctin Formation that interacts with the greenstone in a complex way. The sedimentary dikes are found to be about 3 inches in width and have sediment that is the result of overlying the lava flow . Other sedimentary dikes can be found at the base of flows that lead into

2296-462: The formation occurred between 550 million years ago to 430 million years ago. Neoacadian Orogeny at 375 Ma occurred which caused deformation and metamorphism to the Catoctin Formation. Around 300 million years ago, after the cooling of the Catoctin Formation, a thrust fault cut across the formation. The last big deformation event came from Atlantic rifting that lead to fracturing within the Catoctin Formation around 200 million years ago. Erosion of

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2352-466: The formation. Different colors, such as gray and purple, and textures, such as vitreous and granular, of quartzite , can be found. Cross-bedded in the quartzite layers is conglomerate layers. White quartzite layers and shale beds between those layers can be found in many different layers. The Loudon Formation is part of the Chilhowee Group. This formation is found below the Weverton formation and above

2408-576: The forms of veins and masses. Chlorite will obscure these veins in places where foliation and deformation have happened. Catoctin hydrothermal alteration throughout the formation tends to be fine-grained but there are some places where clasts of feldspar have grown large and taken over most of the rock. Amygdules in the formation are usually elliptical and contain quartz, feldspar, and epidote mainly but also can contain hematite , chlorite , chalcopyrite , malachite , and zeolite . Hand specimens have found epidote and chlorite, with epidote appearing

2464-419: The framework grains in typical sandstones . Alkali feldspar is usually more abundant than plagioclase in sandstone because Alkali feldspars are more resistant to chemical weathering and more stable, but sandstone derived from volcanic rock contains more plagioclase. Plagioclase weathers relatively rapidly to clay minerals such as smectite . The Mohorovičić discontinuity , which defines the boundary between

2520-430: The greenstone through a series of veins . These dikes are the result of lava flow over wet sediments, the steam pressure then forced the sediments up through cracks resulting in the modern-day dikes. Greenstone that is closer to the sedimentary dike is darker and has finer grains, greenstone that is further away is lighter and has coarser grains. The greenstone that is darker and finer is where glass started forming due to

2576-422: The high temperatures from lava that came in contact with the sediments. Amygdules found in the greenstone of the Catoctin Formation are filled with quartz , epidote , albite , calcite , and chlorite . These minerals can occur by themselves or with other minerals. Epidote, quartz, and albite will usually occur together within lots of amygdules, chlorite and albite will also form together with albite forming on

2632-478: The highlands of the Moon . Analysis of thermal emission spectra from the surface of Mars suggests that plagioclase is the most abundant mineral in the crust of Mars. Its name comes from Ancient Greek πλάγιος ( plágios )  'oblique' and κλάσις ( klásis )  'fracture', in reference to its two cleavage angles. Plagioclase is the most common and abundant mineral group in

2688-531: The mafic minerals, and the more sodium-rich the plagioclase. Alkali feldspar appears as the silica content becomes high. Under the QAPF classification , plagioclase is one of the three key minerals, along with quartz and alkali feldspar, used to make the initial classification of the rock type. Low-silica igneous rocks are further divided into dioritic rocks having sodium-rich plagioclase (An<50) and gabbroic rocks having calcium-rich plagioclase (An>50). Anorthosite

2744-413: The mineral's crystal lattice structure. Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or " record -groove" effect. Plagioclase is a major constituent mineral in Earth's crust and is consequently an important diagnostic tool in petrology for identifying the composition, origin and evolution of igneous rocks . Plagioclase is also a major constituent of rock in

2800-402: The most in rocks. Catoctin rock has shown quartz, andesine , and orthoclase to be primary minerals. All of the feldspars have undergone alteration, the orthoclase forms tiny anhedral grains, and quartz forms small grains that are associated with secondary grains. Small grains of hornblende and pyroxene are also found where the pyroxene is associated with biotite . Columnar jointing in

2856-399: The plagioclase crystal structure. Although a calcium ion has a charge of +2, versus +1 for a sodium ion, the two ions have very nearly the same effective radius. The difference in charge is accommodated by the coupled substitution of aluminium (charge +3) for silicon (charge +4), both of which can occupy tetrahedral sites (surrounded by four oxygen ions). This contrasts with potassium, which has

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2912-438: The primary aluminium-bearing mineral in mafic rocks formed at low pressure. It is normally the first and most abundant feldspar to crystallize from a cooling primitive magma . Anorthite has a much higher melting point than albite, and, as a result, calcium-rich plagioclase is the first to crystallize. The plagioclase becomes more enriched in sodium as the temperature drops, forming Bowen's continuous reaction series . However,

2968-424: The rim and chlorite in the center. Amygdules are usually found in areas that are rich with other amygdules. They have an oval shape that has more round upper surfaces and longer bottoms parallel to jointing. The Catoctin Formation is mostly fine grained and has a massive structure in most places except where it is slaty or mylonitic . Throughout the formation, there is quartz , feldspar , and epidote that come in

3024-411: The rock approaches ambient surface temperatures. The resulting exsolution results in very fine lamellar and other intergrowths, normally detected only by sophisticated means. However, exsolution in the andesine to labradorite compositional range sometimes produces lamellae with thicknesses comparable to the wavelength of visible light. This acts like a diffraction grating , causing the labradorite to show

3080-421: The rock are spots of green chlorite and crystals of sphene which are common in the rock. Two common breccias can be found in the Catoctin Formation, one is epidote -amygdaloid breccia and the other is mud-lump breccia. The epidote-amygdaloid breccia pieces of are in a matrix of quartz and epidote . Mud-lump breccia has pieces of paleosol with a quartz filling. Both breccias contain angular fragments, with

3136-429: The same charge as sodium, but is a significantly larger ion. As a result of the size and charge difference between potassium and calcium, there is a very wide miscibility gap between anorthite and potassium feldspar , ( KAlSi 3 O 8 ), the third common rock-forming feldspar end member. Potassium feldspar does form a solid solution series with albite , due to the identical charges of sodium and potassium ions, which

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