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Plagioclase feldspar chemical formula

Virginia Geological Survey - Feldspar

Plagioclase is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na (AlSi 3 O 8), to pure anorthite, Ca (Al 2 Si 2 O 8). Minerals in this series are a homogenous mixture of albite and anorthite Ca2+ can substitute for Na+ because they are similar in size and charge neutrality is maintained by substituting Al3+ for Si4+. Composition of Plagioclase is described in terms of Anorthite with An + Ab = 100. The solid solution equilibrium equation can be written as Na +1 Si +4 <=> Ca +2 Al +3 (Nesse 2012)

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 properly known as the plagioclase feldspar series. This was first shown by the German mineralogist Johann Friedrich Christian Hessel (1796-1872) in 1826 Plagioclase Feldspar. Plagioclase is series of framework silicate minerals in feldspar group. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series, rather than a specific mineral with a particular chemical composition. The series ranges from albite to anorthite endmembers (with respective compositions. The Plagioclase series is a group of related feldspar minerals that essentially have the same formula but vary in their percentage of sodium and calcium. Albite and Anorthite are the end members of the series, with the intermediary minerals Oligoclase, Andesine, Labradorite, and Bytownite. The acclaimed Dana's System of Mineralogy lists the intermediary members as individual minerals, whereas.

Anorthite - Wikipedia

The chemical composition of feldspars includes alcalic metals, aluminum, silica and oxygen. The general chemical formula is a bit complicated (K,Na) 1-x (Ca,Ba) x (Si 3-x Al 1+x O 8). This complexity is caused by presence of both monovalent and divalent cations which require more complicated heterovalent substitution Chemical Formula. (Na,Ca)Al1-2Si3-2O8. Composition. Sodium calcium aluminum silicate. The ratio of sodium to calcium is about 9:1. The amount of aluminum atoms are between 1 and 2, and the amount of silicon atoms are between 3 and 2. In the Plagioclase Feldspar series, Oligoclase contains between 70 and 90 percent Albite (Ab), and between 10.

„Plagioclase Feldspar CaAl2Si2O8 - NaAlSi3O8 „Jadeite - Diopside NaAlSi2O6 - CaMgSi2O6 „Consider a feldspar composition with 40% Anorthite and 60% Albite. Calculate Weight percent Oxides. „40% Anorthite and 60% Albite. „1st step: write the formula The major third component for alkali feldspars is An. The major third component for plagioclase feldspars is Or. Thus, to show graphically the variation in chemical composition of the feldspars, a ternary composition diagram is needed with two independent composition axes and three end member components: Or, Ab, and An (see at-tached diagram) Sodium feldspar chemical formula or albite formula- NaAlSi₃O₈. Calcium feldspar chemical formula- CaAl₂Si₂O₈. Bowen's Reaction Series. Solid solutions between K-feldspar and albite are called alkali feldspar. Solid solutions between albite and anorthite are called plagioclase, or, more properly, plagioclase feldspar

The plagioclase feldspars form a solid solution series between the end members of pure albite (NaAlSi 3 O 8) and pure anorthite (CaAl 2 Si 2 O 8). The alkali feldspars form a solid solution series between pure albite and potassium sanidine (KAlSi 3 O 8). A list of feldspar minerals with their chemical compositions can be seen in Table 1 Ideally, chemical composition of the plagioclase group varies from NaAlSi 3 O 8 (albite) to CaAl 2 Si 2 O 8 (anorthite); however, some potassium is usually present, especially in soda-rich plagioclase. Specific names have been applied to different parts of the compositional range

This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite. In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning Alkali feldspar has the formula KAlSi 3 O 8, potassium aluminosilicate. The formula actually is a blend ranging from all sodium (albite) to all potassium (microcline), but albite is also one endpoint in the plagioclase series so we classify it there Feldspars are a group of rock-forming aluminium tectosilicate minerals, containing sodium, calcium, potassium or barium. The most common members of the feldspar group are the plagioclase (sodium-calcium) feldspars and the alkali (potassium-sodium) feldspars. Feldspars make up about 60% of the Earth's crust, and 41% of the Earth's continental crust by weight What is the chemical formula of feldspar? Chemical composition. All the rock-forming feldspars are aluminosilicate minerals with the general formula AT4O8 in which A = potassium, sodium, or calcium (Ca); and T = silicon (Si) and aluminum (Al), with a Si:Al ratio ranging from 3:1 to 1:1. Click to see full answer

Plagioclase Feldspar: A group of common rock-forming mineral

Feldspar - Feldspar - Chemical composition: All the rock-forming feldspars are aluminosilicate minerals with the general formula AT4O8 in which A = potassium, sodium, or calcium (Ca); and T = silicon (Si) and aluminum (Al), with a Si:Al ratio ranging from 3:1 to 1:1. Microcline and orthoclase are potassium feldspars (KAlSi3O8), usually designated Or in discussions involving their end-member. Plagioclase Feldspar. (001) is a perfect cleavage, and (010) is also quite good. (110) is a very poor cleavage and is not usually seen in thin section. Cleavages intersect at about 93-94o angles. Conchoidal to uneven fracture. Brittle. (Nesse 2012) 1. Color: This mineral is colorless in thin section Feldspar - Feldspar - Identification of specific feldspars: Alkali feldspars can often be distinguished from plagioclase feldspars because most grains of the latter exhibit albite twinning (see above Crystal structure), which is manifested by parallel lines on certain cleavage surfaces, whereas grains of alkali feldspars do not. This criterion is not, however, absolute; some plagioclase. Plagioclase is an important mineral in the feldspar group, and the chemical formula of this mineral is NaAlSi 3 O 8 - CaAl 2 Si 2 O 8. It is also a tectosilicate mineral. The crystal system in this material is triclinic. It appears in white to grey colour

LABRADORITE PHYSICAL PROPERTIES. Labradorite Chemical composition: the composition of the intermediate members of the plagioclase solid solution series are expressed in terms of the two pure end members of this series, albite (NaAlSi 3 O 8) and anorthite (CaAl 2 Si 2 O 8).Using this methodology, the composition of labradorite is thus expressed as Ab 50-30% An 50-70% Feldspar Chemical composition : Potassium, sodium and calcium-aluminum silicates. I. K-feldspar (KALSi 3 0 8) II . Plagioclase feldspar (isomorphous) Crystal system : Monoclinic - triclinic Habit : Prismatic, often twinned Cleavage : Good to perfect Fracture : Conchoidal Hardness : 6 Optic nature : Biaxial ± Refractive index : 1.52 - 1.53 (K. Feldspar. Feldspar is the name given to a group of minerals distinguished by the presence of alumina and silica (SiO 2) in their chemistry. This group includes aluminum silicates of soda, potassium, or lime. It is the single most abundant mineral group on Earth

Chemical Composition - Plagioclase Feldspa

Properties of Feldspar Chemical Properties ┗ Each type of feldspar has a different chemical formula, depending on its composition. The specific chemical formula for this group of compounds is KAlSi 3 O 8 - NaAlSi 3 O 8 - CaAl 2 Si 2 O 8. ┗ These minerals are chemically inert, and have a stable value of pH Plagioclase is typically white, yellow, or reddish-gray, but it can also be blue to black. It occurs in igneous rocks. The minerals albite, oligoclase, andesine, labradorite, bytownite, and anorthite are all plagioclases. Chemical formula: (Na,Ca)Al (Si,Al)Si2O8 Chemical Composition. aluminosilicates of K, Na, Ca Al substitutes for Si 2 main groups can be represented on triangle plagioclase feldspars - Ca-Na alkali feldspars - Na-K in common usage, the terms alkali feldspar and K- feldspar are used interchangeably. Plagioclase Feldspar Physical Color General Plagioclase Information. Chemical Formula: (Na,Ca) (Si,Al)4O8. Composition: Molecular Weight = 270.77 gm. Sodium 4.25 % Na 5.72 % Na 2 O. Calcium 7.40 % Ca 10.36 % CaO. Aluminum 9.96 % Al 18.83 % Al 2 O 3. Silicon 31.12 % Si 66.57 % SiO 2

Plagioclases are triclinic feldspars that form complete isomorphic compounds that are the final members of the Na-plagioclase albite NaAlSi3 O 8 (Ab) and Ca-plagioclase anortite CaAl 2 Si 2 O 8 (An) ( Table 3.13 ). In albite one of four Si 4+ ions is isomorphic substituted with one Al 3+ -ions, and one free (−) valence is related to Na + -ion General Properties Chemical formula: NaAlSi3O8 - CaAl2Si2O8; Triclinic H = 6-6.5; G = 2.60-2.76. Luster: vitreous, may be pearly. Color: white and gray; Streak: white. Color in thin section: colorless. Pleochroism: none. Cleavage: basal {001} cleavage is perfect and {010} cleavage is good; they intersect at an approximately right angle. Optical Properties Interference figure: biaxial (+ or. Type of feldspar (framework silicate) that ranges from sodium to calcium rich. The chemical formula is (Na,Ca)Al(Si,Al)Si 2 O 8. The lower photographs shows two distinctive features of some plagioclase feldspars. 1. Irridescence is the beautiful peacock coloured reflections from cleavage surfaces. 2 33 components with composition can be rationalized on the basis of the evolution of crystal 34 structures and chemistry across this solid-solution join. All plagioclase feldspars have high 35 elastic anisotropy; a* (the direction perpendicular to the b-and c-axes) is the softest direction by 36 a factor of 3 in albite

Plagioclase ranges in composition from Na[AlSi 3 O 8] to calcium Ca[Al 2 Si 2 O 8], or sodium to calcium aluminosilicate. Pure Na[AlSi 3 O 8] is albite, and pure Ca[Al 2 Si 2 O 8] is anorthite. The plagioclase feldspars are named according to the following scheme, where the numbers are the percentage of calcium expressed as anorthite (An) The quartz and feldspars account for 2.3-22.7 wt% of the samples. X-ray diffraction analyses detect both plagioclase feldspar and potassium feldspar. Plagioclase is albite-rich and abundant in the top ~ 7.4 m of the core. Potassium feldspar mainly occurs below ~ 10.4 m Retrieval of standard chemical formulas for minerals from chemical analyses is an exercise in (a) conversion of units of quantity and (b) normalization of sums to match the commonly used formula conventions. The basic steps for formula calculation are: Divide the weight percentage of each oxide by the formula weight of that oxide

Plagioclase - Wikipedi

  1. g tectosilicate
  2. eral name c) the petrologic name for a feldspar of this composition; Hint: Klein, p. 478 (22nd) /471 (23rd) 2) From the chemical analyses of feldspars.
  3. erals have a precise chemical formula. Examples include: quartz, SiO 2. calcite, CaCO 3. Other
  4. The feldspar group has nearly 20 recognized members, but only nine of those are common. These may be divided into two types of feldspar: plagioclase and alkali. Plagioclase feldspar: Each plagioclase grain is made up of stacks of thin crystal with their molecules arranged in opposite directions

Plagioclase Feldspar Physical-Optical Properties

Feldspar Mineral10(d) Composition of Rocks

Plagioclase: The mineral Plagioclase information and picture

Alkali feldspars rank with quartz and plagioclase as the most common minerals in the earth's crust. Chemically they are represented by the formula (K,Na)AlSi 3 O 8.The chemical end member KAlSi 3 O 8 (Or) includes the minerals orthoclase, microcline, and K-rich sanidine; and the end member NaAlSi 3 O 8 (Ab) is the mineral albite.In most natural specimens, there is also a small amount of the. Alkali feldspars have a range of chemical composition and the end members may be written as KAlSi 3 O 8 and NaAlSi 3 O 8. The K-feldspar polymorphs, sanidine, orthoclase, microcline, and adularia have identical chemical compositions. Sanidine is a monoclinic alkali feldspar with small optic axial angle (2V) and commonly occurs in volcanic rocks Sunstone is a plagioclase feldspar that has red, more rarely green or blue glitter which is caused by light reflections from tiny hematite or goethite platelets. The name derives from sunstones believed to have been used for navigation in the Middle Ages. Sunstone is the official gemstone of Oregon, USA. 92 photos

Feldspars - all about the plagioclases and K-feldspar

  1. A plagioclase feldspar with an albite : anorthite molar ratio ranging from 10 : 90 to 30 : 70. May show l abradorescence (compare labradorite ). The original material, a greenish-white substance, was found in a boulder near Bytown (now Ottawa), and later..
  2. SODIUM-PLAGIOCLASE. Chemical Formula: NaAlSi 3 O 8; Mineral Group: non-ferromagnesian silicate; Luster/Color: non-metallic, glassy/ white to light gray; Cleavage: 2 directions at 90 o; Hardness: 6 to 6.5; Other Characteristics: striations, light color POTASSIUM-FELDSPAR. Chemical Formula: KAlSi 3 O 8; Mineral Group: non-ferromagnesian silicat
  3. Petrologists distinguish the position of a plagioclase along this series using two numbers- %Ab (percent albite) and %An (percent anorthite.) Plagioclases are triclinic and have a Moh's hardness of 6 to 6 ½. The pure sodium feldspar, albite has the formula NaAlSi3O8 and the pure calcium endmember, anorthite has the formula CaAl2Si2O2
  4. erals that includes Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine and Sunstone. The Plagioclase Feldspars form a string between Albite and Anorthite. Albite is the sodium-rich end member while Anorthite may be the calcium.
  5. Plagioclase Feldspar: NM; 6 Hardness; Prismatic cleavage (angles about 90 degrees); dark gray with some green; no streak; looks like oil on water in places (oil marks); Chemical Formula: NaAlSi3O8 Muscovite Mica: NM; less than 5.5 hardness; Basal Cleavage; translucent copper clear sheets; light gray streak; breaks off into thin translucent.

Oligoclase feldspar: The mineral Oligoclase information

Plagioclase feldspar: Chemical Composition (Na,Ca)Al 1-2 Si 3-2 O 8: Color: Red, green, yellow, orange, pink, multicolored: Hardness: 6 to 6-1/2 (Mohs) Specific Gravity: 2.69 - 2.70: Refractive Index: 1.560 - 1.568: Proper Care of Andesine Avoid exposing andesine to extreme or prolonged heat. For this reason, boiling and cleaning methods are. Description: Dark colored rocks with very little industrial use. Can not be distinguished with out laboratory analysis Sample 6: Name: Plagioclase feldspar Chemical Composition: NaAlSi3O8 (Albite) to CaAl2Si2O8 (Anorthite) Description: common clast produced during the weathering of igneous and metamorphic rocks. Lighter colored usually but can. Feldspars vary slightly in composition and are classified into two primary branches based on their chemical makeup: Potassium feldspar and plagioclase feldspar. Potassium feldspar gems include orthoclase and microcline . Orthoclase that exhibits adularescence is known as 'moonstone'. Plagioclase feldspars, which are a mixture of calcium and. Diorite has a strict definition based on the QAPF diagram — more than 90% of feldspar is plagioclase, quartz content is lower than 5% and plagioclase contains more sodium than calcium. The latter distinguishes it from gabbro. Main minerals are plagioclase (oligoclase, andesine) and hornblende The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8). Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements

Orthoclase. Orthoclase is frequently known as potassium feldspar or K-spar, and is easily distinguished from the plagioclase feldspars in hand sample by its lack of striations. In thin section, alkali feldspar grains often look like they're sprinkled with dirt (PPL) or tiny confetti (XPL). You can find the explanation on the alkali feldspar page Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks. Chemical composition - KAlSi 3 O 8. Hardness - 6. Specific gravity - 2.6. Transparency - Translucent to opaque (rarely transparent) Colour - Pinkish white, off-white, yellow, or shades of red, orange to brown. Streak - White

Feldspar Chemical Composition - Formula, Structure and

  1. erals that range in chemical composition from pure NaAlSi3 O8, Albite to pure CaAl2 Si2 O8 , anorthite.Anorthite by definition must contain no more than 10% sodium and no less than 90% calcium in the sodium/calcium position in the crystal structure
  2. The composition of coexisting exsolved phases can be found by drawing an isotherm until it intersects the solvus. Such an isotherm at 590°C shows that at this temperature a solid solution having the composition of point B Or 32 Ab 68 coexists with an alkali feldspar solid solution with the composition of point A Or 70 Ab 30. With further.
  3. Albite is the sodic end member of both the plagioclase feldspar series and the alkali feldspar series.. If you have trouble remembering the name of the white feldspar and you're at all familiar with Harry Potter, think of Albus Dumbledore, whose first name, Albus, is Latin for white.It's helped a surprising number of people keep their feldspars straight
  4. eral of two different feldspar series. It is most often associated with the plagioclase series where it is an end member of this series. The plagioclase series comprises felspars that range in chemical composition from pure NaAlSi 3 O 8 to pure CaAl 2 Si 2 O 8
  5. eral with the chemical formula KAlSi308. It has a triclinic or monoclinic crystal system and a white streak, although the colour of the rock itself can be white, pink, grey, or brown. Its lustre is vitreous or glassy and it displays an uneven or conchoidal fracture pattern
  6. ium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements. The
  7. eral with a chemical composition of KAlSi3O8. It is one of the most abundant rock-for

Feldspar minerals make up over 50% of Earth's crust

  1. erals found in more abundance in the mountains include Iron , Quartz , Calcite , Malachite , Azurite , Feldspar , Diorite , and the Lead ores of Galena and Anglesite
  2. erals, principally alu
  3. Chapter 2 THE NATURE O F FELDSPARS FELDSPARS On beginning this reviev, of feldspars and feldspar identification, we may perhaps best first describe... Download PDF 3MB Sizes 2 Downloads 28 View
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  5. These may be divided into two types of feldspar: plagioclase and alkali. Why is K feldspar pink? K-feldspar albite intergrowths known as pertite and albite intergrowths K-feldspar as antipertite. The hardness is from 6 to 6.5 and the relative density of 2.55-2.63
  6. ed color origin. Mg (A and C) is the key trace element that separates Ethiopian from Oregon sunstone. The unknown Cu-bearing feldspar has a much higher Cu and Sr concentration than other natural untreated sunstones (A, D, E, and F)

Plagioclase feldspar SpringerLin

Plagioclase feldspars are the most common feldspar minerals because calcium is somewhat more common in the crust than potassium (3.6 and 2.8 percent of the crust, respectively). Plagioclase feldspars form a continuous solid solution between Ab and An endmembers at high temperature, but the replacement of ions needs to be coupled because of the. plagioclases. The plagioclase feldspar names were created many years ago; these names are no longer considered mineral species, but are still in widespread use and therefore are used here for convenience. Albite refers to a sodium-rich plagioclase containing up to10% of the calcium component. The composition range between 10% and 30% calcium is. feldspars (the response to hydrostatic pressure) [e.g., Angel, 2004] are not sufficient to fully deter-mine the elastic modulus or compliance tensors. Thus, the only primary source of data for the full elastic properties of plagioclase feldspar crystals as a function of composition is the pioneering 1bar ultrasonic measurements of twinned mega Labradorite feldspar is the intermediate to calcic member of the plagioclase feldspar series, the solid solution between albite (Ab: NaAlSi3O8) and anorthite (An: CaAl2Si2O8). It is defined to be plagioclase feldspar with a composition between An50 (half albite and half anorthite) and An70 (70% anorthite and 30% albite). The name labradorite. As the cooling rate decreases, the texture of large plagioclases changes from skeletal to hollow to nearly equant. In this study, we use X-ray map data to obtain a database of 12 275 quantitative chemical analyses. The frequency of An-rich plagioclases showing disequilibrium compositions substantially increases with increasing cooling rate

parent minerals from which kaolin minerals develop are feldspars and muscovite. The transformation of potassium feldspar into kaolin minerals occurs according to the equation: 2 KAlSi3O8 + 3 H2O Æ Al2Si2O5(OH)4 + 4 SiO2 + 2 K(OH) Solubilities of the several chemical species are pH dependent (Mason, 1952). The pH value The three main feldspar minerals are potassium feldspar, (a.k.a. K-feldspar or K-spar) and two types of plagioclase feldspar: albite (sodium only) and anorthite (calcium only). As is the case for iron and magnesium in olivine, there is a continuous range of compositions (solid solution series) between albite and anorthite in plagioclase Plagioclase feldspar is identified by its non-metallic property, they are vitreous to pearly luster; they are in white to dark gray color; they are two good cleavages at nearly 90o. Plagioclase feldspar has striations on some of their cleavage surfaces. Its hardness is 6 Mohs. They are of specific gravity. That is 2.6 to 2.8 Infrared spectra of plagioclase feldspars have been used for qualitative identification and to study the relation between absorption bands and chemical composition (Milkey, 1960; Thompson and Wadsworth, 1957, and others). These investigations, however, were limited to the intermediate (conventional) infrared region

Oligoclase - Wikipediaø Feldspar | The Gemstone List ø

The potassium feldspars are ORTHOCLASE and MICROCLINE. The plagioclase feldspars form a group within a group that varies in composition from ALBITE (ALL-bite), which is sodium-aluminium silicate, to ANORTHITE (an-OR-thite) which is calcium aluminium silicate Feldspar is one of the most abundant minerals in the earth's crust. The feldspars include a range of compositions with the general chemical formula xAl(Al,Si) 3 O 8 where x can be sodium (Na) and/or calcium (Ca) and/or potassium (K). The sodium and potassium-enriched compositions are known as alkali feldspars, which are primarily used in glass manufacture and ceramics 09.FA.35 Plagioclase* (Na,Ca)(Si,Al)4O8 C 1 1 : Other Anorthite Information: See Also: Links to other databases for Anorthite : 1 - Am. Min. Crystal Structure Database 2 - Amethyst Galleries' Mineral Gallery 3 - Athena 4 - EUROmin Project 5 - Ecole des Mines de Paris 6 - Franklin Minerals(Dunn) 7 - Franklin Minerals(Palache) 8 - GeoScienceWorld. There is no chemical gradient between K-feldspar and Ca-feldspar. The potassium feldspars (K-feldspars) (aka alkali feldspars) are those that range in composition from pure K-feldspar to pure Na-feldspar (actually, feldspars with ~even & random mixes of potassium and sodium are rare). The feldspars with Na and/or Ca are the plagioclase. AMAZONITE FELDSPAR BACKGROUND INFORMATION. In the earth's crust, feldspars are more abundant than all other minerals combined. The majority of the common feldspars fall into two groups, the alkali feldspars (KAlSi 3 O 8 through NaAlSi 3 O 8 series), and the plagioclase feldspars (NaAlSi 3 O 8 through CaAl 2 Si 2 O 8 series). Amazonite (KAlSi 3 O 8) is the rare green variety of microcline.

FELDSPAR

ALEX STREKEISEN-Plagioclase

  1. erals are usually light in color and have specific gravities less than 3.0. Common felsic
  2. eralogy is interesting to study. In terms of its chemical composition, feldspar is an alu
  3. Hovis and others (1) investigated the thermal expansion of natural and synthetic AlSi3 feldspars and demonstrated that the coefficient of thermal expansion (α) decreases significantly, and linearly, with increasing room-temperature volume (VRT). In all such feldspars, therefore, chemical expansion limits thermal expansion. The scope of this work now has been broadened to include plagioclase.
  4. plagioclase olivine quartz pyroxene. The fine grained (aphanatic) equivalent of a granite is a: rhyolite gabbro andesite basalt. Grains are alkali feldspar and quartz. The length of the figure is about 2 cm. The diagram above most likely represents a _____ extrusive igneous rock intrusive igneous rock clastic sedimentary rock chemical.
  5. eral polymorphs, each having the same chemical composition, but slightly different crystal structures. All of the feldspar
  6. erals that features Albite, Amazonite, Andesine, Anorthite, Bytownite, Hyalophane, Labradorite, Moonstone, Oligoclase, Orthoclase, Sanidine, and Sunstone. Bytownite is a bright and gorgeous champagne gem that is colored
  7. composition and tectonic setting (Dickinson and from a petrogenetic perspective. Suczek 1979). The proportions of these constit-uents are, however, substantially modified by pro-Stability of Quartz and Feldspars cesses such as chemical weathering (Suttner et al. 1981; Mack 1984). The ''lability'' of plagioclase rel-Introduction
Amazonite - Extraordinary FeldsparUniversity of Minnesota’s Mineral Pages: Gypsum

Feldspars are a group of rock-forming tectosilicate minerals that make up as much as 60% of the upper crust of the Earth [1, 2]. Within the feldspar family, plagioclase feldspars are a very important subset. These form a chemical series between albite, NaAlSi 3 O 8, and anorthite, CaAl 2 Si 2 O 8, as end members Chemical Composition and Nomenclature. Feldspar is the mineral name given to a group of aluminium silicate minerals whose general chemical formula is XAl(Al,Si)Si2O8, where X is potassium, sodium, calcium or barium. The majority of feldspars are classified chemically into alkali feldspars and plagioclase feldspars c) the mineral name e) the petrologic name for a feldspar of this composition; Hint: Klein, p. 478 (22nd) /471 (23rd) 2) From the chemical analyses of feldspars A and B (below) a) express the composition in terms of the endmember feldspar compositions (e.g., Or,Ab,An, where rtytz-100), b) indicate to which feldspar series each belongs, and c.