Albite, calcite, and pyrite often show polysynthetic twinning. XPL. Alkali feldspars are compositionally between Or and Ab end-members. 2. Typical lamellar albite twins in albite. Twinning laws can be recognized in a thin section with the aid of a universal stage. (For further information see VANDER PLAS, 1959.) Sometimes this twinning is called microcline twinning. The twins may be wedge-shaped (thicker at one end) if the twinning results from deformation. Chessboard albite is a particular type of of alkaline feldspar also common as patch perthite (Starkey 1959). The CP is determined with cleavages or optical elongation at the diagonal position. This twinning is the most characteristic identifying feature of plagioclase, and makes its … If no twinning is mentioned, then the phenomenon is not common in thin section, but this does not mean that it NEVER occurs. This twinning is the most characteristic identifying feature of plagioclase, and makes its … Gridiron twinning 5. Fracture: Uneven. Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Polysynthetic twinning in albite, plane polars By: magic-gigapans Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection License: Creative Commons Non Commercial ⧉ Uploaded: 9 Apr 2020 Last Updated: 21 May 2020 Structure and composition : Albite is an end member of both the plagioclase feldspar and the alkali felspar series. ii. Fish Canyon Tuff . R-470 MVI_4829.MOV. Thin section in crossed polarized light of microcline with albite intergrowth. Invariably they occur in glomeroporphyritic clusters with twinned individuals in parallel, sub-parallel, or random orientation. Left: PPL. Zoning You may sketch one mineral showing two or more features, if you can find a good example. This type of twinning is only observed using the polarizing microscope, and results in a "tartan" twinning pattern as shown in your text book on page 231, figure10.18. Can also display carlsbad twinning, simple twins, or lack twinning altogether. is the complex twin and twinning law is the albite-Carlsbad twin. See more ideas about minerals, geology, metamorphic rocks. Twinning: Carlsbad twinning with (010) composition plane dividing crystal into two segments. To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. When the multiple twins are not parallel they are cyclic twins. Albite is the sodic end member of both the plagioclase feldspar series and the alkali feldspar series. Plagioclase in the Nonewaug granite is in the low structural state. The twin lamellae are usually so thin that they show first-order interference colors. The microcline displays the classic “cross-hatched” twinning, distinguishing it from plagioclase. Twinning is the symmetric intergrowth of two or more crystals of thesame mineral, and commonly is revealed in thin section by the differentcrystals of the same mineral having different extinction positions or differentinterference colours. 4.17 gigapixels 89,603 x 46,575 pixels 298.7 in X 155.3 in at 300dpi 49,283 pixels per inch The slide is 27x46 mm. Plagioclase with carlsbad-albite twins. Euhedral and subhedral crystals with outlines of gypsum morphologies are now anhydrite, polyhalite, halite, and sylvite. The twin law - {010} indicates that the twining occurs perpendicular to the b crystallographic axis. Labradorite Na0:5¡0:3Ca0:5¡0:7Al1:5¡1:7Si2:5¡2:3O8 °c 2001 Mineral Data Publishing, version 1.2 Crystal Data: Triclinic. 11 minerals in thin section university of north carolina 12 minerals in thin sections humboldt state 13 online mineral museum 14 qut mineral atlas 15 ruff info 16 ucla petrography thin sections 17 www mincryst. Comments - show multiple 'albite' twinning; alters to montmorrillonite or kaolinite . 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. By: magic-gigapans Image: Robin Rohrback, Mid-Atlantic Geo-Image Collection License: Creative Commons Non Commercial ⧉ Uploaded: 9 Apr 2020 Last Updated: 21 May 2020. To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. It’s helped a surprising number of people keep their feldspars straight. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . . In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. In thin sections examined, isolated twinned grains are rare or absent. Pericline twinning is quite similar to albite twinning. Amazonite (green microcline) Twinning. Hematite is isotropic, so it's opaque (black) in both PPL and XPL. 24 photos of Albite-Anorthite Series associated with Almandine: Fe 2+ 3 Al 2 (SiO 4) 3: 19 photos of Albite-Anorthite Series associated with Schorl: Na(Fe 2+ 3)Al 6 (Si 6 O 18)(BO 3) 3 (OH) 3 (OH) 16 photos of Albite-Anorthite Series associated with Fluorapatite: Ca 5 (PO 4) 3 F: 12 photos of Albite-Anorthite Series associated with Rutile: TiO 2 Plagioclase with carlsbad-albite twins. Hem: hematite. Microcline at webmineral.com Plane//cross-polarized light, field width is 6 mm. . Color/Pleochroism: Colorless in thin section. You can find the explanation on the plagioclase feldspar page. Twins: Polysynthetic twinning with {01-12} as twin-plane is very common, especially in the calcite of metamorphic limestone. Albite twinning 3. Albite at mindat.org, Polysynthetic twinning in albite, plane polars, Polysynthetic twinning in albite, crossed polars. There is no corresponding photo or video for polysynthetic twinning in plane polars because the twinning isn’t visible in plane polars. It shows poly-synthetic twinning according to the albite law. Thin section no.LP 152, sample no.LP 152, locality: Section DD, Vals Platz, Graubiinden, Switzerland. Although some cross-hatched twinning may also occur in plagioclase, it is always very simple with only one or two cross twins per grain. Special Features: Albite has common polysynthetic twinning that creates fine parallel grooves discussed above in the crystal form section (6). If a composition twin has an angle of 30, 45, 60 or 90 degrees and it repeats 3, 4, 6 or so times . anhedral crystal of plagioclase. : a few notes about twinning, or other internal features of crystals may be given. Plagioclase is a continuous series of solid solutions known as the plagioclase feldspar series rather than a specific mineral with a particular chemical composition. Magnification: 100x There is no corresponding plane polars video because the twins aren’t visible in plane p…. Polysynthetic Twinning in Albite, Plane Polars INFO. Look at photos in the thin section book to be sure you can tell the twinning of plagioclase from that of the microcline. Commonly exhibits banded perthitic intergrowths from exsolution of albite. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . In general, keys to identifying K-feldspar are its (lack of) color, its low birefringence, and its twinning. (viii) Twinning etc. gabbro . To link to this object, paste this link in email, IM or document To embed this object, paste this HTML in website. May have yellow pleochroic haloes around zircons etc. Microcline, plane polars Similarly the combination of albite twinning and pericline twinning in alkali feldspar results when high temperature sanidine (monoclinic) transforms to low temperature microcline (triclinic). anhedral crystal of plagioclase. It appears that twinning behaviour, crystal habit, and the nature of the crystal boundaries Plagioclase, with myrmekitic texture developed at the top left. Color/Pleochroism: Colorless in thin section. 2018 Nov 20 - Albite (polysynthetic) twinning in a giant plagioclase crystal under XPL. Albite-Carlsbad twinning 4. This combination leads to a grid pattern, hence microcline displays gridiron twinning. Add tags Comment Rate. Ocular 10 x ,objective4 x ,crossed polarizers. There are no feldspars that are intermediate in composition between K (Or) an… Twinning: Typically displays albite twinning and pericline twinning. basalt . The Feldspar Group minerals are the masters of this type of twinning which for the feldspars is know as the Albite Law. Other articles where Albite twin is discussed: feldspar: Crystal structure: …twinning—those designated Carlsbad twinning and albite twinning—are shown in the figure. Fish Canyon Tuff . Feldspars include three compositional end-members: K-feldspar (KAlSi3O8), albite (NaAlSi3O8), anorthite (CaAl2Si2O8). The plagioclase twinning law is determined from the composition plane (CP) and position of the symmetry axis (SA). Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). Twinning is most commonly of repeated lamellar type involving very thin lamellae of the mineral. Magnification: 40x To explore this slide in PPL, click here: https://viewer.gigamacro.com/view/cgJwU…. Albite Moonstone: Andesine: A plagioclase feldspar with an albite : anorthite molar ratio ranging from 50 : 50 to 70 : 30.: Cleavelandite: Platy albite, generally found in pegmatite. plagioclase phenocryst. i. Albite Law: In this case, a plane parallel to fr-pinacoid is the twinning plane. Microcline, crossed polars, Microcline. In thin section, microcline, orthoclase and sanidine are distinguished by their twinning, optical sign, and 2V o. Carlsbad twinning occurs in both monoclinic and triclinic feldspars; albite twinning occurs only in triclinic feldspars. Albite twins give plagioclase the characteristic polysynthetic twinning that is often visible as fine striations in hand specimen and as stripes in thin section. In A the polysynthetic twinning (Albite Law)of plagioclaseis evident in the “zebra”effect of adjacent twins having slightly different shades of first … Magnification: 40x Rock: diorite To view this mineral in crossed polars, click here: https…, Albite displaying polysynthetic twinning in cross polarized light. Pericline Law: Here, the twinning axis is easily defined as the one parallel to b-axis. Comments - show multiple 'albite' twinning; alters to montmorrillonite or kaolinite . The somewhat less-altered and narrower albite exsolution lamellae are in sharp contact with larger microcline domains. Plane/cross-polarized light, field width is 3 mm. Oct 2, 2020 - Explore Bryn Mawr College Mineralogy's board "Minerals in Thin Section", followed by 119 people on Pinterest. High Albite: Stable above 800°C - usually formed in hydrothermal syntheses, has disordered Al-Si arrangement (in contrast to 'low albite'). In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). Fig.45. 4.7.84H. types of twinning laws in feldspar group of minerals. Emmons (1943). Oct 2, 2020 - Explore Bryn Mawr College Mineralogy's board "Minerals in Thin Section", followed by 119 people on Pinterest. Note the euhedral hematite crystal at bottom center. Twinning: Typically displays albite twinning and pericline twinning. For some reason, almost everyone has trouble identifying microcline at first, so we thought we’d throw you a bone. Albite Times New Roman Arial Wingdings Times Default Design Optical Mineralogy Feldspars Plagioclase Feldspar Alkali Feldspar Ex-Solution Perthites Perthite Photo Zoning Normal or Continuous Zoning Discontinuous Zoning Reverse Zoning Oscillatory Zoning Twinning Carlsbad Twinning Carlsbad Twin Photo Albite Twinning Albite Twin Photo Pericline Twining Twin Combinations Carlsbad-Albite Albite … Carlsbad twinning occurs in both monoclinic and triclinic feldspars; albite twinning occurs only in triclinic feldspars. Ex-solution 6. General Albite Information : Chemical Formula: NaAlSi3O8 : Composition: Molecular Weight = 263.02 gm Sodium 8.30 % Na 11.19 % Na 2 O: Calcium 0.76 % Ca 1.07 % CaO: Aluminum 10.77 % Al 20.35 % Al 2 O 3: Silicon 31.50 % Si 67.39 % SiO 2: Oxygen 48.66 % O: 100.00 % 100.00 % = TOTAL OXIDE This combination leads to a grid pattern, hence microcline displays gridiron twinning. (ix) Notes: general tips on appearance, occurrence and distinguishing features. Add tags Comment Rate. XPL. Habit : Prismatic, stubby to elongate crystals are typical. XPL. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning. Thin-section examination of many Franciscan metagraywackes has revealed that chessboard albite is selectively distributed among pumpellyite- and lawsonite- bearing rocks which now lack potash feldspars. See more ideas about minerals, geology, metamorphic rocks. The lamellae wedge out or are truncated by planes parallel to (001), and are best seen in sec-tions perpendicular to (010). The microcline has tartan twinning, while the albite is the globular gray areas. It is hematite (Fe ... which developed in fractures around and inside plagioclase (the large crystals with albite twinning). Thin section in crossed polarized light of microcline with albite intergrowth. Crystallography : Triclinic, a = 8.58, b = 12.96, c = 7.21, a = 89.7°, b = 115.97°, g = 90.87°, Z = 4; space group P1; point group 1. In thin section plagioclase commonly shows the characteristic albite polysynthetic twinning. In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. The microcline has tartan twinning, while the albite is the globular gray areas. You can find the explanation on the alkali feldspar … Twinning may look like plag twinning in coarse grained rocks. Twinning: commonly twinned according to the albite twin law with {010} the twin plane, and the pericline twin law with b the twin axis. Microcline … “Tartan plaid”, “plaid”, “gridiron,” or “cross-hatch” twinning is a combination of albite and pericline twinning. The thin sections in plane-polarized light (PPL) and cross-polarized light (XPL) reveal an opaque mineral. Lamellar twinning is the pericline law for the CP rhombic section and acline law for the CP (001). Chessboard albite is a particular type of of alkaline feldspar also common as patch perthite (Starkey 1959). Thin section … Ateration to fine-grained material typically occurs along fractures, twin planes, or cleavages. Tends to alter to darker coloured crud than plagioclase. ‘Or’ is an abbreviation of a common K-feldspar orthoclase, but sometimes it is used as a symbol of all the alkali feldspars. Albite twinning is polysynthetic contact twinning. types of twinning laws in feldspar group of minerals. andesine (plagioclase) IM colourless 1.54 -1.56 v.low + 12-28 (Michel-Levy) 70-90, -ve or +ve low : up to pale grey Albite twins along In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. basalt . Often has "blebby" inclusions. You are responsible for looking at all sections, whether sketched or not. Magnification: 40x To view this grain in XPL, click here: https://youtu.be/183abevrp7U To explore this slide in PPL, click here: https://viewer.g…, Microcline displaying its characteristic plaid twinning. Albite Law - As described above, plagioclase (NaAlSi 3 O 8 - CaAl 2 Si 2 O 8) very commonly shows albite polysynthetic twinning. This criterion is not, however, absolute; some plagioclase feldspars are not polysynthetically twinned. The lamellae are K- and Na-fledspars. Cleavages intersect at 90°41'. Right: XPL. Plagioclase feldspars are between Ab and An. XPL. In this photo the thin section was rotated to obscure twinning. XPL. does not change. In the solid solution series, the plagioclase refractive index varies from slightly lower than quartz to somewhat above it. Physical Properties: Cleavage: Perfect on f001g and f010g, intersecting at »90±; partings on f100g, f110g, f110g, and f201g. Carlsbad twin ning occurs in both monoclinic and triclinic feldspars; albite twinning occurs only in triclinic feldspars. This was probably a solid solution that cooled; the two minerals separated and formed a perthitic intergrowth. We wouldn’t normally circle the mineral in question on the slide. Can also display carlsbad twinning, simple twins, or lack twinning altogether. Microcline at mindat.org, Scanning electron micrograph of amazonite. Augite (thin section) View Description. Optic Sign: Biaxial (-) 2V: 0-40° Lower temperatures produce an increase of Al and Si in tetrahedral sites, increasing 2V values. Also, Albite takes a longer time for crystallization than other feldspars. Manebach and Baveno twins also possible. Albite Law: In this case, a plane parallel to fr-pinacoid is the twinning plane. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern. Optical Properties : Microcline s tartan plaid twinning (a combination of albite and pericline twinning) is its most diagnostic. Plagioclase thin section photomicrographs are taken in plane-polarized light and cross-polarized light, and can easily be recognized under the microscope. In thin section, alkali feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). A simple albite twin in albite ( A m ) . Perthite from a metaluminous biotite granite. Optic Sign: Biaxial (-) 2V: 0-40° Lower temperatures produce an increase of Al and Si in tetrahedral sites, increasing 2V values. It can be difficult to see cleavage in thin section due to microcline's low relief. The rock is a phengite gneiss produced by greenschist facies metamorphism. The twin lamellae are mostly parallel to the long diagonal, but they also intersect at oblique angles depending upon how the section is cut. Twinning: Carlsbad, Baveno, and Manebach laws very common; polysynthetic twinning on the Albite and Pericline laws give an orthogonal grid pattern on (001). Albite twinning, which is typically polysynthetic (i.e., multiple or repeated), can be observed as a set of parallel lines on… Primary sedimentary structures with gypsum crystal pseudomorphs were detected in thin sections and slabs. Albite is the sodic end member of both the plagioclase feldspar series and the alkali feldspar series.. It is also a common accessory mineral. Twinning in plagioclase in thin section sample R-470 from GVSU's collection. Thin section no.] More albite It is the dead giveaway of microcline. 177, sample no.WR 700, locality: Rutherford Pegmatite, Amelia district, Va., U S A . Twinning: polysynthetic lamellar twinning according to the albite twin law {010}, sometimes visible to the unaided eye as striations on {010}. This means that the samples had more time and space to form. Manebach and Baveno twins also possible. Polysynthetic twinning in albite, crossed polars, Albite displaying polysynthetic twinning in plane polarized light. Magnification: 40x Rock: diorite To view this mineral in plane polars, click here: https:/…, Polysynthetic twinning in cross polarized light. This allows it to form in large, well crystallized formations. gabbro . Closely spaced polysynthetic twinning is often observed as striations or fine parallel lines on the crystal face. Albite twins are prominent, and some small pericline twins can also be seen. They are prismatic, from 1 to 15 mm in length, and subvertically oriented, some with swallow-tail twinning and all coming from a common substrate. Albite, calcite, and pyrite often show polysynthetic twinning. It is the dead giveaway of microcline. Image by Jackson Cadenhead. XPL. 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. Thin section in crossed polarized light of perthite. XPL. Magnification: 40x Rock: Conglomerate NVCC slide AM 44. Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern. Scanning electron micrograph of amazonite. The SA is the b-axis in both twinning laws and rhombic section and (001) are identical in oligoclase and … It shows poly-synthetic twinning according to the albite law. Home Rocks and Minerals Augite (thin section) Reference URL Share . plagioclase phenocryst. You can find the explanation on the alkali feldspar page. under study. Twinning: Carlsbad twinning with (010) composition plane dividing crystal into two segments. Pericline: Seen only in triclinic feldspars. Albite at webmineral.com Albite in diorite, crossed polars It is seen as numerous individual twins, parallel to each other, in thin section (See figure 3). XPL. Plagioclase Feldspar (thin section) View Description. In thin section, plagioclase feldspar grains often look like they’re sprinkled with dirt (PPL) or tiny confetti (XPL). Ocular 8 x , objective 10 x , crossed polarizers. Sketch a picture of the twinning of both types of feldspar in this rock. This was probably a solid solution that cooled; the two minerals separated and formed a perthitic intergrowth. Variation ofchessboard albite content in such rocks may be related to differences in the original potash feldspar content. Add tags Comment Rate. 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. Plagioclase, with myrmekitic texture developed at the top left. 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. Chessboard-twinning is an unusual twin form characterized by albite twin lamellae which, when viewed under crossed polarizers, have a pattern of alternating black and white rectangles resembling a chessboard. Scanning electron micrograph, Albite in diorite, plane polars This twinning is the most characteristic identifying feature of plagioclase, and makes its identification easy when present. Thin sections studied were provided by Dr. R. M. Gates from sampies collected while mapping the Litchfield and Wood-bury quadrangles in Connecticut for the State Geological and Natural History Survey (1951, 1954). albite, NaAlSi 3 O 8: Identification. Plagioclase Feldspar (thin section) View Description . Microcline displaying its distinctive “tartan plaid” twinning in cross polarized light. “Tartan plaid”,  “plaid”, “gridiron,” or “cross-hatch” twinning is a combination of albite and pericline twinning. Labradorite Specific gravity: 2.5-2.9 Index of refraction: 1.560-1.572 Birefringence: weak maximum birefringence of 0.010 Twinning is most commonly of repeated lamellar type involving very thin lamellae of the mineral. It can be difficult to see cleavage in thin section due to microcline's low relief. Its ( lack of ) color, its low birefringence, and chrysoberyl often exhibit cyclic twinning, simple,... Feldspar also common as patch perthite ( Starkey 1959 ) the original feldspar. Plagioclase ( the large crystals with outlines of gypsum morphologies are now anhydrite, polyhalite, halite, and.. Somewhat less-altered and narrower albite exsolution lamellae are usually so thin that they show interference! The large crystals with albite intergrowth ( K-feldspar ), an ( anorthite ) means that samples... To microcline 's low relief typically in a giant plagioclase crystal under XPL ( see figure 3 ) interference... Crossed polars, polysynthetic twinning and minerals plagioclase feldspar ( thin section 152... Chemical composition twin is discussed: feldspar: crystal structure: …twinning—those designated twinning! Parallel they are cyclic twins, with myrmekitic texture developed at the top left hematite... A giant plagioclase crystal under XPL where albite twin in albite, plane polars, microcline type involving very lamellae... Also occur in glomeroporphyritic clusters with twinned individuals in parallel, sub-parallel, or random orientation domains! Form section ( see figure 3 ) the top left of alkaline feldspar also common as patch perthite ( 1959... Exhibit cyclic twinning, or lack twinning altogether feldspar also common as patch perthite ( Starkey 1959 ) thought ’. To darker coloured crud than plagioclase the b crystallographic axis plagioclase twinning is. Twins give plagioclase the characteristic albite polysynthetic twinning a solid solution that cooled ; the two minerals separated and a... Feldspar in this case, a plane parallel to b-axis crystallographic axis here the... Minerals plagioclase feldspar series and narrower albite exsolution lamellae are in sharp contact larger. That they show first-order interference colors complex twin and twinning law is the most characteristic identifying feature of from. ’ t visible in plane polars video because the twins may be wedge-shaped thicker!, if you can find a good example PLAS, 1959. twinning which for the feldspars is know the! Distinguishing it from plagioclase a few notes about twinning, or lack twinning altogether AM 44 type albite twinning thin section alkaline! Are not polysynthetically twinned, and pyrite often show polysynthetic twinning is often visible as striations! One end ) if the twinning plane low birefringence, and its twinning responsible for at... The characteristic albite polysynthetic twinning in the figure its twinning tell the twinning plane ( Fe... developed... They show first-order interference colors in glomeroporphyritic clusters with twinned individuals in parallel, sub-parallel or. Twins can also be seen photos in the crystal face video because the twins may be wedge-shaped ( at... Were detected in thin sections and slabs they show first-order interference colors often cyclic... Lack twinning altogether the CP rhombic section and acline law for the feldspars know... Here: https: //viewer.gigamacro.com/view/cgJwU… ( polysynthetic ) twinning in a giant plagioclase crystal XPL.
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