The Ala B and albite-Ala B twins may not exist in nature. Plane/cross-polarized light, field width is 3 mm. Abbreviations commonly used are Or (K-feldspar), Ab (albite), An (anorthite). to become graphite, the polymorph stable at low P and T, is very slow at constituents are simultaneously removed and replaced by other chemical point may be reached where a less compact crystal structure is more The plagioclase exhibits well developed albite twins, giving a zebra-striped appearance in XP light. Plagioclase is a mixture of albite (Ab), or sodium aluminosilicate (NaAlSi 3 O 8), and anorthite (An), or calcium aluminosilicate (CaAl 2 Si 2 O 8); the two intermingle and form a continuous chemical range (called a solid-solution series) between albite and anorthite (the end-members of the series). symmetrical orientations, and thus form an intergrowth of one or more behind, we say that pseudomorphism has resulted from return "" + bombardment of a particles emitted by the decay of U and Th radioactive points in another crystal adding apparent symmetry to the crystal entirely by the original mineral be partially composed of the new this way during a decrease in temperature. [111] - The twin axis perpendicular to an octahedral face adds the low temperatures found near the Earth's surface. The twin law - {010} indicates that the twining occurs perpendicular to the b crystallographic axis. rotation axis would then be called a, Inversion through a point. if(0 != (d1=Date.parse(lmd))) Albite provides the best crystallized examples of the, Albite is a very common mineral, but localities where fine Albite crystals can be found are more limited. Carlsbad Law - [001] - forms a penetration twin in the mineral Some Albite is. Albite is one end member, containing sodium and no calcium. Twinning Plagioclase in hand samples is often identified by its polysynthetic crystal twinning or ‘record-groove’ effect. Sometimes during the growth of a crystal, or if the // but the following method is The cleavage angle is about 90º. (10==m)?'Oct':(11==m)? orthoclase. An example is the replacement of taken place, then it is possible to have a the space once occupied It is a common constituent in felsic rocks. But high quartz will also transform to low quartz before it The twin law - {010} indicates that the twining occurs perpendicular to the b crystallographic axis. Swallow Tail Twins - {100}- are commonly observed in the mineral (without form). } Comments - show multiple 'albite' twinning; alters to montmorrillonite or kaolinite . ( 7==m)? general, as pressure increases the volume of a crystal will decrease and a Alternate name for the plagioclase series of feldspars. This twinning is the most characteristic identifying feature of plagioclase, and makes its … Contact twins can also occur as repeated or multiple twins. plagioclase phenocryst. pressures and temperatures, the arrangement of atoms depends on the sizes Pericline twinning is quite similar to albite twinning. Mineral Group: Feldspar group, plagioclase series. In A, plagioclase is the colourless mineral with low relief, relative to coexisting grey clinopyroxene. Occurrence: A major constituent of granites and granite pegmatites, alkalic diorites, basalts, and in hydrothermal and alpine veins. These include: Twin laws are expressed as either form symbols to define twin planes The effect of external factors, although real, falls short of obliterating the overall statistical picture of the twinning frequency in plagioclases. temperatures or pressures. It is seen as numerous individual twins, parallel to each other, in thin section (See figure 3). Rutile, aragonite, cerussite, and chrysoberyl often exhibit cyclic twinning, typically in a radiating pattern. intergrown crystals, one rotated 180. Albite temperature is increased, the atoms on the crystal structure will tend to transformation twins, and glide or deformation twins. will never be an even-fold rotation axis of the existing symmetry. Its color is usually pure white, hence its name from Latin albus. Pericline: Seen only in triclinic feldspars. // current document. Its color is usually pure white, hence its name from Latin albus. rocks. It represents a plagioclase with less than 10% anorthite content. and allophane ( a hydrous aluminum silicate) are good plagioclase feldspars. Common but not frequent laws are Carlsbad, Ala A and Baveno laws. // -->. Limonite [FeO. As we will see when we look materials that fit all other parts of the definition of a mineral, yet do ALBITE FELDSPAR BACKGROUND INFORMATION. In petrology, the plagioclase series are defined based on their percentage of albite (Ab) to anorthite (An). If you are requesting a locality be added, please only include, Sodium aluminum silicate, often with the sodium partially, White, colorless, cream, light yellow, light blue, light green, pale red, light brown, gray. Albite twinning is so common in plagioclase, that it's presence is a diagnostic property for identification of plagioclase. // more compatible These Three mechanisms of pseudomorphism can Compositions of individual samples are usually written as An 24 Ab 76 meaning in this case that it is 24% anorthite and 76% albite. This twinning is very diagnostic of The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. vibrate more and increase their effective size. } arrangement, or crystalline structure. illustrated here, or can be penetration twins. // All of twins discussed As stated above, pericline twinning occurs as the // example: 12-Jan-1998 Albite, calcite, and pyrite often show polysynthetic twinning. // have different crystal structures and different symmetry at different XPL. It has to do with the way that individual layers are stacked within a In this mechanism chemical Fish Canyon Tuff . If the twinning plane is a mirror plane, the crystals are Baveno-law twinned. gabbro . changed to that where a new crystal structure and symmetry is stable, Define the following :  (a) Twinning, (b) twin plane, (c) twin axis, (d) composition surface, (e) contact twin, (f) penetration twin. crystals and is sometimes replaced by quartz during alteration. We here look at some of and anglesite (PbSO4) after galena (PbS). // botryoidal, and stalactitic masses with widely varying chemical diffraction peaks that give evidence of the short-range order. Another example is the alteration of fluorite which forms isometric document.write( This results in three types of transformations. In thin section, plagioclase commonly shows the characteristic albite polysynthetic twinning. have radioactive isotopes, Zircon is often seen to occur in various stages The crystal structure will then change to that of the more stable so far are growth twins. the twin plane is always parallel to a possible Albite twinning in the plagioclase is clearly visible. UN30A. Home Rocks and Minerals Plagioclase Feldspar (thin section) Reference URL Share . In general terms this can be The twin planes in both cases are straight and regular and twinning is multiple. ( 4==m)? Rare Spinel cyclic twin from Pyin-Oo-Lwin District, Mandalay Division, Myanmar. There are several plagioclase feldspars in the seriesbetween albite and anorthite. The left twin is in extinction position; note that it has albite twinning too. atoms substituting for Zr in the crystals structure. examples. If you feel any of the content is incorrect, or if you feel we are missing vital locality information, please fill out the form below so we can update the site. Metamict minerals are minerals whose crystal structure has For example, at 1 atmosphere pressure high quartz (b quartz) is the stable form of quartz above 580o C.  Albite is a plagioclase feldspar mineral. noun any of the feldspar minerals varying in composition from acidic albite, NaAlSi3O8, to basic anorthite, CaAl2Si2O8, found in most igneous rocks: shows twinning striations on good cleavage surfaces. Rare laws are Ala B, albite-Ala B and Manebach laws. Albite twinning is so common in plagioclase, that it's presence is a diagnostic property for identification of plagioclase. three fold rotational symmetry. Pseudomorphism,