Feldspato Alcalino
Monoclinic, triclinic
(K,Na)AlSi3O8
Alkali feldspars are fundamental rock-forming K- and Na-bearing aluminosilicates that occur in many igneous, metamorphic, and sedimentary rocks, and constitute a significant portion of the ‘granitic’ continental crust.
Struttura e chimica
Alkali feldspars are framework silicates characterized by a network of (Si,Al)O4 tetrahedrons linked to one another in all directions. The structure of alkali feldspar can be visualized as a series of ‘rings’ of four tetrahedrons: in these rings, two neighboring tetrahedrons point up and the other two point down. When stacked one on top of each other, the ‘rings’ produce an infinite framework of tetrahedrons, which can be imaged as infinite tetrahedron chains formed along two perpendicular directions. Tetrahedral rings are distorted and deviate from the horizontal, allowing the formation of large sites in the feldspar structures that can accommodate large-radius ions of alkali elements (K+, Na+, and some Ca2+). Other elements that may be present in small amounts include Fe2+, Fe3+, Ba, Ti, Mg, Sr, and Mn.
Alkali feldspars comprise K-feldspar (K[AlSi3O8]) and Na-feldspar o albite (Na[AlSi3O8]). K-feldspars are characterized by a monocline structure at high-temperature, with Al randomly substituting Si in all tetrahedral sites (called T1o, T2o, T2m, and T1m). The resulting structure is monoclinic and known as sanidine. The tetrahedral sites have not exactly the same size and, at low temperature (T < 500 °C), Al – characterized by larger ionic radius than Si – prefers a specific site (T1o), giving rise to a triclinic structure, microcline. This phenomenon is known as order-disorder polymorphism, since the structure of microcline is more ‘ordered’. The transition from high-temperature sanidine to low-temperature microcline occurs at the solid-state during cooling of igneous rocks and requires slow cooling to allow Al to diffuse and organize within the crystal lattice. Volcanic rocks, cooling rapidly from high temperatures, retain the sanidine structure. On the other hand, microcline can be found only in plutonic rocks that cooled down very slowly. In plutonic rocks that cooled slowly but reached T < 500 °C too fast, ordering remains incomplete (with Al concentrated in the T1o and T1m sites), developing a pseudo-monoclinic K-feldspar called orthoclase. Adularia is another partially ordered monoclinic feldspar (like orthoclase), which forms only in hydrothermal veins. Differently from orthoclase, the ordering of adularia occurs at low temperature during crystal growth, resulting in a very different morphology and optical properties.
Albite, the Na-feldspar, has a similar order-disorder polymorphism between high-albite e low-albite. However, both are triclinic and the transition from one structure to the other with temperature occurs at faster rates than in K-feldspar. Consequently, high-albite has been obtained only in the lab and natural albites are all low-albite.
Proprietà
Abito: prismatic, tabular
Durezza: 6 – 6.5
Sfaldatura: {001}, {010} perfect (two planes intersecting close to 90°)
Geminazione: Carlsbad ({hko}, usually {01o} composition plane – [001] twin axis), Baveno ({021}/{0-21} composition plane – [021]/[0-21] twin axis), Manebach ({001} composition plane – [001] twin axis) simple twinning; Albite ({010} composition plane – [010] twin axis), Pericline ({0kl} composition plane – [010] twin axis) polysynthetic twinning
Colore: colorless, white, pink, reddish, green
Lucentezza: vitrea
Struscio: bianco
Alterazione: kaolinite, sericite
In sezione sottile...
α(α^a = 18° microcline, 5° orthoclase and sanidine, 10° anorthoclase): 1.518-1.529
β(β^c = 18° microcline, 20° orthoclase and anorthoclase; β//b sanidine): 1.522-1.533
γ(γ^b = 18° microcline, 5° anorthoclase; γ//b orthoclase; γ^c = 20° sanidine): 1.522-1.539
2Vα: 0-103°
Colore: incolore
Birifrangenza (δ): 0.006-0.010 (first-order greys)
Rilievo: low, lower than quartz
Segno ottico: monoclinic (-), triclinic (+ or -)
[Mindat]
Alkali feldspars constitute a solid-solution between K-feldspar o orthoclase molecule (Or) and albite (Ab), which represents also the sodic end-member of the plagioclase series. Anorthite (Ca[Al2Si2O8]; An) is present in small quantities in alkali feldspars, usually constituting less than 5% of the solid solution and predominantly found in Na-rich alkali feldspars. The solid-solution is based on the exchange of ions, within the structure of alkali feldspars, with the same charge (K+ and Na+) but different radius (K+ = 1.33 Å; Na+ = 0.97 Å). Consequently, the complete solid-solution between K-feldspar and Na-feldspar is possible only at high temperature, as HT structures have larger cation sites. Moreover, the very high-temperature albite (monalbite) is monoclinic, like sanidine. At low-temperature (< 600-700 °C) the structure of albite (triclinic) is incompatible from that of K-feldspar (monoclinic) and the complete solid solution is not possible. Consequently, in natural rocks we find alkali feldspars with K-rich compositions (sanidine, orthoclasee microcline) and Na-rich compositions (anorthoclase, albite). A larger chemical variability in terms of Na and K is found only in sanidine and anorthoclase, since these minerals occur only in volcanic rocks, whose fast cooling allows to preserve the high temperature composition. Sanidine is monoclinic and has potassic composition (> Or37), whereas anorthoclase is triclinic and show sodium-rich composition (< Or37). This limit has been determined experimentally and in natural rocks sanidine composition tends to be even more potassic (> Or67). In plutonic rocks, orthoclase and microcline are nearly purely K-rich and albite occurs as part of the plagioclasio series, with significant proportion of Ca. Eventual (K, Na)-feldspar compositions unmix at the slow cooling rates of plutonic rocks, commonly producing perthites (i.e. irregular lamellae or intergrowths of Na-feldspar in K-feldspar) and antiperthites (exsolved lamellae of K-feldspar in plagioclase).
Geminazione
Alkali feldspars obey to several twinning laws, including simple twinning between two crystals and polysynthetic twinning, involving several crystals. Carlsbad twinning occurs when two crystals are paired through a 180 ° rotation along the long axis (c-axis): the twins can be twinned along a crystal face (contact twin) or be interpenetrated with one another (interpenetrated twin). Other common twin laws include Baveno (approximate 45° rotation around the c-axis) and Manebach twinning (180° rotation around the short axis or a-axis). Polysynthetic twinning in alkali feldspars occurs only in triclinic alkali feldspars (i.e. microcline and anorthoclase). The most common laws are Albite e Pericline twinning: the former is a 180° rotation normal to the intermediate axis (b-axis), the latter a 180° rotation parallel to the b-axis. In triclinic feldspars, these two polysynthetic twinning laws occur together and, since they are oriented at nearly 90° from one another, they produce the typical cross-hatched (or tartan) extinction of microcline and anorthoclase.
Above: Carlsbad twinning recognizable in the external morphology of alkali feldspar (orthoclase) megacrystals in the Monte Capanne monzogranite. Sant’Andrea, Elba, Italy. [see post]
Caratteristiche di terreno
Feldspars show tabular or prismatic habit, appearing stocky and not much elongated. Their habit is well visible in volcanic and plutonic igneous rocks, where they tend to be euhedral or subhedral. Feldspar have two prismatic sfaldatura systems intersecting at 90° and, for this reason, they have vitreous (glass-like) luster on broken surfaces, as they tend to break along cleavage planes. Carlsbad twinning – common in alkali feldspars – is easily identified as a surface that runs roughly parallel to the long axis of the crystal, separating twins with different orientation (i.e. reflecting light at different angle). All feldspars have hardness around 6 on the Mohs scale (harder than metal but weaker than quarzo). In the field, the color of feldspars can be used to distinguish alkali feldspar from plagioclasio, although with some care. Both can be white, but alkali feldspar tends to show pale red to salmon color, whereas plagioclase tends to be ‘milk white’. The presence, within a rock, of feldspars with different color, one pinker and the other whiter, hints to the presence of both alkali feldspar and plagioclase. Transparent alkali feldspar occurs only in volcanic rocks (sanidine). Orthoclase and microcline tend to be opaque and often show irregular perthites with different color. Transparent sanidine can be distinguished from quartz because of its vitreous luster, tabular habit, and presence of cleavage planes. Quarzo, indeed, shows conchoidal fracture and lacks cleavage systems. Green alkali feldspar (variety: amazonite) occurs and can be confused with Ca-plagioclase. In this case, looking for perthites can be helpful for its identification. Plutonic rocks may contain either orthoclase or microcline. However, the two cannot be distinguished based on macroscopic observations only.
Above: trace of the Carlsbad twinning plane and intersecting cleavage planes in an alkali feldspar (orthoclase) megacrystal in monzogranite. Sant’Andrea, Island of Elba, Italy. [see post] A close-up of the feldspar cleavage is available below.
Alkali feldspars in thin section
Alkali feldspars are colorless and show very low relief at PPL. At CPL they show low first-order-grey interference colors. All alkali feldspars show two cleavage planes intersecting close to 90° on sections parallel to the c axis. Euhedral crystals are prismatic or tabular, with elongated prismatic sections and rectangular to square-like basal sections. Alkali feldspars commonly alter to phyllosilicates (sericite, kaolinite), appearing therefore ‘dirty’ under the microscope. In plutonic and metamorphic rocks, they can show perthite unmixing, i.e. irregular lamellae of albite showing different orientation and slightly different color compared to the hosting feldspar. Alkali feldspars can be confused with quarzo (especially in fine-grained aggregates). However, they show lower relief compared to quartz. Twinning is very common in alkali feldspars and the type of twinning represents the main tool to distinguish them under the microscope: sanidine and orthoclase show only simple twinning between two crystals (most commonly Carlsbad twinning), which forms during their growth. Anorthoclase and microcline show ‘tartan extinction’, related to the presence of both Albite and Pericline polysynthetic twinning. Microcline and anorthoclase are similar but the cross-hatching of anorthoclase is more fine-grained and has a different orientation. Anyway, sanidine and anorthoclase occur only in volcanic rocks whereas orthoclase and microcline occur are confined to plutonic and metamorphic rocks. Adularia is morphologically very similar to microcline but lacks tartan twinning and rather show crystals with variable angle of extinction (zonal extinction). Alkali feldspars are easily distinguishable from plagioclasio, which shows only polysynthetic Albite twinning.
Sanidine
Sanidine occurs only in volcanic rocks. It forms tabular to prismatic crystals commonly showing Carlsbad twinning.
NX
N//
NX
N//
Above: Sanidine crystals in a trachyte. Note the characteristic prismatic habit and Carlsbad twinning. The tiny prismatic grains in the groundmass are also sanidine crystals (field of view = 7mm). Photo © Alessandro da Mommio (alexstrekeisen.it)
Orthoclase
Orthoclase is the partially ordered alkali feldspar that may occur in plutonic and metamorphic rocks. Like sanidine, it is prismatic or tabular and shows well-developed Carlsbad twinning. Orthoclase commonly shows perthites, appearing as irregular lamellae of albite within its crystals.
NX
N//
NX
N//
Microcline
Microcline is the completely ordered triclinic alkali feldspar that occurs only in plutonic and metamorphic rocks. Microcline is easily recognizable due to the characteristic cross-hatched or tartan extinction. Perthite unmixing is also common.
NX
N//
Anorthoclase
Anorthoclase is a triclinic alkali feldspar that is easily recognizable due to its cross-hatched twinning patterns. The ‘cross-hatched’ extinction is more fine-grained that that of microcline. Moreover, anorthoclase occurs only in Na-rich volcanic rocks.
Adularia
Adularia is a low-temperature alkali feldspar that occurs only in veins, associated to other hydrothermal minerals. Its primary feature is the large optical variability, with crystals showing patchy or zonal extinction patterns.
Ritrovamento
Alkali feldspars are fundamental constituents of many acid igneous rocks, such as granites, syenites, granodiorites, and their volcanic counterparts. They also commonly occur in pegmatites and in other hydrothermal rocks (e.g. adularia). Acid volcanic rocks commonly contain sanidine or anorthoclase, whereas acid intrusives typically show large crystals of microcline or orthoclase. Alkali feldspars are also present in feldspathoid-bearing rocks (e.g. monzosyenites, foidolites) and in the groundmass of some alkali-rich basic rocks. Alkali feldspars are relatively resistant to weathering and, therefore, common constituents of detrital sedimentary rocks such as sandstones. In metamorphic rocks, they may occur also as detrital grains (especially in low-grade metasediments) or form as metamorphic minerals. The most common metamorphic reaction that produces alkali feldspars is the dehydration of K-bearing phyllosilicates like white mica and biotite in amphibolite- to granulite-facies metamorphic rocks. Partial melting of metasediments in migmatites may also form alkali feldspars (both from metamorphic reactions and crystallization of melt). In metamorphic rocks, alkali feldspars undergo the same phase transformations that happens in igneous rocks, with microcline being stable at low temperature (T < 450 °C) and sanidine at higher temperatures. This has been used by some workers as indication of metamorphic grade (i.e. microcline-sanidine isograd). During exhumation, high-T sanidine should change to microcline, but the transformation from a monoclinic to a triclinic structure is energetically hindered and high-T feldspars in metamorphic rocks commonly occur as orthoclase.
Brown, W. L., & Parsons, I. (1989). Alkali feldspars: ordering rates, phase transformations and behaviour diagrams for igneous rocks. Mineralogical Magazine, 53(369), 25-42.
Hovis, G. L. (1986). Behavior of alkali feldspars; crystallographic properties and characterization of composition and Al-Si distribution. American Mineralogist, 71(7-8), 869-890.
Mora, C. I., & Valley, J. W. (1985). Ternary feldspar thermometry in granulites from the Oaxacan Complex, Mexico. Contributions to Mineralogy and Petrology, 89(2-3), 215-225.
Parsons, I., Fitz Gerald, J. D., & Lee, M. R. (2015). Routine characterization and interpretation of complex alkali feldspar intergrowths. American Mineralogist, 100(5-6), 1277-1303.
Schairer, J. T. (1950). The alkali-feldspar join in the system NaAlSiO4-KAlSiO4-SiO2. The Journal of Geology, 58(5), 512-517.
Smith, P., & Parsons, I. (1974). The alkali-feldspar solvus at 1 kilobar water-vapour pressure. Mineralogical Magazine, 39(307), 747-767.
Risorse
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
Traduzione in corso!
Le pagine in Italiano dovrebbero essere disponibili nuovamente nel giro di qualche mese.
Italiano 
English