Does Potassium Feldspar Have Lamellae?

Plagioclase feldspars

The play of colors visible in some feldspar of labradorite composition is due to very fine-grained exsolution lamellae known as Bøggild intergrowth. The specific gravity in the plagioclase series increases from albite (2.62) to anorthite (2.72–2.75).

Does plagioclase have exsolution?

Plagioclase feldspars exhibit exsolution microtextures in three compositional ranges, the Peristerite, Bøggild and Huttenlocher ‘miscibility gaps’ shown in orange on Fig. 4 and in white in Fig. 8. These intergrowths are coherent and usually below the resolution of an optical microscope.

Which type of feldspar has Perthitic texture?

Alkali feldspars show irregular perthitic features (‘patch’ perthite) and exhibit striking granophyric intergrowth with quartz, particularly near miarolitic cavities, which are formed by exsolution of the magmatic vapour phase at low confining pressures.

What is exsolution lamellae?

Exsolution lamellae are fine crystals that were dissolved entirely in their host mineral structures at high temperature (e.g., clino- and orthopyroxenes) and/or pressure (e.g., majorite-pyroxenes), but are produced when their host minerals lose the solubility as a result of cooling to low temperatures (T-controlled …

What minerals contain Perthitic texture?

The largest documented single crystal of perthite was found in Hugo Mine in South Dakota and measured about 10.7 m (35 ft) x 4.6 m (15 ft) x 1.8 m (5 ft 11 in). The gem varieties of potassium feldspar, amazonite and moonstone are variant colored perthites.

What are Perthite and Antiperthite?

A perthite is an intimate intergrowth of sodic and potassic feldspar resulting from subsolidus exsolution (unmixing of two minerals). … An antiperthite is an intergrowth arising due to exsolution where potassic feldspar is present as blebs or lamellae within a sodic feldspar.

What is microcline feldspar?

Microcline (KAlSi3O8) is an important igneous rock-forming tectosilicate mineral. It is a potassium-rich alkali feldspar. Microcline typically contains minor amounts of sodium. It is common in granite and pegmatites. … Sanidine is a polymorph of alkali feldspar stable at yet higher temperature.

Is hornblende an alkali feldspar?

The typical color index (M) of an alkali feldspar granite ranges between 0 and 20 %. Biotite and hornblende representing the commonest mafic minerals. If M < 10% the rock can be named alaskite (i.e. a leucocratic alkali feldspar granite). Muscovite, oxides, and other accessories may be present.

Which mineral has the exsolution lamellae structure?

pyroxene structure

The lamellae are exsolved along specific crystallographic directions, producing oriented intergrowths with parallel and herringbone texture.

What is the difference between alkali feldspar and plagioclase feldspar?

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.

What is potash feldspar?

A feldspar is typically referred to as ‘potash’ if there is significantly more potassium than sodium (typically there will be 2-5% Na2O). … Potassium feldspar melts at around 1200C (higher than soda feldspar) producing a more a viscous melt than does sodium feldspar.

Does orthoclase feldspar have striations?

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.

What mineral has striations visible?

Striations can also be a growth pattern or mineral habit that looks like a set of hairline grooves, seen on crystal faces of certain minerals. Examples of minerals that can show growth striations include pyrite, feldspar, quartz, tourmaline, chalcocite and sphalerite.

Where is microcline feldspar found?

Microcline is found at Baveno, Italy; Kragerø, Nor.; Madagascar; and, as amazonstone, in the Urals, Russia, and Florissant, Colo., U.S. For detailed physical properties, see feldspar (table).

What is the difference between orthoclase and microcline?

Orthoclase is a polymorph of other minerals that share the same chemistry, but have different crystal structures. … The differences between these minerals are minor in hand samples. Microcline tends to be deeper-colored and is the only one that can be, but is not always, a blue-green color (amazonite).

Which type of Lustre is exhibited by nepheline?

Nepheline is an important feldspathoid mineral. It is white, grey or yellow in colour with a vitreous lustre and a poor cleavage.

What type of rock is Sanidine found in?

Sanidine is the high temperature form of potassium feldspar with a general formula K(AlSi3O8). Sanidine is found most typically in felsic volcanic rocks such as obsidian, rhyolite and trachyte.

What is Myrmekitic texture?

myrmekite, irregular, wormy penetration by quartz in plagioclase feldspar; these wartlike, wormlike, or fingerlike bodies may develop during the late stages of crystallization of igneous rocks if the two minerals (quartz and feldspar) grow simultaneously in the presence of a volatile phase.

What type of rock is anorthite?

Anorthite is a rare compositional variety of plagioclase. It occurs in mafic igneous rock. It also occurs in metamorphic rocks of granulite facies, in metamorphosed carbonate rocks, and corundum deposits. Its type localities are Monte Somma and Valle di Fassa, Italy.

What is graphic texture in geology?

Graphic texture refers to the regular intergrowth of two minerals, one of them generally serving as a host and the other appearing on surfaces of the host as striplike or cuneiform units with grossly consistent orientation; the graphic intergrowth of quartz in alkali feldspar is…

Does feldspar have cleavage?

As a group, feldspars range from transparent to nearly opaque, have nonmetallic lustres—typically vitreous to subvitreous on fractures and pearly or porcelaneous on cleavage surfaces, exhibit two cleavages—one perfect, the other good—at or near 90° to each other, and have a Mohs hardness of approximately 6.


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