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Feldspars are a group of closely related minerals that together are the most abundant mineral in the Earth's crust. A thorough knowledge of the feldspars is what separates geologists from the rest of us. Feldspars are hard minerals, all of them with a hardness of 6 on the Mohs scale. This lies between the hardness of a steel knife 5. In fact, feldspar is the standard for hardness 6 in the Mohs scale. Feldspars usually are white or nearly white, though they may be clear or light shades of orange or buff.

Natural glass is the major constituent of a few volcanic rocks-e. Macerals are macerated bits of organic matter, primarily plant materials; one or more of the macerals are the chief original constituents of all the diverse coals and several other organic-rich rocks such as oil shales.

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In the classification of igneous rocks of the International Union of Geological Sciences IUGSthe feldspars are treated as two groups: the alkali feldspars and the plagioclase feldspars. The alkali feldspars include orthoclasemicroclinesanidineanorthoclaseand the two-phase intermixtures called perthite.

The plagioclase feldspars include members of the albite - anorthite solid-solution series.

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Strictly speaking, however, albite is an alkali feldspar as well as a plagioclase feldspar. Article Media.

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Thank you for your feedback. Introduction Chemical composition Crystal structure Physical properties Common properties of the group Identification of specific feldspars Origin and occurrence Uses. Author of Stones; coauthor of Gems, Gravels and Granites.

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See Article History. Aluminum makes three bonds trivalentcalcium makes two divalent and sodium and potassium make one monovalent.

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So the identity of X depends on how many bonds are needed to make up the total of One Al leaves one bond for Na or K to fill.

Two Al's leaves two bonds for Ca to fill.

Jun 01,   Feldspar, as alternative dosimeter, has successfully been used to date young sediments (e.g. Riedesel et al., ); however, its signal is less light sensitive and often suffers from a signal loss over time, known as anomalous fading (Spooner, ). Both properties make feldspar less suitable for dating Holocene doursim.com: Dominik Brill, Marco Cisternas. Apr 29,   Post?infrared (pIR) stimulated luminescence dating of sedimentary feldspar largely avoids the effects of anomalous fading that affect conventional infrared stimulated luminescence (IRSL) dating. Here we test against independent age control the recently developed feldspar post-IR IRSL approach to the dating of sediments, which appears to avoid signal instability. A physical model explaining our observations is discussed, and the method is shown to be accurate back to doursim.com by:

So there are two different mixtures that are possible in the feldspars, a sodium-potassium series and a sodium-calcium series. The first is alkali feldspar and the second is plagioclase feldspar.

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Alkali feldspar has the formula KAlSi 3 O 8potassium aluminosilicate. The formula actually is a blend ranging from all sodium albite to all potassium microclinebut albite is also one endpoint in the plagioclase series so we classify it there. This mineral is often called potassium feldspar or K-feldspar because potassium always exceeds sodium in its formula.

Potassium feldspar comes in three different crystal structures that depend on the temperature it formed at. Microcline is the stable form below about C.

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Orthoclase and sanidine are stable above C and C, respectively. Outside the geological community, only dedicated mineral collectors can tell these apart.

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But a deep green variety of microcline called amazonite stands out in a pretty homogeneous field. The color is from the presence of lead.

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The high potassium content and high strength of K-feldspar make it the best mineral for potassium-argon dating. Alkali feldspar is a crucial ingredient in glass and pottery glazes.

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Microcline has a minor use as an abrasive mineral. The plagioclase feldspars are named according to the following scheme, where the numbers are the percentage of calcium expressed as anorthite An :. The geologist distinguishes these under the microscope.

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One way is to determine the mineral's density by putting crushed grains in immersion oils of different densities. Albite's specific gravity is 2. The really precise way is to use thin sections to determine the optical properties along the different crystallographic axes. The amateur has a few clues.

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An iridescent play of light can result from optical interference inside some feldspars. In l abradorite, it often has a dazzling blue hue called labradorescence.

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If you see that it's a sure thing. Bytownite and anorthite are rather rare and unlikely to be seen. An unusual igneous rock consisting of only plagioclase is called anorthosite.

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Feldspar, any of a group of aluminosilicate minerals that contain calcium, sodium, or potassium. Feldspars make up more than half of Earth's crust, and professional literature about them constitutes a large percentage of the literature of mineralogy. Learn more about the properties and uses of feldspar . Aug 10,   The high potassium content and high strength of K-feldspar make it the best mineral for potassium-argon dating. Alkali feldspar is a crucial ingredient in glass and pottery glazes. Microcline has a minor use as an abrasive mineral.

Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U.

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