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About Feldspar

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alkali feldspar crystal

Potassium feldspar forms stubby crystals

Andrew Alden photo

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.

How to Tell Feldspar

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 (5.5) and the hardness of quartz (7). 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. They usually have a glassy luster.

Feldspar is what's called a rock-forming mineral, very common and usually making up a large part of the rock. In sum, any glassy mineral that's slightly softer than quartz is very likely to be a feldspar.

The main mineral that might be confused with feldspar is quartz. Besides hardness, the biggest difference is how the two minerals break. Quartz breaks in curvy and irregular shapes (conchoidal fracture). Feldspar, however, breaks readily along flat faces, a property called cleavage. As you turn a piece of rock in the light, quartz glitters and feldspar flashes.

Other differences: quartz is usually clear and feldspar is usually cloudy. Quartz appears in crystals more commonly than feldspar, and the six-sided spears of quartz are very different from the generally blocky crystals of feldspar.

What Kind of Feldspar?

For general purposes, like picking granite for a countertop, it doesn't matter what type of feldspar is in a rock. For geological purposes, feldspars are quite important. For rockhounds without laboratories, it's enough to be able to tell the two main types of feldspar, plagioclase (PLADGE-yo-clays) feldspar and alkali feldspar.

The one thing about plagioclase that's usually different is that its broken faces—its cleavage planes—almost always have fine parallel lines across them. These striations are signs of crystal twinning. Each plagioclase grain, in reality, is typically a stack of thin crystals, each with its molecules arranged in opposite directions. Plagioclase has a color range from white to dark gray, and it's typically translucent.

Alkali feldspar (also called potassium feldspar or K-feldspar) has a color range from white to brick-red, and it's typically opaque.

Many rocks have both feldspars, like granite. Cases like that are helpful for learning to tell the feldspars apart. The differences can be subtle and confusing. That's because the chemical formulas for the feldspars blend smoothly into each other.

Feldspar Formulas and Structure

What is common to all the feldspars is the same arrangement of atoms, a framework arrangement, and one basic chemical recipe, a silicate (silicon plus oxygen) recipe. Quartz is another framework silicate, consisting only of oxygen and silicon, but feldspar has various other metals partly replacing the silicon.

The basic feldspar recipe is X(Al,Si)4O8, where X stands for Na, K or Ca. The exact composition of the various feldspar minerals depends on what elements balance the oxygen, which has two bonds to fill (remember H2O?). Silicon makes four chemical bonds with oxygen; that is, it's tetravalent. Aluminum makes three bonds (trivalent), calcium makes two (divalent) and sodium and potassium make one (monovalent). So the identity of the X depends on how many bonds are needed to make up the total of 16.

One Al leaves one bond for Na or K to fill. Two Al's leaves two bonds for Ca to fill. 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.

Alkali Feldspar in Detail

Alkali feldspar has the formula KAlSi3O8, potassium aluminosilicate. The formula actually is a blend ranging from all sodium (albite) to all potassium (microcline), but 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 400° C. Orthoclase and sanidine are stable above 500° C and 900° C, respectively.

Outside the geological community, only dedicated mineral collectors can tell these apart. But a deep green variety of microcline called amazonite stands out in a pretty homogeneous field. The color is from the presence of lead.

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.

Plagioclase in Detail

Plagioclase ranges in composition from Na[AlSi3O8] to Ca[Al2Si2O8]—sodium to calcium aluminosilicate. Pure Na[AlSi3O8] is albite, and pure Ca[Al2Si2O8] is anorthite. The plagioclase feldspars are named according to the following scheme, where the numbers are percentage of calcium expressed as anorthite (An):

  • Albite (An 0–10)
    Oligoclase (An 10–30)
    Andesine (An 30–50)
    Labradorite (An 50–70)
    Bytownite (An 70–90)
    Anorthite (An 90–100)

The geologist distinguishes these under the microscope. One way is to determine the mineral's density by putting crushed grains in immersion oils of different densities. (Albite's specific gravity is 2.62, anorthite's is 2.74, and the others fall in between.) 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. An iridescent play of light can result from optical interference inside some feldspars. In labradorite it often has a dazzling blue hue called labradorescence. 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. A noteworthy occurrence is in New York's Adirondack Mountains; another one is the Moon.

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