Feldspar Value, Price, and Jewelry Information

Comments

Feldspars are the most common minerals at the Earth's surface. In fact, if the entire composition of the Earth's crust were regarded as a single mineral, it would calculate out almost exactly as a feldspar.

The feldspars are complex aluminosilicate minerals containing K, Na, and Ca, with some rarer types rich in Ba. The structures of these species are very similar. However, most feldspars crystallize from a melt in igneous rocks. The structures at high temperatures are different from those at low temperatures. In addition, the various compositions that may exist at high temperatures may not be stable at low temperatures. When a feldspar cools, it may segregate internally into separate mineral crystals, one type oriented within the other according to the symmetry of the host crystal. The specific type of intergrowth, composition of the minerals involved, and the size of the included crystals all depend on the original high-temperature composition and the cooling history of the feldspar, which may be very complex. It is easy to see why it may take years for a mineralogist simply to understand the complexities of the feldspar group, let alone contribute new data.

These complications, while both troublesome and intriguing to mineralogists, are not critical to gemological discussions. We may therefore simplify the discussion to a summary of the basic feldspar species and their properties, insofar as these are relevant to gemstones.

Potassium Feldspars: These all have the composition KAISi₃O but differ in structure. Orthoclase is monoclinic; sanidine and anorthoclase are also monoclinic, but the distributions of atoms within the structures are distinctive and different from each other and orthoclase. Microcline is triclinic. The properties are summarized in the table below. Sanidine and Anorthoclase contain appreciable sodium.

Plagioclase Feldspars: The term plagioclase indicates a solid-solution series, ranging in composition from albite (NaAISi₃O₈) to anorthite (CaAl₂Si₂O₈); for convenience the series was long ago arbitrarily divided into six distinct species as follows: albite (Ab); oligoclase (Og); andesine (Ad); labradorite (La); bytownite (By); anorthite (An). The series is divided according to the relative percentages of albite vs. anorthite:

The optical parameters vary nearly linearly with composition, but because of the structural complexities, X-ray diffraction work is usually advised in identification of plagioclase feldspar. Most plagioclase crystals are twinned according to various laws related to the crystal structures and also the distribution of atoms in the structures. Zoning is common and is due to variation in the growth history of the crystals and to the fact that, in a magma, the composition of the melt changes as crystallization proceeds and minerals are extracted from the molten mass. The properties of a plagioclase crystal may therefore vary widely within a small grain. Plagioclases also are often clouded, that is, contain dust like particles of other minerals, including spinel, rutile, garnet, magnetite, clinozoisite, muscovite.

In many cases faceted gems are identified as a feldspar in the plagioclase series, but the finder does not have the instrumentation needed to pin down the species. This is accomplished by a combination of optical and X-ray analysis. A few plagioclase gems have been well-characterized, however, and reported in the literature.

Compositions within the feldspar group are complicated by the fact that K may enter the plagioclase structure or Na the orthoclase structure. The resulting compositions are known as ternary (three-component) feldspars. In addition, as in the plagioclase series itself, high-temperature mixed feldspar compositions are stable, but at low temperatures un mixing occurs, that is, a segregation of the potassic and sodic molecules into separate feldspar phases, one distributed within the other. This creates such oddities as perthites (mixtures of albite with oligoclase or orthoclase), sunstone, moonstone, and peristerites, which are albite-oligoclase mixtures. The presence of feldspar lamellae in another feldspar gives rise to the Schiller effect, an iridescence due to light refraction. Schiller is best developed in labradorites, creating a lovely color play in shades of green, blue, gold and yellow. The color may be uniform or vary within a single feldspar crystal.

Most feldspar crystals are tabular and flattened and (in the case of plagioclase) usually complexly twinned. All the plagioclases are triclinic, and all the feldspars have excellent cleavage in two directions. The luster is vitreous, inclining to pearly on the cleavages. Feldspars are sometimes massive, cleavable, or granular.

Microcline may be colorless, white, pink, yellow, red, gray, or green to blue-green. The latter color is popular in gem circles, and the blue-green variety known as amazonite is widely cut into cabochons, beads, and carvings. Orthoclase is usually colorless, white, gray, yellow, reddish, and greenish, whereas sanidine is colorless, pinkish, or brownish. The plagioclases are all colorless, white, or gray, although the drabness is often broken by spectacular Schiller effects. Moonstones may be colored by impurities such as goethite (brown).

Comments by Don Clark, CSM IMG

Feldspars are our most common mineral, making up over 99% of the earth's crust. Their formation is one of the most complex in nature. Formed from igneous melts, both their crystal habits and chemistry vary according to the temperature at which they are formed and it often changes during cooling.

When a feldspar cools, it may internally segregate into separate crystals with different properties. These are not easily distinguished, but a single stone may have layers of orthoclase, plagioclase and other feldspars. This process is called unmixing and is how blends are created. The layering of different species results in twinning.

Color zoning is also common, but it has slightly different cause. The chemistry of the magma in which they form, changes as the crystallization proceeds. This also contributes to differing blends, but is the sole cause of zoning.

It is the twinning of different species, with layers of different optical properties, that creates the spectacular phenomena these gems are famous for. When you see a beautiful moonstone or labradorite, it is a result of this complex formation process.

In general, we speak of microcline, orthoclase and plagioclase feldspars. While microcline and orthoclase are unique species, plagioclase is a solid state series with six individual species. As with garnets, these are rarely found in their pure states, but are blended with other species.

The number of combinations are high and identification requires careful attention. You can find blends, not only within the plagioclase series, but plagioclase and any other feldspar. They also mix with other minerals, like emerald and jadeite.

Hints for Easier Identification of Feldspars by Don Clark, CSM IMG

Labradorescence is a feature of labradorite, from which it gets its name. You will find labradorescence in translucent to opaque stones, but not transparent ones. Schiller is only found in transparent gems. These are sunstones.

Most plagioclase are twinned and zoning is common.

Also remember to distinguish between orthoclase, (OR-tho-klase,) which is an individual species and oligoclase, (oh-LIG-oh-klase,) which is in the plagioclase series.

Moonstone is an orthoclase feldspar and amazonite is microcline. All other feldspars are in the plagioclase series.

Exceptions: Moonstone is usually an orthoclase feldspar, but occasionally it is a labradorite. The RI will easily separate them..

Labradorite is the most common faceted feldspar.

Exception: Sunstone can be labradorite or oligoclase. To distinguish them, both the optic sign and the RI vary.

Exception: There are a few transparent orthoclase, but the RI will easily separate them.     You may find transparent gems in the plagioclase series that are described as something other than oligoclase or labradorite, (I.E. albite or andesine.) These will be valued the same as sunstone.

The primary data to separate species is the refractive index. You will often be using spot readings, which can be inaccurate. Fortunately, you can use the stone's appearance for much of your diagnostics.     There is an overlap in the RI is between the low of oligoclase and the high of orthoclase and microcline. To put it another way, the overlap is between sunstone, moonstone and amazonite. These stones look considerably different, so use their appearance to verify your data.

Exception: This does not apply to transparent gems, but you can get more accurate RI readings on transparent stones.

Varieties of Feldspar We Recommend Reading About

1. Orthoclase

Orthoclase is best known for moonstone. It is occasionally a transparent, faceted gem.

Note that moonstone is occasionally a labradorite. If you find a moonstone that does not have this RI or SG, see the note under labradorite.

2. Microcline

The only microcline you are likely to encounter is amazonite.

 3. Plagioclase

Sunstone and labradorite are the important plagioclase gems.

Plagioclase is not a single species, but a solid state series that runs from albite, (NaAlSi3O8) to anothrite, (CaAl2Si2O8). They are described as six species ranging from albite, to oligoclase, andesine, labradorite and bytonite to anothrite; this in order of their relative percentages of albite and anothrite.

K may enter plagioclase and Na the orthoclase structure. These are called ternary, (three component,) feldspars. Albite/orthoclase and albite/oligoclase mixtures are possible in the same crystal, which are called perthites. Sunstone, moonstone and peristerite are examples of albite/oligoclase feldspars.

It is not always possible to make a positive distinction between the species with standard gemological instruments. Hence, we usually list plagioclase gems as either oligoclase or labradorite. This is a standard procedure that is used by most major gemological associations.

Oligoclase

Labradorite (LAB-ra-door-ite)

There is also a labradorite moonstone, which is found only in Madagascar. It has an RI of 1.550 - 1.553, which is lower than other labradorites, but higher than orthoclase moonstones. The birefringence is .008 - .010 and the specific gravity is 2.70.

5. Plagioclase Series Gems

• Formula: varies between NaAlSi3O8 and CaAl2Si2O8
• Crystallography: Triclinic
• Colors:
• Labradorite usually dark GRAY or BLACK, also COLORLESS GREEN YELLOW ORANGE BROWN BROWNISH RED
• OligoclaseCOLORLESS YELLOW ORANGE BROWN LIGHT GREEN GRAY
• Transparency: Transparent to Opaque

• Pleochroism: Usually none
• Yellow stones; colorless and light yellow
• Red-orange and blue-green bicolored; bluish green/light red-violet/reddish orange.
• Bluish green; bluish green/light orange/colorless
• Red-orange; orange/ light reddish purple
• Orange; orange/reddish orange
• Yellowish green; bluish green/light orange
• Blue green; red-violet/reddish orange/bluish green
• Violet; red-violet/reddish orange/bluish green
• UV long: Inert to weak patchy white
• UV short: Inert to weak patchy white
• Absorption Spectrum: Not diagnostic
• Polish Luster: Vitreous to pearly
• Fracture: Uneven to splintery
• Fracture Luster: Pearly to vitreous
• Cleavage: Perfect and easy in two directions, parting common
• Phenomena: Labradorescence, adventurescence. Rare chatoyancy or asterism in labradorite
• Toughness: Poor
• Size:     Labradorite to large     Oligoclase to 5 carats
• Rarity: Common
• Heat Sensitive: Yes
• Enhancements: Some red andesine/labradorite feldspars are diffusion treated. Detect with immersion. Common, stable.

Identifying Characteristics

• Labradorite, labradorescence, repeated twinning, black needle like inclusions
• Sunstone, reddish or golden platelets with metallic luster

Variety Names

• Spectrolite, labradorite with strong labradorescence
• Sunstone, labradorite or oligoclase with schiller.
• Sunstone also refers to transparent labradorite.
• Adventurine, labradorite or oligoclase with adventurescence
• Albite, usually colorless but sometimes yellow, pink, gray or reddish. Translucent albite is sometime colored green by chrome jadeite. It is also a component of trapiche emeralds.
• Peristerite is primarily oligoclase with a complex mixture of feldspars. It has iridescence that is either blue or white.
• Sanadine is uncommon and rarely seen as a gem. While occasionally brown, most examples are colorless     While the properties are closer to microcline, it is usually classed as a plagioclase. RI 1.516 to 1.526, SG 2.57 - 2.58, birefringence .003 - .005
• Perthite is a blend of microcline, albite and oligoclase. It is usually brown and white. May have gold or white iridescence.

Misnomer

Rainbow moonstone, colorless labradorite with multicolored labradorescence

Phenomenal Feldspar Gems by Don Clark, CSM IMG

Phenomenal terms are often used incorrectly. You need to both familiarize yourself with their correct usage and be aware that others may be using them indiscriminately. See Phenomenal Gems for more information.

	SchillerSchiller results from reflections off inclusions. It is only found in transparent gems.
	LabradorescenceLabradorescence is a flash of color caused by the interference of light. It is found on the surface of opaque stones.
	AdularescenceAdularescence is a light floating inside a stone. It is found in transparent and translucent gems.

Summary of Properties

Orthoclase - Moonstone

Microcline - Amazonite

Oligoclase - Sunstone

Labradorite - Sunstone and Labradorite