Nepheline
A variety of Feldspathoid Group
What is Nepheline?
Nepheline is a rock-forming mineral which sometimes occurs in recent lava flows as glassy crystals. This mineral is usually white in color, often with a gray or yellowish tint. It is often very large and found in unattractive specimens. Its main uses are for mineral collections and raw material for special kinds of ceramics and glass.
Uses & Applications
Nepheline is used to lower the melting temperature when making ceramics and glass, which also gives it resistance to getting scratched or broken. Nepheline is also used in rock wool insulation, roofing granules, building stone, and in bathrooms to make toilet bowls or sinks.
Healing Properties
Nepheline is believed to provide a gentle energy that can help children or adults suffering from nightmares, improve one's love life, and bring a sense of harmony to the home. Many believe it has the power to eliminate emotional pain and relieve mental anguish. Keep it in the bedroom to improve intimate relationships. Carry it with you to open the flow of earth's energy and allow the full enjoyment of every day life.
Optical Properties
- Refractive Index
- 1.529-1.547
- Birefringence
- 0.003-0.005
- Optical Character
- Uniaxial negative
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Key Characteristics
Characteristics of Nepheline
Nepheline crystals are rare and belong to the hexagonal system, usually having the form of a short, six-sided prism terminated by the basal plane. The crystals appear to have more symmetry than they actually possess, but unsymmetrical etched figures produced artificially on the prism faces indicate that the crystals are hemimorphic and tetartohedral, the only element of symmetry being a polar hexad axis. It is found in compact, granular aggregates, and can be white, yellow, gray, green, or reddish. The hardness is 5.5 – 6, and the specific gravity 2.60 – 2.65. It is often translucent with a greasy luster. The low index of refraction and the feeble double refraction in nepheline are nearly the same as in quartz; but since in nepheline the sign of the double refraction is negative, while in quartz it is positive, the two minerals are readily distinguished under the microscope. An important determinative character of nepheline is the ease with which it is decomposed by hydrochloric acid, with separation of gelatinous silica (which may be readily stained by coloring matters) and cubes of salt. For this reason, a clear crystal of nepheline becomes cloudy when immersed in acid. The mineral is prone to alteration to zeolites (especially natrolite), sodalite, kaolin, or compact muscovite.
Formation of Nepheline
Nephelinite is a rock-forming mineral found in silica-poor igneous rocks. These include nepheline syenite, foidite, and phonolite. It is often found along with leucite, sodalite, potassium feldspars, and sodium-rich plagioclase, amphiboles, or pyroxenes, but almost never in association with quartz. Notable outcrops of nepheline-bearing rocks are found on the Kola Peninsula; in Norway and South Africa; and at Litchfield, Maine; Magnet Cove, Arkansas; and Beemerville, New Jersey, in the United States. Syenites found near Bancroft, Ontario contain large deposits of high-purity nepheline. Elaeolite (a name given by M. H. Klaproth 1809, from Greek words for oil [ἔλαίον] and stone [λίθος]; German: Fettstein) is a massive form of translucent nepheline with a darker color and greasy luster.
Composition of Nepheline
The aluminosilicate backbone of nepheline has a fairly open structure of interlocked six-member rings. This resembles the structure of tridymite, with aluminum substituting for every other silicon atom. This structure produces one nearly hexagonal interstitial site and three irregular interstitial sites per unit cell. In ideal nepheline, the hexagonal sites are occupied by potassium ions and the irregular sites by smaller sodium ions, yielding an atomic ratio of sodium to potassium of (3:1). This corresponds to an ideal weight percentage of K2O of 8.1% The range of compositions seen in natural nepheline is 3% to 12% K2O. Small amounts of calcium may be present as well. At elevated temperature, nepheline forms a complete solid solution series with kalsilite, KAlSiO4. At temperatures below about 1,000 °C (1,830 °F), there is a wide miscibility gap between nepheline and kalsilite, similar to the miscibility gap between microcline and albite. A composition falling in this gap will experience exsolution as it cools, where nepheline and kalsilite separate into separate microscopic layers (lamellae).
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Quick Facts
Physical Properties
- Color
- White, grey, yellowish
- Hardness (Mohs)
- 5.5 - 6
- Density
- 2.64 g/cm³
- Streak
- White
- Luster
- Greasy, Vitreous
- Crystal System
- Hexagonal
Chemical Properties
- Chemical Formula
- Na3K(Al4Si4O16)
- Elements
- Al, K, Na, O, Si
Also Known As
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