Nepheline
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Introduction to Nepheline
Crystal Structure and Physical Properties
Nepheline, also known as nephelite, is a feldspathoid mineral that is classified as a silica-undersaturated aluminosilicate with the chemical composition (Na3K)Al4Si4O16. This mineral forms in intrusive and volcanic rocks that are low in silica, as well as in their associated pegmatites. Nepheline plays an essential role in the geology of certain igneous rocks, particularly those with an alkaline nature.
Optical and Chemical Properties
Nepheline crystals are relatively rare and belong to the hexagonal crystal system, typically appearing as short, six-sided prisms terminated by basal planes. Interestingly, artificially produced etched patterns on prism faces show that these crystals are hemimorphic and tetartohedral, featuring a polar hexad axis as the sole element of symmetry. Nepheline is most commonly found in compact, granular aggregates and is known for its varied colors, including white, yellow, gray, green, and reddish shades. The hardness of nepheline ranges between 5.5 and 6 on the Mohs scale, and its specific gravity is between 2.56 and 2.66, making it a moderately dense mineral with a unique greasy luster.
Chemical Variations and Alterations
The mineral’s optical properties include a low index of refraction and weak double refraction, similar to that of quartz. However, nepheline can be distinguished from quartz by its negative optical sign, while quartz has a positive one. This difference allows for easy identification under a microscope. A notable characteristic of nepheline is its reaction to hydrochloric acid. When immersed in acid, nepheline decomposes easily, producing gelatinous silica that can be stained by coloring agents, along with the formation of salt cubes. As a result, a clear nepheline crystal will become cloudy when placed in acid.
Alteration and Substitution Products
While natural nepheline always contains sodium and potassium in a roughly 3:1 atomic ratio, artificially prepared crystals have the composition NaAlSiO4. The corresponding potassium compound, KAlSiO4, is known as kaliophilite, which can also be synthesized in the laboratory. Some researchers have suggested that the true formula for nepheline is better represented as (Na,K)AlSiO4, reflecting its orthosilicate structure.
Nepheline is particularly prone to alteration, both in natural environments and under laboratory conditions. Various substitution products of nepheline have been synthesized, and in nature, nepheline often alters to form minerals such as natrolite, sodalite, kaolin, or compact muscovite. Other pseudomorphs, such as gieseckite and liebenerite, have also been observed as alteration products of nepheline.
Sample from Myanmar
Varieties of Nepheline
Two main varieties of nepheline can be distinguished, based on their external appearance and occurrence. These varieties are analogous to sanidine and common orthoclase feldspar, respectively.
Glassy Nepheline
This variety is characterized by small, colorless, transparent crystals and grains with a vitreous luster. Glassy nepheline is commonly found in alkali-rich volcanic rocks, such as phonolite, nepheline-basalt, and leucite basalt, as well as in certain dike rocks, like tinguaite. The finest crystals of glassy nepheline can be found in the crystal-lined cavities of ejected blocks from Monte Somma, Vesuvius, alongside minerals like mica, sanidine, and garnet.
Elaeolite
The other variety, known as elaeolite, consists of large, rough crystals or irregular masses. Elaeolite is generally opaque or only slightly translucent and exhibits a greasy luster, with colors that range from reddish and greenish to brownish or gray. Elaeolite is an essential component of certain alkaline plutonic rocks, particularly those from the nepheline syenite series, which are prominently found in southern Norway.
Frequently Asked Questions
Identifying Nepheline vs. Quartz
Nepheline is softer than quartz and differs in its optic sign. Nepheline has a negative optical sign, while quartz has a positive one. Nepheline occurs in alkali-rich, silica-poor igneous rocks, such as nepheline syenite, foidite, and phonolite, and can also be found in some contact-metamorphosed rocks.
Nepheline vs. Plagioclase
No, nepheline is not a plagioclase. It is a feldspathoid mineral that forms in silica-undersaturated igneous rocks, including plutonic nepheline syenites and volcanic phonolites. Nepheline is colorless in plane light and has a relief similar to K-feldspar, sodic plagioclase, and other feldspathoids, with which it frequently coexists.
Crystal Healing Properties of Nepheline
Throughout history, various cultures have attributed healing properties to crystals. Nepheline is believed to promote calmness, especially in children, helping to banish nightmares and alleviate sleep difficulties. It is also said to assist in managing temper tantrums and reducing stress-related disorders. These healing attributes are not scientifically proven, but many people use nepheline for its metaphysical properties.
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