Faldshpatoidy (silicates): Nepheline (Eleolit)
Diagnostic Card.
On the picture. Nepheline crystals with brownish yellow idocrase of Monte Somma, Province of Naples.
K Na 3 (Al Si O 4)4
Crystal system hexagonal
Hardness of 5.5-6
Specific gravity 2.56-2.66
Cleavage appears difficult
fracture conchoidal
Colour colorless, differently colored
Gloss glass
nepheline crystals usually is short-, "cut" the small faces of the hexagonal pyramid and completes basal pinacoids. Habit, moreover, can be tabular. Nepheline can also occur in the form of transparent grains and vitreous dense massive aggregates. Painting colorless, light gray, yellow or light green. The mineral can have all the transitions from transparent to almost opaque, his brilliance - from glass to fat. In addition, it is hard, lightweight, has no appreciable cleavage, with conchoidal fracture, brittle.
Chemical composition. sodium oxide (Na2O) of about 13- 16%, alumina (Al2O3) 32-33%, silicon dioxide (SiO2) 43,5-45%; moreover, potassium oxide (K2O) 5.5-6.5%, calcium oxide (CaO) 0.5-1.8%. Form crystals. Prismatic, short-columnar, tolstotablitchatye. The crystal structure. Silicate with an infinite three-dimensional skeleton (SiAl) O4. Class symmetry. Hexagonal pyramid - 6. Cleavage. Good for the (1010) (0001). Aggregates. Dense, drain.
Diagnostic features.
Acid is easily decomposed to form silica gelatinous residue. It melts easily. Behavior in acids. Soluble in HCl. Most often, the simple hexagonal prism with a rough surface, short-columnar; ingrown crystals and accrued. Cleavage is imperfect; density 2.60-2.65; gray, yellowish, greenish, bluish, dark green, brownish-red and colorless. This theoretical structure in the formula changes in natural conditions, so slow cooling crystals grow turbid, with rapid - remain transparent.
Origin.
Being very abundant mineral, nepheline occurs in both intrusive and extrusive rocks in; especially large clusters it may form in the nepheline syenite. Mostly found in dark and extrusive rocks to their pegmatites; rarely in the form of fine crystals in pores accrued volcanic bombs, for example in Italy (Vesuvius), Germany (Eifel mountains), Sweden and Norway. Deposits of the Kola Peninsula (Russia) and in Norway.
Deposits and application.
The lavas of the Lazio region, this mineral forms beautiful crystals; it is also found in volcanic products Monte Somma in the province of Naples. Its crystals were found in the volcanic area of the Eifel massif in Germany; nepheline widespread in northern Norway, the Urals and Kola Peninsula (Russia), Greenland and Canada. In the case of large deposits of nepheline it can be used in industry for the production of glass and ceramics.
Nepheline. Fragment pegmatite made nepheline, microcline and sodalite. In October m-p. Don, Azov, Ukraine. About 1 m sample:. Min. Museum them. AE RAS Fersman (N88784. Gift Ukrainian geologists). Photo: © AA Evseev.
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