Feldspathoid group of minerals

The feldspathoids are anhydrous framework silicates. The amount of silica in them is lesser than felspars. Nepheline, leucite, and sodalite are the important members of this group, which are being discussed in the below lines.

(1). Nepheline:

Nepheline contains Sodium or Potassium with AlSiO4. Its chemical formula is (Na,K)AlSiO_4. Some important physical properties of nepheline are as under;

1. Physical Properties of Nepheline:
   • It is vitreous greasy luster.
   • It is found in a hexagonal crystal system.
   • It is cut in poor cleavage. The cleavage is not so good.
   • It has a specific gravity from 2.60 to 2.63.
   • It is found naturally in white, grey, and yellowish colours.
   • It is a mineral of massive habit.
   • Its hardness is 5.5 to 6.0 on moh’s scale.
   • It has transparent as well as opaque diaphaneity.
   • Nepheline is the characteristic mineral of alkaline plutonic rocks, particularly nepheline syenites and nepheline gneisses.
   • It is a mineral of brittle tenacity.
2. Uses of Nepheline:
   • It is used as a substitute for felspar for the manufacturing of glass and ceramics.
3. Occurrence of Nepheline: 
   • Nepheline occurs in beautiful crystal form with mica, garnet, and sanidine feldspar on Monte Somma, Vesuvius, Italy.

(2). Leucite Mineral:

Leucite is another important rock-forming mineral of the feldspathoid group which includes potassium and aluminum tectosilicate and has a chemical formula KAlSi2O6. The name leucite has been derived from the Greek leucos for white, in allusion to its color. Specimens can also be colorless or gray. Crystals are common and can be up to 31 ⁄2 in (9 cm) wide. More often, leucite occurs as massive or granular aggregates or as disseminated grains. It is tetragonal at temperatures below 1,155°F (625°C) and cubic with trapezohedral crystals at higher temperatures. The trapezohedral form is preserved as the mineral cools and develops tetragonal symmetry. Leucite is found in potassium-rich and silica-poor igneous rocks. It is found with nepheline, sodalite, natrolite, analcime, and sodium and potassium-rich feldspars, and occurs worldwide. Physical properties of Leucite are as under;

1. Physical Properties of Leucite:
   • It has an indistinct cleavage.
   • It is found either in colorless form or white, yellow grey, and grey colors.
   • It is found in translucent to transparent diaphaneity.
   • Fracture: Brittle – Conchoidal – Very brittle fracture producing small, conchoidal fragments
   • It hardness is 5.5 – 6 at moh’s scales.
   • It has vitereous lustre.
   • It has a specific gravity 2.45 to 2.50.
   • It is non-magnetic in nature.
   • It has a white streak.
   • It is a mineral of brittle tenacity.
2. Occurrence of Leucite:
   • It is found naturally in igneous rocks along with Nepheline. All the minerals of feldspathoid groups are found in potassium-rich, silica-poor rocks of recent lavas.
3. Uses of Leucite:
   • As Leucite possesses a high potassium content, therefore it is used as a fertilizer in countries where it is found in abundance, for instance, in Italy.
   • It is also used as a source of commercial alum.

(3). Sodalite:

Sodalite is the third major mineral of Feldspathoid group. It is comparatively rare than Nepheline and Leucite. It is found in blue and blue-voilet colors. Its chemical composition is Na₄Al₃Si₃O₁₂Cl. It is included in semi-precious gemstones. Its blue color helps to diagnose it. Its physical properties are as under:

1. Physical Properties: 
   • It is found in cubic crystal systems.
   • It has a poor cleavage and has an even fracture.
   • It has a massive habit.
   • It has a vitreous luster.
   • It is mostly found in blue color, but in rare cases it is also found in white, grey and green colors.
   • Its specific gravity ranges from 2.15 to 3.0.
2. Uses of Sodalite:
   • A semiprecious gemstone often used to produce cabochons, beads, and tumbled stones. An ornamental stone often used to make small sculptures. “Sodalite granite” is a rare nepheline syenite with minor to abundant sodalite that is cut as a dimension stone for interior use. A less costly alternative to lapis lazuli.
3. Occurrence: 

• • Sodalite is a general of the feldspathoid group, that is why it is naturally occurred along with Nepheline in syenite and ”sodalite granite rock”. Both are found in igneous rocks of recent lavas.