Feldspathoids are silicate minerals that are chemically similar to feldspars but deficient in silica. They form in place of feldspars when magma lacks sufficient silica. Common feldspathoids include leucite, nepheline, sodalite, cancrinite, and analcime. Some feldspathoids contain unusual anions like chloride, carbonate, sulfate, or sulfide. Leucite is characteristic of potassium-rich volcanic rocks. Nepheline occurs in both plutonic and volcanic alkaline, silica-poor rocks. Sodalite forms blue veinlets and masses and is found in nepheline syenites and phonolites.

1. Feldspathoid Group

2. Introduction Feldspathoids are anhydrous framework siicates that are chemically similar to the feldspars. The main difference being that they are deficient of silica. Species in this group form in place of feldspars when a magma lacks enough silica to form the later, hence the feldspathoid name

3. Species include leucite, nepheline, sodalite, cancrinite and analcime. Some of the members of feldspathoid group contain unusual anions. Sodalite contains Cl and Cancrinite incorporates CO3. Noselite houses SO4 and Lazurite SO4, S, and Cl ions. These large anions and anionic groups are located in large interstices of the structure.

4. Leucite- KAl Si2 O6 Name : From the Greek leukos , “white”. Crystallography- Tetragonal below 605 ºC. Isometric above 605 ºC. Usually trapezohedral crystals which at low temperature are pseudomorphs of high temperature. Most leucites are close to KAl Si2 O6. small amounts of Na may replace K. Distinguish from analcite by te common presence of complex twinning and by having a higher refractive index.

5. Paragenesis Characteristic mineral of potassium rich basic lavas such as leucite- basanite, leucite tephrite, leucite melilite basalt and leucite ankaramite. It also an essential constituent of the potasium rich ultrabasic rocks, ugrandite(olv+augite+leucite) and katungite, (melilite +leucite glass with or without leucite)

6. Nepheline group: Na Al SiO4 Characteristic mineral of the alkaline rocks and occurs as a primary phase in many plutonic , hypabyssal and volcanic rocks of extremely varied mineralogical and chemical composition. The nephelines of nepheline-syenites and nepheline gneisses do not vary greatly in composition. And in these rocks nepheline is associated with low temperature feldspars of microcline albite series.

7. It is coarsely crystallized with a greasy lustre. Nepheline readily decomposed by HCl. Occurs almost exclusively in magmatic, silica poor, alkaline rocks. Occurs both plutonic and volcanic rocks and in pegmatites associated with nepheline syenites. It is used extensively in the glass and ceramic industry.

8. Sodalite group- Na8 (AlSiO4)6 Cl2 Isometric system. Na8 (AlSiO4)6 Cl2 Occurs in grayish blue, and ink-blue veinlets and as clusters and massive aggregates. Crystals rare, more commonly massive and granular. Distinguished by its characteristic blue color and translucency. Roughly resembles azurite but azurite is readily distinguished by its lower hardness and effervescence in acid.

9. Macroscopic crystals are sometimes observed in nepheline syenites and phonolites. Exceptional transparent crystals have been found in the lavas of Mt. Vesuvius (Italy) Sodalite occurs in both volcanic and plutonic rocks of the nepheline syenite family More commonly in the plutonic types, generally in association with nepheline.

10. Nosean- has a composition Na8 (AlSiO4)6 SO4 occurs in extrusive alkaline igneous rocks. Huyne- another member of the sodalite group. (Na-Ca)4-8 (Al6 Si6 O24) (SO4, S)1-2 It differs chemically from nosean in having a much higher proportion of Ca and in being richer in the sulfate radicle.

11. Lazurite- Na8 (AlSiO4)6 SO4 Characterized by its dark blue colour. Lapis Lazuli is a mixture of lazurite, calcite and pyroxenes. Lazurite was formerly used for blue paint pigment and is highly valued decorative stone. Natural lazurite can be considered as a sulfide bearing huyne. It occurs in Lapis Lazuli in metamorphosed limestone or at contacts of pegmatites with limestone.

12. Cancrinite - Na6 Ca2 (Al Si O4)6 (CO3)2 Hexagonal system. Carbonate rich variety. Occurs in nepheline syenites and as an alteration product of nepheline. Cancrinite occurs commonly as a secondary mineral particularly replacing nepheline.

13. IJOLITE A Medium to coarse grained, ultra alkaline plutonic igneous rock, consisting of essential nepheline, aegirine or aegirine-augite with or without garnet and accessory apatite, sphene and cancrinite. The rock can be considered an undersaturated alkali syenite

14. Nepheline syenite A medium to coarse grained igneous rock consisting of essential alkali feldspar, nepheline, pyroxene and amphibole with accessory sphene, apatite, titanomagnetite, ilmenite, and zircon. The rock is , in effect, a syenite which is undersaturated in silica

15. Nepheline Monzonite An undersaturated monzonite containing essential nepheline in addition to the monzonite mineral assemblage.

16. Nephelinite A fine grained often porphyritic, ultra-alkaline extrusive igneous rock consisting of essential nepheline, titanaugite and titanomagnetite with a ccessory apatite, sphene. Sodalite and analcite may also found in the groundmass. These are essentially super-undersaturated basalt with nepheline taking the place of all the plagioclase