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  1. 1. Feldspathoid Group
  2. 2. Introduction <ul><li>Feldspathoids are anhydrous framework siicates that are chemically similar to the feldspars. </li></ul><ul><li>The main difference being that they are deficient of silica. </li></ul><ul><li>Species in this group form in place of feldspars when a magma lacks enough silica to form the later, hence the feldspathoid name </li></ul>
  3. 3. <ul><li>Species include leucite, nepheline, sodalite, cancrinite and analcime. </li></ul><ul><li>Some of the members of feldspathoid group contain unusual anions. </li></ul><ul><li>Sodalite contains Cl and Cancrinite incorporates CO3. </li></ul><ul><li>Noselite houses SO4 and Lazurite SO4, S, and Cl ions. </li></ul><ul><li>These large anions and anionic groups are located in large interstices of the structure. </li></ul>
  4. 4. Leucite- KAl Si2 O6 <ul><li>Name : From the Greek leukos , “white”. </li></ul><ul><li>Crystallography- Tetragonal below 605 ºC. </li></ul><ul><li>Isometric above 605 ºC. </li></ul><ul><li>Usually trapezohedral crystals which at low temperature are pseudomorphs of high temperature. </li></ul><ul><li>Most leucites are close to KAl Si2 O6. small amounts of Na may replace K. </li></ul><ul><li>Distinguish from analcite by te common presence of complex twinning and by having a higher refractive index. </li></ul>
  5. 5. Paragenesis <ul><li>Characteristic mineral of potassium rich basic lavas such as leucite- basanite, leucite tephrite, leucite melilite basalt and leucite ankaramite. </li></ul><ul><li>It also an essential constituent of the potasium rich ultrabasic rocks, ugrandite(olv+augite+leucite) and katungite, (melilite +leucite glass with or without leucite) </li></ul>
  6. 6. Nepheline group: Na Al SiO4 <ul><li>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. </li></ul><ul><li>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. </li></ul>
  7. 7. <ul><li>It is coarsely crystallized with a greasy lustre. </li></ul><ul><li>Nepheline readily decomposed by HCl. </li></ul><ul><li>Occurs almost exclusively in magmatic, silica poor, alkaline rocks. </li></ul><ul><li>Occurs both plutonic and volcanic rocks and in pegmatites associated with nepheline syenites. </li></ul><ul><li>It is used extensively in the glass and ceramic industry. </li></ul>
  8. 8. Sodalite group- Na8 (AlSiO4)6 Cl2 <ul><li>Isometric system. Na8 (AlSiO4)6 Cl2 </li></ul><ul><li>Occurs in grayish blue, and ink-blue veinlets and as clusters and massive aggregates. </li></ul><ul><li>Crystals rare, more commonly massive and granular. </li></ul><ul><li>Distinguished by its characteristic blue color and translucency. </li></ul><ul><li>Roughly resembles azurite but azurite is readily distinguished by its lower hardness and effervescence in acid. </li></ul>
  9. 9. <ul><li>Macroscopic crystals are sometimes observed in nepheline syenites and phonolites. </li></ul><ul><li>Exceptional transparent crystals have been found in the lavas of Mt. Vesuvius (Italy) </li></ul><ul><li>Sodalite occurs in both volcanic and plutonic rocks of the nepheline syenite family </li></ul><ul><li>More commonly in the plutonic types, generally in association with nepheline. </li></ul>
  10. 10. <ul><li>Nosean- has a composition Na8 (AlSiO4)6 SO4 </li></ul><ul><li>occurs in extrusive alkaline igneous rocks. </li></ul><ul><li>Huyne- another member of the sodalite group. </li></ul><ul><li>(Na-Ca)4-8 (Al6 Si6 O24) (SO4, S)1-2 </li></ul><ul><li>It differs chemically from nosean in having a much higher proportion of Ca and in being richer in the sulfate radicle. </li></ul>
  11. 11. <ul><li>Lazurite- Na8 (AlSiO4)6 SO4 </li></ul><ul><li>Characterized by its dark blue colour. </li></ul><ul><li>Lapis Lazuli is a mixture of lazurite, calcite and pyroxenes. </li></ul><ul><li>Lazurite was formerly used for blue paint pigment and is highly valued decorative stone. </li></ul><ul><li>Natural lazurite can be considered as a sulfide bearing huyne. </li></ul><ul><li>It occurs in Lapis Lazuli in metamorphosed limestone or at contacts of pegmatites with limestone. </li></ul>
  12. 12. Cancrinite - <ul><li>Na6 Ca2 (Al Si O4)6 (CO3)2 </li></ul><ul><li>Hexagonal system. </li></ul><ul><li>Carbonate rich variety. </li></ul><ul><li>Occurs in nepheline syenites and as an alteration product of nepheline. </li></ul><ul><li>Cancrinite occurs commonly as a secondary mineral particularly replacing nepheline. </li></ul>
  13. 13. IJOLITE <ul><li>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 </li></ul>
  14. 14. Nepheline syenite <ul><li>A medium to coarse grained igneous rock consisting of essential alkali feldspar, nepheline, pyroxene and amphibole with accessory sphene, apatite, titanomagnetite, ilmenite, and zircon. </li></ul><ul><li>The rock is , in effect, a syenite which is undersaturated in silica </li></ul>
  15. 15. Nepheline Monzonite <ul><li>An undersaturated monzonite containing essential nepheline in addition to the monzonite mineral assemblage. </li></ul>
  16. 16. Nephelinite <ul><li>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 </li></ul>