UNDERSTANDING of FELDSPATHOID.pptx

UNDERSTANDING of FELDSPATHOID

1. UNDERSTANDING FELDSPATHOIDS AN EXPLORATION OF ALKALI MINERALS JAYASHREE SAHOO AND NILADREE SEKHAR SAHA

2. INTRODUCTION The feldspathoids are family of rock-forming minerals consisting of aluminosilicates of sodium, potassium, or calcium and having too little silica to form feldspar. There is considerable structural variation, so it is not a true group. Feldspathoids take the places of feldspars in igneous rocks that are undersaturated with respect to silica or that contain more alkalis and aluminium than can be accommodated in the feldspars. Feldspathoids commonly occur with feldspars, and also with amphiboles, olivine, and pyroxenes, but never with quartz or other polymorphs of silica.

3. CRYSTAL STRUCTURE AND CHEMICAL COMPOSITION The structure of the feldspathoids is similar to the feldspars and consist of four and six-member rings linked laterally to form a three-dimensional tetrahedral framework. The presence of six-member rings requires that the structures be somewhat more open than the feldspars, so the specific gravity of these minerals is less. They have compositions that are, for the most part, equivalent to silica-deficient feldspars. These minerals commonly crystallize from magmas that are relatively low in SiO 2  or that contain more Na, K, and Al than can fit into feldspars. These minerals have large openings in their atomic arrangements that allow the minerals to contain significant amounts of large anions and molecular anions, including chlorine, carbonate, and sulfate.

4. OCCURRENCE Feldspathoids may occur along with feldspars in igneous rocks. They do not occur in igneous rocks containing original free silica i.e.,  in rocks that contain quartz of the same generation. They are restricted to quartz-free rocks because they will react with quartz to produce feldspars by reactions such as: NaAlSi 2 O 6 ∙H 2 O (Analcime) + SiO 2  = NaAlSi 3 O 8  (Albite) KAlSi 2 O 6  (Leucite) + SiO 2  = KAlSi 3 O 8 (Orthoclase) NaAlSiO 4  (Nepheline) + 2 SiO 2  = NaAlSi 3 O 8 (Albite)

5. CLASSIFICATION OF PHANERITIC IGNEOUS ROCKS A QAPF diagram is a double quasi-ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid ( Foid )".  Foid , a contraction of the term  feldspathoid , is applied to any igneous rock containing up to 60% modal feldspathoid minerals. For example, a syenite with significant nepheline present can be termed a nepheline bearing syenite or a nepheline syenite, with the term nepheline replaceable by any foid mineral. Such terminology is used in the QAPF classification of igneous rock.

6. VARITIES The most common rock-forming feldspathoids that occur in magmatic igneous rocks are: Nepheline Group Nepheline-  Na 3 (Na,K)[Al 4 Si 4 O 16 ] Kalsilite- K[AlSiO 4 ] Leucite- K[AlSi 2 O 6 ] Sodalite Group Sodalite- Na 8 [Al 6 Si 6 O 24 ]Cl 2 Nosean- Na 8 [Al 6 Si 6 O 24 ]SO 4 Haüyne- (Na,Ca) 4-8 [Al 6 Si 6 O 24 ](SO 4 ,S) 1-2 Lazurite- (Na,Ca) 8 Al 6 Si 6 O 24 (SO 4 ,S,Cl) 2 Cancrinite – Vishnevite  (Na,Ca,K) 6-8 [Al 6 Si 6 O 24 ](CO 3 ,SO 4 ,Cl,OH) 1-2 ·1-5H 2 O

7. NEPHELINE Nepheline is a silica-undersaturated aluminosilicate, Na 3 ( Na,K )[Al 4 Si 4 O 16 ] , that occurs in both intrusive and volcanic rocks with low silica, and in their associated pegmatites . It is often found in mica schist and gneiss. An important determinative character of nepheline can be decomposed by hydrochloric acid, with separation of gelatinous silica and cubes of salt. Colorless hexagonal prisms of nepheline. Capo di Bova, Via Appia Antica, Rome, Italy. Physical properties • Crystal system – Hexagonal • Colour- Colourless, white, purplish brown, yellowish, dark green • Cleavage- Perfect prismatic, and poor basal. • Fracture- Subconchoidal • Lusture - Vitreous or greasy • Hardness-  5.5 - 6 • Specific Gravity- 2.6+ (average) • Streak- white

8. Optical properties • Color : colorless or rich in inclusions (clouded aspect) • Form: hexagonal or stubby prismatic section • Shape: anhedrical , short prismatic hexagonal • Cleavage: poor basal (001) • Interference colors : first order grey • Relief: low. • Birefringence: Very low 0.003 - 0.006 • Optical Sign: Uniaxial (-) • Alteration- Nepheline may alter to either clay minerals or analcime or sodalite or cancrinite by the addition of water, silica or other volatiles Kalsilite: Kalsilite and nepheline have similar structure, habit, and optical properties. This makes the recognition of kalsilite from nepheline extremely difficult without chemical analyses. Kalsilite is a very rare feldspathoid. Nepheline crystal in a foidite from Cape Verde. PPL image , 10x (Field of view = 2mm Nepheline crystal in a foidite from Cape Verde. XPL image , 10x (Field of view = 2mm

9. LEUCITE Leucite (from the Greek word leukos meaning white) is a rock-forming mineral composed of potassium and aluminium tectosilicate K[AlSi 2 O 6 ] . Small amounts of Al may replace K. Leucite is common in some volcanic rocks in which it crystallises with a cubic crystal structure at high temperature (ca. 900°C), forming isometric trapezohedral crystals. Upon cooling to 700-600°C, it transforms into a tetragonal modification which is stable at room temperature, and forms characteristic polysynthetic twin lamellae. The transformation is reversible. For the presence of this mineral, it is necessary that the silica percentage of the rock should be low, since leucite is incompatible with free quartz and reacts with it to form potassium feldspar. Because it weathers rapidly, leucite is most common in lavas of recent and Tertiary age.

10. Physical properties • Crystal system – Tetragonal, pseudo-cubic; cubic at temperatures of 500-600°C • Form and Habit - {211} trapezohedron form common; less common as cube {100} and rhombic dodecahedron {1l0}; also found as disseminated grains • Colour- White or pale grey • Shape: polygonal crystals (or sub-rounded) • Cleavage- Very poor on {11O} • Fracture- Subconchoidal • Lusture - Vitreous on fracture surfaces • Diaphinity - Translucent to opaque • Hardness-  5.5 - 6 • Specific Gravity- 2.47-2.50 • Streak- white Colorless pseudocubic crystal showing trapezohedral faces. Alban Hills, Rome Province, Latium, Italy. Leucite crystals. Poggio Nibbio , Vico Lake, Viterbo Province, Latium, Italy.

11. Optical properties • Color : colorless • Form: Normally forms trapezohedron crystals, with octahedral to roughly circular outline in thin section • Cleavage: poor • Interference colors : Very low, no more than dark first-order gray • Relief: low • Birefringence: Isotropic to low • Optical Sign: Mostly isotropic, but may be uniaxial - ve . Sign impossible to obtain because of twinning • Twinning - Repeated twinning on {110} is common as a type of cross-hatching under XPL • Alteration- Mixture of nepheline and feldspar, called pseudo-leucite, can replace leucite in some rocks. Leucite phenocryst. Vulsini volcano, Italy. PPL image, 10x (Field of view = 2mm) Leucite phenocryst. Vulsini volcano, Italy. XPL image, 10x (Field of view = 2mm)

12. USES While not as widely utilized as common feldspars, feldspathoids have niche applications and play essential roles in geological studies and certain industries. Feldspathoids, such as nepheline, leucite, sodalite, and cancrinite, have various uses: 1. Building Materials 2. Gemstones 3. Catalysts 4. Decorative Stones 5. Ceramics 6. Scientific Research 7. Mineral Collecting

13. CONCLUSION In conclusion, feldspathoids represent a fascinating group of minerals with distinctive structures and diverse geological implications. From their role in the formation of alkali-rich igneous rocks to applications in construction materials, gemstones, and catalyst research, feldspathoids contribute significantly to both the natural world and various industries. Understanding their unique properties enhances our comprehension of Earth's geological processes and enriches fields ranging from materials science to mineralogy. As we delve deeper into the intricate world of feldspathoids, their significance becomes ever more apparent, bridging the realms of scientific inquiry and practical applications.

14. REFERENCES W EBSITES Feldspathoid: https://en.wikipedia.org/wiki/Feldspathoid - Wikipedia Feldspathoid: https://www.britannica.com/science/feldspathoid - Britannica Feldspathoids: https://geologyistheway.com/minerals/feldspathoids/ - Geology is the way Feldspathoid: https://www.alexstrekeisen.it/english/vulc/feldspathoid.php - Alessandro Da Mommio BOOKS Introduction to Mineralogy – William . D . Nesse Mineralogy - Dextral Parkins An Introduction to the Rock-Forming Minerals – Deer, Howie & Zussman

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