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 SiO2 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:
NaAlSi2O6∙H2O (Analcime) + SiO2 = NaAlSi3O8 (Albite)
KAlSi2O6 (Leucite) + SiO2 = KAlSi3O8 (Orthoclase)
NaAlSiO4 (Nepheline) + 2 SiO2 = NaAlSi3O8 (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- Na3(Na,K)[Al4Si4O16]
Kalsilite- K[AlSiO4]
Leucite- K[AlSi2O6]
Sodalite Group
Sodalite- Na8[Al6Si6O24]Cl2
Nosean- Na8[Al6Si6O24]SO4
Haüyne- (Na,Ca)4-8[Al6Si6O24](SO4,S)1-2
Lazurite- (Na,Ca)8Al6Si6O24(SO4,S,Cl)2
Cancrinite–Vishnevite (Na,Ca,K)6-8[Al6Si6O24](CO3,SO4,Cl,OH)1-2·1-5H2O

7. NEPHELINE
Nepheline is a silica-undersaturated aluminosilicate, Na3(Na,K)[Al4Si4O16], 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[AlSi2O6]. 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
WEBSITES
• 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

15. THANK YOU