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Topic 3: Feldspar Group
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups
Hassan Z. Harraz
hharraz2006@yahoo.com
2012- 20...
OUTLINE OF TOPIC 3:
 Introduction
 Structures
 Compositions
 Chemistry
 Properties
 Phase Relationships
 Feldspar R...
 This group of minerals consists of framework tectosilicates.
 Feldspar is the most common rock-forming mineral (~ 60% o...
21 November
2015
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 4
Structure
• Felsdpars are framework silicates where
...
21 November
2015
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 5
Framework silicate (feldspars): all corners shared; ...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 6
 Feldspars are tectosilicates
with every oxygen ato...
21 November
2015
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 7
Low Sanidine and Orthoclase
are more ordered. For th...
21 November
2015
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 8
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 9
Looking down c-axis
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 10
Compositions
 A second group of alumino-silicates,...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 11
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 12
Alkali Feldspars
Sanidine
The K-feldspars or alkali...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 13
Alkali Group
[Orthoclase] →[Albite]
1) Sanadine (K,...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 14
Perthite (albite exsolution in microcline)
Triclini...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 15
Limited Solid Solution and
Subsolidus Exsolution:
e...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 16
TWO-COMPONENT SYSTEM WITH SOLID SOLUTION,A EUTECTIC...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 17
Two-Component System with a Solvus
Pressure Effects...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 18
Plagioclase Feldspars Series
 Essential minerals i...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 19
Plagioclase series: complete solid solution
 Beca...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 20
Plagioclase Feldspars Series (Cont.)
 In the plagi...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 21
Properties
 Feldspar minerals are usually:
i) They...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 22
Optical and Physical Properties
Plagioclase
 Colou...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 23
Compositional Zoning
(Oscillatory)
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 24
Plagioclase Composition from Albite Twins
Albite tw...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 25
Alkali Feldspars
 Colour: Colourless
 Form: Pheno...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 26
Chemistry
 The feldspar group approximates a terna...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 27
Plagioclase Feldspar Solid Solution Series
• There ...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 28
Alkali Feldspar Solid Solution Series
1) Volcanics ...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 29
Alteration
The feldspar chemically weather to kaol...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 30
Phase Relationships
First examine the Alkali Feldsp...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 31
Alkali Feldspar: Miscibility gap
 Wrong size “bric...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 32
This diagram (Fig 6-4a) is for 1 atms and a dry mel...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 33
Remember the Plagioclase
solid solution series (An-...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 34
Eskola Experiment
• Took Ca plagioclase, added crus...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 35
This phase diagram is similar to the Forsterite-
En...
21 November
2015
Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 36
Now lets look at the 3-D version of this system :
(...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 37
Let’s look at an example of a quartz rich melt. Ini...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 38
In general:
a) Silica saturated compositions end
cr...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 39
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 40
Feldspathoids (Si-poor)
Common in Alkaline (Si-unde...
21 November
2015 41
Hydrous Tectosilicates
Analcime (Scapolite Gp)
NaAlSi2O6·H2O
Natrolite (Zeolite Gp)
Na2Al2Si3O10·2H2O
...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 42
Feldspar Resources
 Feldspar raw materials are roc...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 43
Feldspar Resources (Cont.)
a) Pegmatite
b) Feldspar...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 44
Possible substitutes of feldspar
Feldspar substitu...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 45
Feldspar Products Terminology: The supplier and us...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 46
Pegmatites:
i) Potash Feldspars:
 The presence of ...
21 November
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 47
Feldspar Beneficiations and Processing Technologies...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 48
Mineral processing
• Feldspars are either selective...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 49
In the feldspar process, one may distinguish three
...
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 50
Feldspar Application
Generally, feldspar is used i...
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 51
Glazes
Defination:
• Glazes are thin glassy coating...
21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 52
Feldspar (Naturally occurring forms of devitrified ...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 53
i) Fluxes
 The Na2O, K2O and CaO in the above form...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 54
ii) Stabilizer
 Aluminum oxide (Al2O3) (Stabilizer...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 55
iii) Glass Former
 Silica silicon dioxide (SiO2) (...
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Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 56
References
Bourne, H.L., 1994, Glass raw materials:...
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Lecture 3 feldspar group and its application

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Introduction; Structures; Compositions; Chemistry; Properties; Phase Relationships; Feldspar Resources; Possible substitutes of feldspar; Feldspar Products Terminology; Feldspar Beneficiations; Processing Technologies; Feldspar Application; Glazes;Fluxes; Stabilizer; Glass Former

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  • It is a good topic and lecture, I am in Feldspar Mining, having a very big Feldspar deposit in India, want to market world wide.mgkmines@yahoo.com
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Lecture 3 feldspar group and its application

  1. 1. Topic 3: Feldspar Group Prof. Dr. H.Z. Harraz Presentation Feldspar Groups Hassan Z. Harraz hharraz2006@yahoo.com 2012- 2013
  2. 2. OUTLINE OF TOPIC 3:  Introduction  Structures  Compositions  Chemistry  Properties  Phase Relationships  Feldspar Resources  Possible substitutes of feldspar  Feldspar Products Terminology  Feldspar Beneficiations  Processing Technologies  Feldspar Application 21 November Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 2
  3. 3.  This group of minerals consists of framework tectosilicates.  Feldspar is the most common rock-forming mineral (~ 60% of the earth’s crust) (Kauffman and Van Dyk, 1994).  The mineral name feldspar is derived from the German words feld + spar. The word "feld" is "field" in German and "spar" is a term for light colored minerals that break with a smooth surface. The feldspars are by far the most abundant group of minerals and are found in igneous, metamorphic and many sedimentary rocks and thus can be found throughout different geological environment. It is more commonly found in igneous and metamorphic rocks. Feldspar minerals are essential components in igneous, metamorphic and sedimentary rocks, to such an extent that the classification of a number of rocks is based upon feldspar content. Introduction 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 3
  4. 4. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 4 Structure • Felsdpars are framework silicates where each silica tetrahedra share all corners with its four neighbouring tetrahedra. • A portion of the tetrahedra contain Al3+ instead if Si4+. • The charge is balanced by incorporating Na+, Ca2+ or K+.
  5. 5. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 5 Framework silicate (feldspars): all corners shared; no octahedral sites.
  6. 6. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 6  Feldspars are tectosilicates with every oxygen atom shared by adjacent silicon or aluminum tetrahedra.  The tetrahedra are arranged in four-member rings that are stacked to form “crankshafts” parallel to the a-axis of the monoclinic or triclinic structure.  The crankshafts are joined together in an open structure with large voids to hold the alkali metals K+ or Na+, or the alkaline earth ion Ca++ .
  7. 7. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 7 Low Sanidine and Orthoclase are more ordered. For these minerals to be monoclinic, the center of symmetry in each ring must be preserved. At still lower temperatures, the Al+3 will be completely ordered: always on the two t1 tetrahedra. This ordering will destroy the center of symmetry and the mineral will become triclinic Microcline. High Sanidine is fully disordered with a statistically random Al-Si distribution: each tetrahedron has, on averaging over a reasonable volume, 0.25 Al atoms and 0.75 Si atoms. The Al+3 can be anywhere. Looking down the a-axis
  8. 8. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 8
  9. 9. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 9 Looking down c-axis
  10. 10. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 10 Compositions  A second group of alumino-silicates,  tetrahedra form three-dimensional frameworks with Ca, Na and K as the balancing cations  Feldspar is a common name that applies to a group of minerals with a general chemical formula of: xAl(Al,Si)3O8 where: x can be sodium (Na) and/or calcium (Ca) and/or potassium (K). Chemically, the feldspars are silicates of aluminium, containing sodium, potassium, iron, calcium, or barium or combinations of these elements. Most deposits offer sodium feldspar as well as potassium feldspar and mixed feldspars. The term feldspar encompasses a whole range of materials. The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi3O8), Albite (NaAlSi3O8) and Anorthite (CaAl2Si2O8). Compositions of major elements in common feldspars can be expressed in terms of three endmembers:  Potassium-Feldspar (K-spar) endmember : KAlSi3O8  Albite endmember NaAlSi3O8  Anorthite endmember CaAl2Si2O8  Feldspar are subdivided into:  K-Na bearing alkali feldspars and  Ca-Na solid-solution series called the plagioclase feldspars.
  11. 11. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 11
  12. 12. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 12 Alkali Feldspars Sanidine The K-feldspars or alkali felspars:  Microcline: Potassium aluminum silicate  Sanidine: Potassium sodium aluminum silicate  Orthoclase: Potassium aluminum silicate
  13. 13. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 13 Alkali Group [Orthoclase] →[Albite] 1) Sanadine (K,Na)AlSi3O8  monoclinic  high temperature  Al randomly distributed  often see carlsbad twinning 2) Microcline (K,Na)AlSi3O8  triclinic  much more ordering of Al in the structure  any heating will randomly redistribute the Al  often see “tartan” twinning
  14. 14. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 14 Perthite (albite exsolution in microcline) Triclinic K-spar “Microcline” Albite At low temperatures solid solution (ss) is unstable, ss exsolves to Albite + Microcline. We say the two phases are immiscible
  15. 15. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 15 Limited Solid Solution and Subsolidus Exsolution: e.g. Alkali Feldspar Increasing Pressure
  16. 16. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 16 TWO-COMPONENT SYSTEM WITH SOLID SOLUTION,A EUTECTICAND A SOLVUS PLAGIOCLASE AND ALKALI FELDSPAR Subsolidus Solvus  Perthitic Exsolution SOLID SOLUTION WITH A EUTECTIC
  17. 17. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 17 Two-Component System with a Solvus Pressure Effects Increasing Pressure
  18. 18. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 18 Plagioclase Feldspars Series  Essential minerals in most igneous, sedimentary, and metamorphic rocks  Complete (temperature dependant) solid solution between: Albite (NaAlSi3O8) Anorthite (CaAl2Si2O8) Minor solid solution of K+ increasing with increasing Ab content 18
  19. 19. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 19 Plagioclase series: complete solid solution  Because of the similarity in ionic radius between Na+1 (0.95 Å) and Ca+2 (0.99 Å)  Isomorphous solid solution between end members  Coupled substitution  (CaxNa1-x)(Al1+xSi3-x)O8  Ex: Ca.20Na.80(Al1.2Si2.8)O8 • Oligoclase Plagioclase Feldspars Series (Cont.)
  20. 20. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 20 Plagioclase Feldspars Series (Cont.)  In the plagioclase series, there is usually high and low temperature structural states.  At high temperatures the structure remains the same over the complete solid solution. e.g., anorthite has the same structure as high albite. This would be the case for rapidly quenched volcanic rocks.  At lower temperatures the series is often not continuous but contains structural divisions (due to position of Al in the crystal structure). An0-3, Ab97-100 low albite An3-22, Ab78-97 Peristerites An22-73, Ab27-78 Intermediate Structure An73-100 Ab0-27 Anorthite Structure Peristerites: sub-microscopic intergrowth of Na rich and Ca rich phases with alternate low albite structure and intermediate structure. Optically peristerites produce a “schiller” (iridescent) effect on cleavage surfaces.  If you reheated these low temperature inhomogeneous mixtures you would get a member of the series with high albite structure.  All the Plagioclase is triclinic but in High Albite, the position of the Al is random, while in Low Albite the Al position is more ordered and fixed.
  21. 21. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 21 Properties  Feldspar minerals are usually: i) They usually crystallise in the monoclinic or triclinic system. The crystalline structure of feldspars consists of an infinite network of SiO2 and AlO4 tetrahedra. ii) white or very light in color and iii) have a hardness of 6 on the Mohs’ Scale of Hardness and iv) perfect to good cleavage (plane of breakage) in two directions. v) Another physical property of the feldspar group is that the mineral has good cleavage in two directions. The cleavage breaks are about 86o for plagioclase feldspars (albite, oligoclase, andesine, labradorite and anorthite), at 90o for orthoclase and 89.5o for microcline. vi) Feldspar weathers to kaolin which is the main clay mineral used in ceramics and fine pottery. Feldspars are primarily used in industrial applications for their alumina and alkali content. Most of the products we use on a daily basis are made with feldspar: glass for drinking, glass for protection, fibreglass for insulation, the floor tiles and shower basins in our bathrooms, and the tableware from which we eat. Feldspar is part of our daily life.
  22. 22. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 22 Optical and Physical Properties Plagioclase  Colour: Colourless  Form: Laths or euhedral xtls  RI, Bire, 2V and Optic Figure: All vary systematically with composition.  Cleavage: 3 directions (two good)  Extinction: Varies with composition.  Twinning: Albite twinning present 22
  23. 23. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 23 Compositional Zoning (Oscillatory)
  24. 24. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 24 Plagioclase Composition from Albite Twins Albite twins in Plagioclase reveal solid solution composition. Albite Twinning
  25. 25. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 25 Alkali Feldspars  Colour: Colourless  Form: Phenocrysts or euhedral to anhedral crystals  Relief: Low  Birefrincence: Weak  Cleavage: 3 directions (2 good)  Interference figure: Biaxial negative  2VAngle: Varies with composition  Extinction: Varies with composition and axial plane  Twinning: Carlsbad, Albite and Pericline can be present
  26. 26. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 26 Chemistry  The feldspar group approximates a ternary system consisting of three components: Albite, Anorthite and Orthoclase.  There are continuous and limited solid solution between the three groups.  Plagioclase -solid solution (continuous) between Anorthite (CaAl2Si2O8) and Albite (NaAlSi3O8).  Alkali feldspars -solid solution between Albite and Orthoclase (KAlSi3O8) is limited (dependent on temperature).
  27. 27. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 27 Plagioclase Feldspar Solid Solution Series • There is a complete solid solution between Albite and Anorthite. • Intermediate phases are Oligoclase, Andesine, Labradorite and Bytownite. • The amount of water in the melt PH2O will change the crystallization range of plagioclase e.g., PH2O of 5000 bars will drop the crystallization temperature of Anorthite from 1553°C to 1234°C.
  28. 28. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 28 Alkali Feldspar Solid Solution Series 1) Volcanics -rapidly quenched, at very high temperatures get High Albite→Sanadine. It would appear to be uniform (homogeneous) in thin section. e.g: Or25 →Or60 on Fig 46. However, submicroscopically, every crystal has two components: Laths of plagioclase in a sanadine host. X-ray diffraction would show two peaks. 2) Plutonic -called PERTHITES if they can be seen (cryptoperthite appears homogeneous). They consist of Na rich plagioclase intergrowths in K rich feldspar host (Fig 34 handout).  low albite -Microcline  Or20→Or80 Perthite  most common Perthite is orthoclase or microcline host with albite lamellae. Called Antiperthite If you have Alkali feldspar in a plagioclase host.
  29. 29. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 29 Alteration The feldspar chemically weather to kaolinite. Sericite (a mica) is also an alteration product of feldspars. The feldspars often have a “dirty’ or “dusty” appearance when altered.
  30. 30. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 30 Phase Relationships First examine the Alkali Feldspar Series (Or-Ab)
  31. 31. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 31 Alkali Feldspar: Miscibility gap  Wrong size “bricks” results in alkali feldspar polymorphs  Single alkali feldspars formed at high temp exsolve (unmix) at lower temp (if slowly cooled)  Orthoclase-rich-> perthite  Albite-rich-> antiperthite • Alkali Feldspar: Miscibility gap – Because of dissimilarity in size between the Na+1 (0.95 Å) and K+1 (1.33 Å) complete solid solution occurs only at high temp
  32. 32. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 32 This diagram (Fig 6-4a) is for 1 atms and a dry melt (no H2O). a) There is a minimum temperature E (eutectic) that all melts move towards. Example: Melt of composition O cools and hits the liquidus at point P. Start to crystallize crystals of composition C. As cooling continues the melt becomes more Ab rich. The final product is homogeneous crystals of phase D. Only a melt of eutectic composition would ever crystallize E. b) What happens when the temp drops below the solidus? There is no longer complete miscibility in the alkali feldspar series. c) At lower cooling temperatures we get immiscing in the middle range of feldspar compositions into 2 separate solid phases. This occurs below the area called the solvus and where immiscing occurs, defines the solvus line. d) Any quick cooling would freeze the reactions and immiscing would not occur. e) The exsolution of K rich phase results lamellae. The slower the cooling, the more time for diffusion and exsolution and the larger the lamellae. Figure 6-4b and Figure 20. When a melt contains H2O the fields of the phase diagrams change drastically. i) The leucite field disappears with increasing H2O. Only occurs in volcanics (quenched). ii) Crystallization temperatures are reduced. iii) Composition changes continuously along liquidus-solidus during cooling. iv) On Figure 6-4b a liquid L cools to N. At point N crystals of M form. The liquid cools and changes to E while the crystals change to composition D. This is the limit of the solid solution.  At E, the eutectic you get crystallization of two solids D and F until all the melt is used up. Cooling continues. The composition of 2 stable, co-existing phases would occur along the solvus from D→G→J and F→H→K. Na and K end members. Get lamellae. Perthite intergrowths should occur, but are dependent on the kinetics of diffusion of K and Na ions in the solid media. Kinetics are slow with lower temperatures. Slow cooling in nature usually does not produce perthitic intergrowths. Usually two separate feldspars will occur.
  33. 33. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 33 Remember the Plagioclase solid solution series (An-Ab), Figure 4-4. • If water is added, the whole curve is depressed by 300°C. If pressure fluctuates, you get zoned crystals Ca Na
  34. 34. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 34 Eskola Experiment • Took Ca plagioclase, added crushed SiO2, H2O and soda (NaHCO3) and heated to 300°C for three days. • The result was that much of the plagioclase had converted to albite. • Therefore, in many low temperature and metamorphic rocks, albite is a stable end member. [Implications to hydrothermal, seawater regional metamorphism]. • Ca plagioclase is the high temperature end member.  Now lets look at the system NaAlSiO4-KAlSiO4-SiO2 There is no solid solution between these end members. They are simple binary eutectic systems.
  35. 35. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 35 This phase diagram is similar to the Forsterite- Enstatite-Silica system we looked at before (previous handout Figure 13-7). Depending on the original composition we can end up with K- feldspar and Quartz or K-feldspar and Leucite. BUT, we can not end up with Leucite and Quartz because they are not stable together (the same way olivine and quartz can not occur together). Note, however, that there is the complication of incongruent melting in the Leucite-K-feldspar portion of the right diagram. Point R is a Peritectic. Leucite is not stable in the presence of melt below 1150°C and will undergo the reaction: KAlSi2O6 + SiO2 → KAlSi3O8 (leucite) (quartz) (k-feldspar)
  36. 36. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 36 Now lets look at the 3-D version of this system : (NaAlSiO4-KAlSiO4-Si2O) Examine Figure 8-13 and note some of its complications: 1) Structure is temperature dependent (i.e., from orthoclase to albite). 2) As discussed before, there is limited solid solution at low temperatures but almost complete solid solution at high temperatures. Under low pressure, dry conditions, can get quite a large portion of leucite (Figure 8-14).
  37. 37. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 37 Let’s look at an example of a quartz rich melt. Initially tridymite (quartz) crystallizes. The composition moves along the line to a minimum point where feldspar +tridymite precipitate at T1. After this temperature, the phases are governed by what we say in Figure 6-4 or Figure 20 (ie are pressure PH2O dependent). As temperature drops and moves along the hoop shaped solvus, lamellae form and change composition. This is dependent on ion mobility and temperature. In plutonic conditions (slow cooling) formation of perthite is favoured. If the PH2O is raised, the solidus and liquidus are depressed and the leucite field would shrink and disappear at 5000 atm (Figure 8-14). Also, the addition of iron into the system would cause pyroxene or iron oxides to crystallize (Figure 8-15).
  38. 38. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 38 In general: a) Silica saturated compositions end crystallization at the minimum along the SiO2-Feldspar join (Peralkaline, oversaturated rocks). b) Silica undersaturated compositions end crystallization at the Ternary Eutectic (syenites and trachyte) (Figure 8-12).
  39. 39. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 39
  40. 40. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 40 Feldspathoids (Si-poor) Common in Alkaline (Si-undersaturated) igneous rocks Leucite – KAlSiO4 Nepheline – (Na,K)AlSiO4 Sodalite – Na8(AlSiO4)6Cl2
  41. 41. 21 November 2015 41 Hydrous Tectosilicates Analcime (Scapolite Gp) NaAlSi2O6·H2O Natrolite (Zeolite Gp) Na2Al2Si3O10·2H2O Heulandite (Zeolite Gp) CaAl2Si7O18·6H2O Stilbite (Zeolite Gp) NaCa2Al5Si13O36·14H2O Scapolite
  42. 42. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 42 Feldspar Resources  Feldspar raw materials are rocks with the prevalent portion of minerals of the feldspar group or their mixtures in such a form, quantity and quality, which allow their industrial processing.  Feldspars are a group of monoclinic (orthoclase, sanidine) and triclinic (microcline, plagioclases) potassium and sodium-calcium alumosilicates, and together with quartz they represent the most common rock forming minerals.  For industrial use are suitable are potassium feldspars (orthoclase, microcline) and acid plagioclases (albite, oligoclase, andesite).  Suitable feldspar resources are dyke rocks (pegmatites, aplites), igneous rocks (granites) and sediments (feldspar bearing sands and gravel), eventually also residues of incompletely kaolinized rocks. The major impurities are high content of iron in the feldspar structure (unremoveable) or in the form of admixture (removeable).  Most often, commercial feldspar are mined from pegmatite or feldspatic (sand) Placer deposits.  Aplite, which is a fine-grained igneous rock with the same mineralogical composition as granite, also is mined frequently for its feldspar content.  Alaskite is a granitic rock with few or no dark minerals, and a texture ranging from granitic to pegmatic. The average mineral composition of the alaskites is oligoclase feldspar (40%), quartz (25%), microcline feldspar (20%), and muscovite mica (15%). Alaskite also contains minor accessory minerals such as biotite, garnet, epidote, apatite and pyrite.  Another source of feldspar is phonolite, the effusive equivalent of nepheline syenite, which contains feldspathoids and biotite? Phonolite is produced commercially in West Germany.  In the same country, rhyolite is also exploited for feldspar.  Amongst the numerous rocks in which they are present, feldspars are particularly abundant in igneous rocks like granite, which contains from 50 to 70% of alkaline feldspar. Granite is however rarely used for its feldspatic content.
  43. 43. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 43 Feldspar Resources (Cont.) a) Pegmatite b) Feldspar (sand) placer c) Aplite d) Nepheline Syenite e) Phonolite f) Granite Feldspar from Pegmatite: The initial production of feldspar was from pegmatite bodies. Pegmatites are bodies of rock commonly of granitic composition that consist mainly of unusually large crystals or masses of quartz, feldspar, and mica. Some pegmatites also contain potentially economic minerals such as fluorspar, beryl, and spodumene. The constituent minerals may occur segregated in zones, an advantage in commercial extraction. Pegmatites crystallize during the last stages of injection of granitic magma. The magmatic fluids are rich in water and cool so slowly that the crystals grow larger than usual. The grain size can range from less than an inch to more than a foot but rarely greater than three feet. Pegmatites range in size from small lens, one to a few feet in thickness, to large tabular bodies tens of feet thick and hundreds of feet long. Past production came from the larger pegmatites. Mining was labour intensive. Ore was hand cobbed (hand-size pieces are broken by hammer) and sorted by hand. Conventional open-mining methods including removal of overburden, drilling and blasting, loading, and transport by trucks are used to mine ores containing feldspar. The ore is crushed by primary and secondary crushers and ground by jaw crushers, cone crushers, and rod mills until it is reduced to less than 841 µm (20 mesh). A froth flotation process is used for most feldspar ore beneficiation. Feldspar from Placer deposits: Beach sand and alluvial deposits may contain economic quantities of feldspar, and are mine in Spain. Under certain conditions feldspar decomposes to koalinitic clay; if the alteration is only partial, the deposits may contain feldspar, kaolin and quartz.  Feldspar-quartz mixture  Potassium feldspar  Sodium feldspar
  44. 44. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 44 Possible substitutes of feldspar Feldspar substitutes are materials having alkali metals confined to other minerals than feldspars, like nepheline syenites or nepheline phonolites (in the Czech Republic). These replace feldspars as a melting agent. In other applications (fine abrasives, filler in rubber, plastics and paints), feldspars can be replaced by bauxite, corundum, diatomite, garnet, magnetite, nepheline syenite, olivine, perlite, pumice, silica sand, staurolite, ilmenite, barite, kaolin, mica, wollastonite, calcined alumina hydrate, clays, talc, spodumene, pyrophyllite or their mixtures.
  45. 45. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 45 Feldspar Products Terminology: The supplier and user of feldspar worldwide normally have agreed on a standardized definition of the feldspar products which are placed in the world market.  According to that harmonised definition, feldspar products are aluminosilicates with:  SiO2 content > 58%,  Alkali content (Na2O+K2O) > 2%,  Alumina content (Al2O3) > 3% and  loss on ignition < 3%. Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content. On those elements we can distinguishThree families: 1) Feldspatic sand: with an alkali content (Na2O+K2O) < 6% or alumina content (Al2O3) < 8% 2) Pegmatite: with an alkali content (Na2O+K2O) >6% and alumina content (Al2O3) comprised between 8 and 14% 3) Feldspar: with an alkali content (Na2O+K2O) >6% and alumina (Al2O3) content > 14%. A further distinction can be made between Sodium, Potassium and mixed feldspars, depending on the type of alkali they contain. Note:  For the same purposes there are also used feldspar substitutes, which are rocks with alkali metals confined to some other minerals (mostly nepheline - anhydrous sodium-potassium alumo-silicate).  Nepheline syenites are particularly used abroad to substitute for feldspar raw materials.
  46. 46. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 46 Pegmatites: i) Potash Feldspars:  The presence of potash feldspar in a glaze or clay body has a more refractory effect on the ceramic surface compared to equivalent amounts of soda feldspar.  Although potash feldspar actually begins its melt at a lower temperature than soda feldspar, once the melt begins, the formation of leucite crystals causes a slower and more viscous flow. ii) Soda Feldspars:  Kona F-4 and NC-4 feldspars contain a fair amount of potassium oxide, and their total sodium content is not as high as the total content of potassium in potash feldspars.  These feldspars are hybrids that incorporate some qualities of both potash and soda feldspars.  This is especially evident when they’re compared to stronger sodium materials, such as nepheline syenite. Hence, it’s often possible to substitute some soda feldspars for potash feldspars without causing a dramatic surface change. Nepheline Syenite:  Nepheline syenite is a low-silica, high-soda, high-alumina mineral referred to as a feldspathic rock.  The fluxing power and shrinkage rate of nepheline syenite depends on the grade number.  The finest grades (A400 and 700) have the greatest melting power and shrinkage rate and are used in electrical porcelain and by manufacturers of ceramic wares.  Grade A270 has a medium melting and shrinkage rate and is the most commonly used form of nepheline syenite in ceramic studios and schools.  Grades A40-A200 (used by glass manufacturers) are the coarsest grades and produce the lowest melting and shrinkage rates. Cornwall stone (feldspathic rock):  Cornwall stone, also a feldspathic rock, contains more silica and less melter oxides than do the feldspars. Since silica has a high melting point, Cornwall stone has a higher melting temperature than the feldspars and appears stiffer and less melted when fired alone to stoneware temperatures.  This is especially apparent at the lower stoneware temperatures. Even the potash feldspars show more fusion at the cone 5-6 oxidation firing temperatures than does Cornwall stone, so this would not be a first choice as a glaze core at these firing temperatures unless a stiffer surface is desired.
  47. 47. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 47 Feldspar Beneficiations and Processing Technologies  Feldspars are either selectively mined or processed by flotation and/or magnetic separation, in order to remove the accessory minerals (e.g. quartz, mica, rutile, etc.) present in the ore.  The feldspar may then undergo a milling step which allows to adapt the particle-size to the intended use.  The degree of refining and possible milling is very dependent upon the final use of the product.  For a number of uses, it is perfectly acceptable, and even advantageous, that the product retains some accessory minerals, e.g. quartz, while at the other extreme some applications require extremely pure and fine-grounded grades.  Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content.  Normally feldspar concentrates should meet the following requirements:  To maintain a proper ratio of (K2O+Na2O) to SiO2  To be low in iron content
  48. 48. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 48 Mineral processing • Feldspars are either selectively mined or processed by optical, flotation and/or electrostatic separation, in order to remove the accessory minerals (e.g. quartz, mica, rutile, etc.) present in the ore. The feldspar then undergoes a comminution step. The degree of refining and possible comminution is very dependent upon the final use of the product. For a number of uses, it is perfectly acceptable, and even advantageous, that the product retains some accessory minerals, e.g. quartz, while at the other extreme some applications require extremely pure and fine grounded grades. Basically, the two properties which make feldspars useful for downstream industries are their alkali and alumina content. • The flotation process is only used by AKW, INCUSA, and SP Minerals. The feldspar recovered by flotation only represents about 10 % of the European feldspar production. The flotation process is essential to get a high quality grade (low iron content and high alumina content) required for some specific and important applications (e.g. TV/computer screens). For instance, although the Italian producer Maffei is the biggest producer in Europe, the three abovementioned companies supply the Italian market with these high quality grade products. • The essential use of the flotation process may be explained by the following figure: In Sections I and III a primary mechanical separation (hydrocycloning, centrifugation) can be achieved. Figure Feldspar particle vs. recovery graph [39, IMA, 2002] In Section II, either optical, flotation or electrostatic separation can be used to separate feldspar from quartz, depending on both the intrinsic characteristics of the raw material, and the final product requirements.
  49. 49. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 49 In the feldspar process, one may distinguish three different flotation steps, namely the micas flotation, the oxides flotation, and the feldspar flotation. Each of these requires a different reagent regime. The following flow sheet shows the steps involved in the recovery of feldspar.
  50. 50. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 50 Feldspar Application Generally, feldspar is used in the manufacture of glass products (70%), in ceramics and other products (30%). Feldspar is also used Filler (in paint, in mild abrasives, urethane, latex foam, and as a welding rod coating). In the manufacture of ceramics, feldspar is the second most important ingredient after clay. Feldspars are an important glaze raw material used as the main flux in ceramic industries. Feldspar is a source for the simultaneous introduction of SiO2, Al2O3, Na2O, K2O and CaO and is the most suitable material for introducing alkaline oxides into glazes. Feldspars are used as fluxing agents to form a glassy phase at low temperatures and as a source of alkalis and alumina in glazes. Feldspars play an important role as fluxing agents in ceramics and glass applications, and also are used as functional fillers in the paint, plastic, rubber and adhesive industries. Because of their low melting point, feldspars are used as a melting agent in ceramic mixtures, glass batches, glazes, enamels and also as casting powders in the last years. Feldspar does not have a strict melting point, since it melts gradually over a range of temperatures. This greatly facilitates the melting of quartz and clays and, through appropriate mixing, allows modulations of this important step of ceramic making. Feldspars melt at ~1150oC. The feldspathic glass they produce surrounds the refractory clay particles and fills up the pores between them. Due to the free fluxes they contain, feldspathic glasses will also bind to the surfaces of the refractory particles thus helping to bind the ceramic body together. The more feldspathic glass a ceramic body contains, the denser the fired body will be. They improve the strength, toughness, and durability of the ceramic body, and cement the crystalline phase of other ingredients, softening, melting and wetting other batch constituents.
  51. 51. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 51 Glazes Defination: • Glazes are thin glassy coatings usually 0.15 to 0.5 mm thick formed in place on a ceramic body, after blending the raw materials, spreading the mixture on the surface and firing at a high temperature. Important and uses: • Glazes are usually applied to make the bodies non- porous, smooth, glossy, mechanically stronger and chemically more resistance. • They improve the aesthetic appearance of ceramic ware glazes are required to fit different ceramic bodies, to mature at different temperatures and to exhibit various specific properties, which explains the great variety of grazes.
  52. 52. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 52 Feldspar (Naturally occurring forms of devitrified glass) Potassium Feldspar (Orthoclase or microcline) K2O Al2O3 6SiO2 Sodium Feldspar (Albite) Na2O Al2O3 6SiO2 Lime Feldspar (Anorthite) CaO Al2O3 2SiO2 Flux Stabilizer Glass Former Each of the three components of feldspar is discussed below
  53. 53. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 53 i) Fluxes  The Na2O, K2O and CaO in the above formulas are called alkaline metal oxides because they are strong bases when added to water. These oxides are used as fluxes.  Fluxes have very active molecular structures at high temperature, and they attach to and combine with the surface molecular structure of otherwise hard crystalline materials, causing the surface molecules in the crystals to "dissolve".  This exposes deeper layers of the crystal to the dissolving action of other flux molecules and so on until the entire crystal melts away. In other words, fluxes cause crystalline structures to melt at lower temperatures than would otherwise be possible, a bit like water melts a cube of sugar at room temperature.  Without fluxes present, none of the other constituents in the ceramic body would be able to melt at normally attainable temperatures, and the fabrication of pottery would have been beyond the reach of prehistoric peoples.
  54. 54. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 54 ii) Stabilizer  Aluminum oxide (Al2O3) (Stabilizer).  Aluminum oxide exists in two separate forms within clay and porcelain bodies.  When chemically combined in molecular form with the other constituents of feldspars, aluminum oxide acts as a stabilizer, and is a part of the glass melt.  Aluminum atoms can bond with silicon via a shared oxygen atom and can thus be an integral part of the amorphous silicon matrix. In this form, it does NOT affect the transparency of the glass.  However, aluminum oxide is also added to clays as a separate constituent in the form of kaolinite. Because of the large amount of flux contained in the feldspar, some of the kaolinite also melts into a glass, like the feldspar itself. But the by-product left over when the kaolinite melts is a precipitate of pure crystalline aluminum oxide called alumina.  The alumina crystals remain unmelted (i.e., they are refractory particles) and scattered throughout the glass melt, and in this form, aluminum oxide causes the glass to become cloudy or opaque.
  55. 55. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 55 iii) Glass Former  Silica silicon dioxide (SiO2) (Glass Former)  Silica is silicon dioxide, The SiO2 portion of the feldspar formulas shown above.  Like alumina, silica also exists in two entirely separate forms within clay and porcelain bodies.  When chemically combined with flux and aluminum oxide, as it is in feldspar, silica exists as a molecular component in the amorphous melted glass gel.  Silica also exists as unmelted crystalline particles of quartz scattered throughout the glass melt. This form is part of the refractory substructure which supports clay and porcelain bodies.
  56. 56. 21 November 2015 Prof. Dr. H.Z. Harraz Presentation Feldspar Groups 56 References Bourne, H.L., 1994, Glass raw materials: in Carr, D.D. and others, eds., Industrial minerals and rocks (6th edition): Littleton, CO, Soc. for Mining, Metallurgy and Exploration, Inc., p. 543-550. Kauffman, R.A. and Van Dyk, D., 1994, Feldspars: in Carr, D.D. and others, eds., Industrial minerals and rocks (6th edition): Littleton, CO., Soc. for Mining, Metallurgy, and Exploration, Inc., p. 473-481. Lesure, F.G., 1968, Mica deposits of the Blue Ridge in North Carolina: U.S. Geol. Sur. Prof. Paper 577, 129 p. Potter, M.J., 1991, Feldspar, and Nepheline Syenite, and Aplite: Annual Report, US Dept. of Interior, US Bureau of Mines. Potter, M.J., 1996, Feldspar and Nepheline Syenite: Minerals Yearbook, US Dept. of Interior, US Geological Survey; (//minerals.er.usgs.gov/minerals). Potter, M.J., 1997, Feldspar: Mineral Commodity Summaries, US Dept. of Interior, US Geological Survey; (//minerals.er.usgs.gov/minerals). USM 1993, Short Course on : Industrial Processing of Kaolin, quartz/silica sand and Feldspar, 20th-22nd. April 1993, School of Materials and Mineral Resources Engineering in Collaboration with The Department of Mining and Metallurgical Engineering, University of Queensland, Australia.

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