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Tectosilicates – (Framework Silicates)Infinite 3-dimensional network of (SiO4)4or (Si3Al)O81- - (Si2Al2)O82building blocksSuch minerals account for some 64% ofthe Earths crust.All oxygen atoms are shared between twoSiO44- tetrahedron.Quartz is useful as a source of Si and for silica. It is used inelectronics as an oscillator, and is pizoelectric.
(SiO2)Infinite tetrahedral network
SiO2 GroupThe SiO2 stucture is electrically neutral – it does not requireadditional cation or anion species.SiO2 – quartzt ini itsit simplesti l t form.fHowever, we discuss the “SiO2 group” because thereare nine SiO2 polymorphs.polymorphsStishoviteTetragonal4.35 g/cm3C iCoesiteMMonoclinicli i3 013.01Low (α) QuartzHexagonal (Trigonal)2.65High (β) QuartzHexagonal (Trigonal)2.53KeatiteTetragonal2.50Low (α) TridymiteMono. or Ortho.2.26High (β) TridymiteHexagonal2.22Low (α) CristobaliteTetragonal2.32High (β) CristobaliteIsometric2.20
The stable polymorph is determined by energy considerations.HighHih T formsf– greatert latticel ttienergy, lower D and RI.High P forms – closer packing,packinghigher symmetry, higher D andhigher RI.Lower the T and P the lowerthe symmetry of the SiO2p ypolymorph.pQuartz, tridymite andcristobalite are related viareconstructive inversions.Inversions between high andlow are displacive.
QQuartzC i t b litCristobalite
TridymiteCoesite
Optical PropertiesQuartzQt iis commonlyl whitehit totcolourless in hand speciman andcolourless in thin section.Low relief and low 1st order birefringence– sometimes tingedg yellow.yUniaxial (Trigonal) positive andlength fastfast.Chemically very stable mineral, andwith medium-highmedium high hardness (7)mechanically stable.Ubiquitous in the Geologicalenvironment (12% of crust).
Feldspar GroupThe feldspar group minerals are the most common silicates in the Earth’s Crust.The great majority of them can be expressed in terms to a ternary system.systemCaAl2Si2O8Anorthite (An)There is an “old” system forf ldfeldsparnomenclature,lbasedb d inicomposition.Or37-100 SanidineOr10-37 AnorthoclaseAb90-70 OligoclaseAb70-5070 50 AndesineAb50-30 LabradoriteAb50-10 BytowniteNaAlSi3O8Albite (Ab)Alkali FspKAlSi3O8Orthoclase (Or)
Physical PropertiesFeldspars show good cleavage int directionstwodi tiatt 90 .90 Often, plagioclase is lighter, towhite, in colour and the alkalifeldspars often appear to have asalmon pink colour.colourHardness is around 6-7.Found most intrusive igneous andmany thermal and regionallymetamorphosed rocks.
The structure of feldspar is similar to that of the SiO2 polymorphs,consisting of an infinite network of inter-connected, via bridging oxygenatoms tetrahedra.atoms,tetrahedraHowever, in contrast to the SiO2 group, the tetrahedra may be AlO4 aswell as SiO4.Minerals are rendered electrically neutral as a result of being “stuffed”with alkali or alkali-earth element cations in available voids.(001)(001)
Feldspars have an idealised structural formula A(T1T2)O8For unambiguous feldspar characterisation, information onfeldspar composition must be coupled with the “structuralstate” of the crystallographic lattice.statelatticeWhat does it mean – the structural state of the crystallographic lattice?Alkali Feldspars (K,Na)[AlSi3O8]MicroclineLow SanidineHigh SanidineOrthoclaseAnorthoclaseHigh albiteLow AlbiteHigh AlbiteTriclinicTriclinicTri/Mono- clinic2V 34-103 0-54 60-50
What does that all mean?Feldspars are a classic example of the Alavoidance principle.Which, itself, is measure of order in the lattice.How does orderingvary in this diagram?SandineMicroclinec oc eTemmperatureHighOrthoclaseSandineLowVery FastOrthoclaseCooling RateMicroclineMicroclineVery SlowHow doescrystallographicstructure vary withordering?What induces changei theinh structurallsymmetry?
Composition of Alkali FeldsparsThe alkali feldspars, consist oftwo endend-members,members, NaAlSi3O8 –KAlSi3O8. However completesolid solution only occurs at hightemperatures.pAreas of importance on thisdiagram include:The Liquid FieldLiquid SolidSingle Solid PhaseMiscibility Gap (immiscibility)Unmixed Phases
Fast vs Slow Cooling and Feldspar CompositionFast cooling in the alkali feldsparsystem results in growth of a onephase feldspar. These feldspars arethe“high”feldspars,withdisordered structures.Feldspars that growgro in slowslo to veryerslow cooling systems, at lowtemperatures, separate into twophases: these are “low”low feldspars,feldsparsof albite microcline compositionsand have ordered structures.The unmixed feldspars result in perthite (albite in orthoclase), or morerarely, antiperthite (orthoclase in albite).
Composition of PlagioclaseThe plagioclase feldspars, consistoff two end-members,dbNaAlSiN AlSi3O8– CaAl2Si2O8. Complete solidsolution occurs across the wholecomposition range.rangeThe phase diagram, therefore,l k slightlylooksli htl different:difftThe Liquid FieldLiquid SolidSingle Solid PhaseThe consequence is that plagioclase feldspar may have a compositionthat lies anywhere along the solid solution.
Identifying FeldsparsBoth feldspar groups are biaxial (monoclinic to triclinic – high to low).Feldspars have moderate tolow relief, low birefringence((1st order ggreys),y ) and mayy beoptically ve or –ve.Crystals are often prismatic,but can elongate, particularly inhigh-strain rocks.A poorlyl definedd fi d cleavagelmayalso be seen in some grains.Differentiating feldspars from quartz, in such examples is difficult.
TwinningTwins result when different domains of a single crystal have differentatomic orientations. The domains share atoms along a common surface.They are observed in cross polarised light only.Twins are NOT intergrowths – twin planes are continuous.Simple Twins2 domains with acommon plane of atoms.Complexp Twins 2 individual domains(polysynthetic twinning)
A more complexcomplepolysynthetic twinning isobserved in the triclinic formsof K-feldspar – so called“tartan twinning”.The different types of twins observed in different minerals are known bytheir various twin lawslaws.K-Fsp contain simple twins related bypCarlsbad,, Baveno,, albite or periclinelaws, which are defined by orientationto crystallographic lattice.Combining albite and pericline, resultsin tartan.Twinning is not restricted to thefeldspars. It is also seen in somepyroxenes, calcite, and thefeldspathoids leucite and nepheline.feldspathoids,nepheline
Leucite and NephelineThe feldspathoid group minerals are also anhydrous tectosilicates.Chemically,y, theyy are similar to feldspars,p , but contain less SiO2.Subsequently the tend to form from melts rich in alkalis (Na and K), andpoor is SiO2.Leucite (KAlSi2O6) is tetragonal, with K accommodated in large 12coordinated cavities in the structure.Nepheline (Na,K)AlSiO4) is hexagonal.While leucite is not particularly common, nepheline is found in manyalkaline rocks. Care must be taken because its optical properties are similarto those of quartz – the one exception is that it is optically negative!NB NN.B.Neitheri h mineralil iis ever foundfd iin theh presence off quartz.
Infinite 3-dimensional network of (SiO 4)4-or (Si 3 Al)O 8 1-- (Si 2 Al 2)O 8 or (Si 2-building blocks Such minerals account for some 64% of the Earths crust. All oxygen atoms are shared between twoAll oxygen atom
