Phyllosilicate minerals. Serpentine group Clay minerals group Mica group Chlorite group

Transcription

1 Phyllosilicate minerals Serpentine group Clay minerals group Mica group Chlorite group

2 Phyllosilicate Phyllosilicate is characterised by extended sheet of SiO 4 tetrahedra of which 3 in 4 oxygens are joined with adjacent tetrahedra leading to Si:O ratio of 2:5 Minerals in this group are generally platy or flaky habit with one perfect cleavage. They are soft, rather low SG, may exhibit flexibility or even elasticity. Most minerals in this group contain (OH) group in centre of 6- fold rings of tetrahedra at same height as unshared apical oxygens of SiO 4 tetrahedra.

3 External ions joined to Si 2 O 5 sheet by coordinated to two oxygens and one (OH). Size of triangle between two oxygens and one (OH) is nearly the same as triangle face of XO 6 octahedron (X = Mg or Al). Therefore, XO 6 can join to (Si 2 O 5 OH) 3- by fit its triangle face to triangle spaced. As a result, lizardite or kaolinite structure is formed.

4 If cations in octahedral sheet are divalent (Mg, Fe 2+ ), it has brucite geometry also known as trioctahedral. Likewise, if cations are trivalent as in corundum and gibbsite structure, also known as dioctahedral. It is, therefore, employed for classified structure of phyllosilicate into two types.

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6 Brucite, Mg(OH) 2, consists of two (OH) planes between Mg is coordinated in octahedra..if two (OH) groups are replaced by two apical oxygen of Si 2 O 5 sheet, then we get... This means that other side of brucite structure is not attached to Si 2 O 5 sheet. This structure present in lizardite, Mg 3 Si 2 O 5 (OH) 4, structure. Equivalent structure with dioctahedral sheet is kaolinite, Al 2 Si 2 O 5 (OH) 4. These structure are made of one tetrahedral t and one octahedral o sheet (t-o layers). The arrangement is clue for classifying phyllosilicate structure, which leads to t-o layer and t-o-t layers.

7 These t-o layers are electrically neutral and linked to each other by van der Waals bonds, which is very weak bond and lead to excellent cleavage, easy gliding, and greasy feeling. Substitution of Al for Si in tetrahedral sites of Si 2 O 5 sheets cause free electrical charge (HOW?) occurred on surface of t-o-t layer. This electrical charge could be neutralised by introducing of univalent cations (K & Na) as interlayer cation. As a result, structure is more tighten together, easiness of gliding is minimised, hardness is increased, and slippery feel is lost. Trioctahedral Dioctahedral KMg 3 (AlSi 3 O 10 )(OH) 2 Phlogopite KAl 2 (AlSi 3 O 10 )(OH)2 Muscovite

8 If Al substitute for Si up to half of Si in tetrahedral sites of Si 2 O 5, two charges for t-o-t layers become available and it can link interlayer cation like Ca 2+, Ba + (smaller degree). It is directly affect physical properties of micas e.g. strongly linked by ionic bond decrease quality of cleavage, hardness is increased, flexibility seems to totally gone, density is higher it is known as brittle micas

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12 Antigorite, Lizardite and Chrysotile: Mg 3 Si 2 O 5 (OH) 4 Hexagonal and monoclinic, where orthorhombic polymorphs of antigorite and chrysotile are also found. Xtals, except as pseudomorphs, unknown. Antigorite and lizardite are commonly massive and fine-grained; chrysotile is fibrous. Chrysotile asbestos occurs in very narrow tubular fibers, often hollow. Serpentine group Garnierite (Ni,Mg) 3 Si 2 O 5 (OH) 4 is apple-green Ni-bearing serpentine formed as alteration product of Ni-rich peridotites.

13 Fe and Ni may substitute Mg, where Al for Si. Antigorite (trioctahedral) structure is analogous to kaolinite (dioctahedral). The mistif in its structure cause corrugated layer parallel to {001} Chrysotile mistif is responsible for curvature of t-o layer forming cylindrical tubes. Lizardite, matrix material in serpentine, is extremely fine-grained and exhibits platy morphology. Recognized by its variegated green colour and its greasy lustre or fibrous habit. Serpentine is commonly found as alteration of Mg-silicates, particularly, olivine, pyroxene, and amphibole. Example for olivine; 2Mg 2 SiO 4 + 3H 2 O Mg 3 Si 2 O 5 (OH) 4 + Mg(OH) 2

14 Frequently associated with magnesite, chromite, and magnetite. Found in both igneous and metamorphic rocks, usually in disseminated particles or make up the whole rock mass. Serpentine as a rock name, is applied to rock masses made up mostly of mixture of fine-grained antigorite and lizardite. Use as ornamental stone and dimension stone. Verde antique marble is beautiful variegated colour of serpentine mixed with white marble. Bowenite is transparent yellowgreen variety of serpentine used as jade substitute. Chrysotile, as asbestos, fibrous and its flexibility nature allows it to be made into felt or woven into clothes and other fabrics for fire-proofing and insulators agains heat and electricity

15 Yellowish green, fibrous antigorite from East Broughon, Quebec, Canada Dave Barthelmy

16 Vein filling lizardite, Trinitiy ultramafic complex, California

17 Asbestiform of serpentine: chrysotile also known as white asbestos

18 Clay mineral group Clay refers to naturally occurring material composed of fine-grained materials, which are plastic at appropriate water contents and will harden when dried or fired. XRD techniques shown group of crystallin substances, clay minerals, which are essentially hydrous aluminum layers of silicates. Mg or Fe may substitute for Al; alkalies or alkali earths may present as essential components.

19 Kaolinite: Al 2 Si 2 O 5 (OH) 4 Triclinic, very minute, thin, rhombic or hexagonal plates, usually claylike masses, either compact or friable Little variation in chemical composition (Al 2 O , SiO , H 2 O 14%). It consists of Si 2 O 5 sheet bonded to a gibbsite sheet. Dickite and nacrite are similar compositions but exhibit different way of t-o arrangement. Recognised by its claylike structure, but cannot be distinguished from other clay minerals without X-Ray test.

20 Common constituent of kaolin or clay. Secondary mineral formed by weathering or hydrothermal alteration of aluminum silicates, particularly feldspar. Found mixed with feldspar in under gone weathered rocks and could form entire deposits where completely altered. One of common product of decomposition of rocks found in soils and transported by water to deposit with other material like quartz formed clay beds.

21 Use as raw material for producing brick, paving brick, drain tile, and sewer pipe. High-grade clay, china clay or kaolin, has many uses in addition to produce china and pottery, including filler in paper, rubber industry, and manufacture of refractories. The great benifit of clay for ceramic products is owing to special properties that it can be molded easily when wet, and when it is heated, some water is driven off, resulting hard and durable surface

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23 Similar species including dickite and nacrite, halloysite that shows two forms: one with kaolinite composition (Al 2 Si 2 O 5 (OH) 4 ), one with Al 2 Si 2 O 5 (OH) 4.2H 2 O. Second type dehydrates give the first type with loss of interlayer water molecules. Smectite group refers to t-o-t layers clay of both dioctahedral and trioctahedral group that show special capacity of absorbing water molecules between sheets, leading to expansion in structure. Dioctahedral members are montmorillonite, beidelite, and nontronite. Trioctahedral members are hectorite and saponite.

24 Montmorillonite is dominant clay mineral in bentonite, altered volcanic ash. Bentonite can expand several times to its original volume when placed in water. How can we use it??? llite is alkali-deficient mica close to muscovite composition but illite has less Al replaced Si. It contains more water and K may be replaced by Ca and Mg. Illite is major constituent in many shales.

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26 Talc Mg 3 Si 4 O 10 (OH) 2 Triclinic. Rare xtals, usually tabular with rhombic or hexagonal outline. Foliated and in radiating foliated groups. Compact and massive known as steatite or soapstone. Pure talc contains MgO 31.7%, SiO %, H 2 O 2.8%. Small amount of Al or Ti could replace Si, where Fe may substitute some Mg. Minnesotaite (Fe 3 Si 4 O 10 (OH) 2 ) commonly found in low grade metamorphic iron ore deposits could be the other end-member of complete solid solution. Trioctahedral t-o-t structure in talc similar to that dioctahedral t-o-t in pyrophyllite where neutral t-o-t layers joined together by weak residual bonds. Characterised by its micaceous habit, cleavage, softness, and greasy feel.

27 Talc is secondary mineral formed by alteration of magnesium silicates like olivine, pyroxenes, and amphiboles (also found as pseudomorphs after these minerals). Characteristic of low-grade metamorphic rocks. Use in powdered form as ingredient in paint, ceramics, rubber, insecticides, roofing, paper, and foundry facings. Talcum powder is the most familiar use. Also use as ornamental stone for carving.

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29 Pyrophyllite Al 2 Si 4 O 10 (OH) 2 Triclinic, not in distinct xtals. Foliated, sometimes in radiatin lamellar aggregates, or granular to compact. Identical with talc in appearance. Little variation from ideal formula; Al 2 O , SiO , H 2 O 5.0%. Dioctahedral t-o-t structure held together by weak van der waal bonds. Recognised by its micaceous habit, cleavage, and greasy feel. XRD techniques are needed for positive identification. Pyrophyllite is comparatively rare minerals. Found in metamorphic rocks; frequently with kyanite. Use for the same purpose as talc but cheaper than talc. Considerable part of agalmatolite, Chinese carve small images, is pyrophyllite.

30 t o tlayers in pyrophyllite

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32 Mica group Micas composed of t-o-t layers with interlayer cations and little or no exchangeable water. They are monoclinic minerals with β close to 90, so their monoclinic symmetry is not easily observed.

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34 Muscovite: KAl 2 (AlSi 3 O 10 )(OH) 2 Monoclinic, rare isolated xtals; usually tabular with prominent {001}. Prim faces {110} at nearly 60 give some plates diamond-shaped outline, as a result imitating orthorhombic symmetry. Prism faces are roughened by horizontal striations and frequently taper. [310] penetration twins. Foliated in large to small sheets, also aggregated into plumose or globular forms. Cryptocrystalline and compact massive also found.

35 Minor replacement of Na, Rb, Cs for K; Mg, Fe 2+, Fe 3+, Li, Mn, Ti, Cr for Al; and F for OH. Paragonite, NaAl 2 (AlSi 3 O 10 )(OH) 2, is isostructural with muscovite and cannot be distinguished without X-Ray techniques. Large miscibility gap occurs between muscovite and paragonite at low-moderate temperature. Characterised by its highly perfect cleavage and light colour. Common rock-forming mineral. Characteristics of granites and granite pegmatites. Form as major constituents of certain mica schist.

36 Almandine garnet in albite with muscovite from pegmatite, west of Cathance Road, Topsham District, Sagadahoc County, Maine

37 Biotite: K(Mg, Fe) 3 (AlSi 3 O 10 )(OH) 2 Monoclinic, rare tabular or short prismatic with prominent {001} and pseudohexagonal outlines. Usually irregular foliated masses or often disseminated scales/ scales aggregates. Dark green or brown to black or rarely light yellow, perfect cleavage {001}, folia flexible and elastic, splendent (metallic) lustre. Considerable substitution of Fe 2+ for Mg, Fe 3+ and Al or Mg and Al for Si. Also substitution of Na, Ca, Ba, Rb and Cs for K. Recognised by micaceous cleavage and dark colour

38 Biotite is found in wide spectrum of igneos rocks (granite pegmatitesgranite-diorite-gabbro-peridotite). In metamorphic rocks, biotite forms in both contact and regional metamorphism over a wide range of temperature. Xtallisation of biotite in argillaceous rocks P-T conditions (metamorphic grade) for biotite zone e.g. biotite-chlorite, biotite-muscovite Glauconite: authigenic mineral found green pellets in marine sedimentary rocks Vermicultie: alteration of biotite with structure of talc interlayered by water molecules. Vermiculite expands into wormlike forms when heated thus expanding condition is used in heat & sound insulator. vermiculite Heated vermiculite

39 t o t type layer with interlayer of H 2 O in vermiculite

40 Lepidolite: K(Li, Al) 2 3 (AlSi 3 O 10 )(O,OH,F) 2 Monoclinic and hexagonal depends on polytype. Xtals are small plates or prisms with hexagonal outline. Commonly coarse- to fine-grained scaly aggregates. Pink and lilac to greyish white, perfect cleavage {001}, pearly lustre. Li substitution Al in octahedron site that can contain up to wt% Li 2 O. Also substitution of Na, Rb and Cs for K. Recognised by micaceous cleavage and lilac to pink colour Lepidolite is relatively rare, found in pegmatites associated with Li-bearing minerals e.g. pink-green tourmaline, amblygoinite and spodumene. Often intergrown with muscovite in parallel position.

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43 Chlorite group Chlorite: (Mg,Fe) 3 (Si,Al) 4 O 10 (OH) 2 (Mg,Fe) 3 (OH) 6 Triclinic or monoclinic, rare distinct xtal, usually foliated massive or minute scales aggregates Green of various shades rarely yellow, white, rose-red. Perfect cleavage {001}, flexible but not elastic t-o-t layers interleaved with a brucitelike sheet. 1/3 of Mg 2+ is replaced by Al 3+ [Mg 2 Al(OH) 6 ] 1+ and becomes (Mg,Al) 6 (Si,Al) 4 O 10 (OH) 8 when added t-o-t layers and brucitelike sheet.

44 A 5-6 Z 4 O 10 (OH) 8 general formula A = Al, Fe 2+, Fe 3+,Li, Mg, Mn and Ni Z = Al, Si, Fe 3+ Giving rise to extensive solid solutions and mineral members of this group e.g. chamosite, clinochlore, pennantite and sudoite Recognised by green colour, micaceous habit and cleavage. Chlorite is common in metamorphic rocks. It is a diagnostic mineral of greenschist facies. The assemblage quartzalbite-chlorite-sericite-garnet is found in pelitic schist. It is also commonly found with actinolite & epidote. In igneous rock, chlorite forms as alteration product of Mg-Fe silicates like pyroxenes, amphiboles, biotite and garnet.

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46 t o t type layer with interlayer of brucite e.g. chlorite

47 Chlorite schist from the Michigamme Mine, Michigan, USA

48 Metamorphic facies