Accessory minerals Prof. RNDr. Milan Novák, CSc. Al[2]SiO[5] modifications and other Al-rich minerals Thesis: • Introduction • Al[2]SiO[5 ]modifications [• ]Alteration of Al[2]SiO[5] • Related minerals • Other Al-rich minerals 1. Introduction • Minerals Al[2]SiO[5 ]are very important accessory and locally also rock-forming minerals typical for Al-rich rocks from low- to high-grade metamorphic to various acid magmatic. Along these most common Al-rich minerals also several paragenetically and structurally related minerals occur (e.g., staurolite, boralsilite and mullite), or originated during low-T alteartions (e.g. diaspore, pyrofylite). ^ 2. Al[2]SiO[5][] [• ]Minerals Al[2]SiO[5] [ ] Sillimanite Al^6 Al^4 O SiO[4] ortorombic Pbnm, 2/m2/m2/m. Andalusite Al^6 Al^5 O SiO[4] ortorombic Pnnm, 2/m2/m2/m Kyanite Al^6 Al^6 O SiO[4] triclinic C1, 1 • Sillimanite a 7.486, b 7.657, c 5.7729 Å, V 331.6 Å^3, Z 4 • Andalusite a 7.795, b 7.896, c 5.558 Å, V 342.1 Å^3, Z 4 • Kyanite a 7.112, b 7.844, c 5.574 Å, α 88.9, β 101.1, γ 105.9, V 292.9 Å^3, Z 4 • Chemical composition is commonly close to the ideal formula. minor and trace elements: Sillimanite B^3+, Mg, Fe^3+ Andalusite Mn^3+, Fe^3+, Cr^3+ Kyanite Cr^3+, V^3+, Fe^3+ 2. Al[2]SiO[5][] 2. Al[2]SiO[5] • Commonly close to ideal formula except Mn^3+ and partly in Fe^3+ in andalusite where - kanonaite Mn^3+^6 Al^5 O SiO[4] is further member of this group. Entering of Al instead Si into T-site in sillimanite tends to mullite. • Andalusite-kanonaite – Mn^3+ - full miscibility. • Sillimanite-mullite – Al - limited miscibility.. • Sillimanite-boralsilite – B - miscibility exists but not clear. • Minor and trace elements commonly enter both Al-sites, but more into octahedral site (Mn^3+ in Mn-andalusite), B enters T-site. 2. Al[2]SiO[5] • Properties: • Sillimanite – grey, white, yellowish • Andalusite - pink, red, brown-red, grey, green pleochroic • Kyanite - blue, grey colorless • Hardness 6-7, kyanite 7-5, • density = 3,2-3,6 (kyanite). • Variety: fibrolit – fibrous sillimanite viridine – green Mn-andalusite chiastolite – andalusite se with sectorial zoning 2. Al[2]SiO[5] • Tripl point Al[2]SiO[5] • All modifications have triple point and its position in PT diagrams is extremekly important for geological implications. • Richardson et al. (1969) • Holdaway (1971) • Robie a Hemingway (1984) • Holland and Powell (1985) • Pattison (1989) 2. Al[2]SiO[5] 2. Al[2]SiO[5] 2. Al[2]SiO[5] • From all figures it is clear that the position of the triple point is still not strictly defined. Recently triple point of Holdaway (1971) or from Pattison (1989) is used. Chiefly the position of the univariant reaction andalusite=sillimanite is problematic. • Problem of fibrolite fibrous sillimanite-fibrolite is more abundatnt then prismatic sillimanite. Its stability likely overlaps field of andalusite in PT diagrams due to distinct grain-size used in experiments. 2. Skupina Al[2]SiO[5] • Occurrences: Al-rich rocks with low Na+K/Al. • Sillimanite – regional metamorphosed rocks of medium- to high grade, migmatites • Andalusite – contakt and regional metamorphic rocks (Branná), granites, pegmatites (Dolní Bory) • Kyanite – regional metamorphic rocks of medium- to high grade, (Frymburk, Kovářová, Branná), granulites 3. Al[2]SiO[5 ]- alterations 3. Al[2]SiO[5 ]- alterations 4. Related minerals • Kanonaite Mn^3+ Al O SiO[4] ortrombic, Pnnm, 2/m2/m2/m, a 7.959, b 8.047, c 5.616 Å, V 359.6 Å3, Z 4 substitution: Mn^3+ - Al dark green with strong pleochroism, in Mn-rich rocks 4. Related minerals 4. Related minerals • Mullite Al[4+2x]Si[2-2x]O[10-x] ortorombic, Pbam, 2/m2/m2/m, a 7.5416, b 7.6942, c 2.8875 Å, V 167.5 Å3, Z 2, substitution: 2Al O - 2Si q • Fine-graind fibrous in HT contact metamorphism and ceramics 4. Related minerals • Boralsilite Al[16]B[6]Si[2]O[37] ortorombic substitution: 2B q - 2Si O • Extremely rare 4. Related minerals • Staurolite General formula A[4]B[4]C[18]D[4]T[8]O[40]X[8] Monoclinic (pseudoortorombic) A = Fe^2+, Mg, ( > 2) M(4A),M(4B) B = Fe^2+, Zn, Co, Mg, Li, Al, Fe^3+, Mn, T(2) C = Al, Fe^3+, Cr,V,Mg,Ti M(1A),M(1B),M(2) D = Al,Mg, ( > 2) M(3A), M(3B) T = Si, Al T(1) X = OH,F,O O(1A), O(1B) End-member compositions A B C D T O X [4] Fe^2+[4]^ Al[16] Al[2][2] Si[8 ]O[40 ](OH)[2]O[6] [2]Fe^2+[2 ][4]^ Al[16] Al[2][2] Si[8 ]O[40 ](OH)[6]O[2] [4] Fe^2+[4]^ Al[16] [4] Si[8 ]O[40 ](OH)[8] [4] Fe^2+[4]^ Al[16 ]Al[2][2] Si[4]Al[4 ]O[40 ](OH)[6]O[2] [4] Fe^2+[4]^ Al[12]Mg[4 ]Al[2][2] Si[8 ]O[40 ](OH)[6]O[2] [4] Li[4]^ Al[16] Al[2][2] Si[8 ]O[40 ](OH)[6]O[2] 4. Related minerals • Properties: Color: brown to black pleochroic refractory indices: a 1.736-1.747, b 1.742-1.753, g 1.748-1.761, density: 3.74-3.83, hardness: 7-71/2 4. Related minerals 4. Related minerals occurrences: Al-rich metamoprhic rocks • Typical features of natural staurolites: high Fe: decrease in X[Mg] • Metapelites of medium-grade tourmaline > cordierite > chlorite > biotite > staurolite > garnet typically minor Li: decrease in X[Li] • Metapelites of medium-grade staurolite > cordierite > biotite > muscovite > garnet, tourmaline, chloritoid commonly high Zn: • ratio Zn/Fe in staurolite 10x to 100x highre then in associated minerals (garnet, biotite, chlorite). 5. Additional Al-rich minerals • Safiríne (Mg,Fe^2+,Fe^3+,Al)[8]O[2] (Al,Si)[6] O[18] monoclinic, space group P2[1]/m • Rare accessory phase in Al-rich Si-poor metamorphic rocks • Propeeties: Color: green, blue, grey strongly pleochroic: Indexy lomu: a 1.701-1.726, b 1.703-1.728, g 1.705-1.734, Density: 3.40-3.58 g/cm^3^, hardness: 71/2 Typical substitution MgSi Al[-2] 5. Additional Al-rich minerals • Corundum Al[2]O[3] trigonal R 3 c • Color: grey, bluish, blue, red, yellow and others (ruby, sapphire) Density: 4,0-4,1 g/cm^3^, hardness: 9 5. Additional Al-rich minerals 5. Additional Al-rich minerals • Spinels Spinel MgAl[2]O[4] hercynite FeAl[2]O[4 ]Gahnite[ ]ZnAl[2]O[4] • Cubic Fd3m Color: grey, bluish, blue, red, yellow, black, green and others Density: 3,6-4,5 g/cm^3^, hardness: 7-8 substitutions: Mg-Fe^2+-Zn-Mn, Al-Fe^3+-Cr 5. Additional Al-rich minerals 5. Additional Al-rich minerals • Yoderite Mg[2](Al,Fe^3+)[6]Si[4]O[18 ](OH)[2] monoclinic, space group P2[1]/m, class 2/m. • Rare mineral known only from quartz-kyanite-talc schists (~ 10 kbar, ~ 800 ^o C). Yoderit je indicator of high fO[2] in deep part of earth crust. • Color: dark violet to emerald green pleochroic Indexy lomu: a 1.689, b 1.691, g 1.715, Density: 3.39 g/cm^3,^ hardness: 6 5. Additional Al-rich minerals • : • Kornerupine (Mg,Fe) (Al,Mg,Fe)[9] (Si,Al,B)[3] (O,OH,F)[22] • Grandidierite (Mg,Fe) Al[3]O[2] (BO[3])SiO[4] • Werdingite (Mg,Fe)[2] Al[12] (Al,Fe)[2] Si[4] (B,Al)[4] O[37] [ ] In high-grade Al,B-rich rocks 6. Conclusions Al-rich mineráls are important for geological implications due to: • Al[2]SiO[5] modifications are important for estimation of PT-conditions. • Presence of Al-rich rocks incicates: - high Al in rock – interpretation of source sedimentary rock) - ratio Al/Si. - Low activity of alkalis. 3. Late alterations of Al-rich minerals indicate PTX-conditions of these proceses