Feldspars. Structure. The feldspars are by far the most abundant group of minerals and are found in igneous, metamorphic and many sedimentary rocks.

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1 Feldspars The feldspars are by far the most abundant group of minerals and are found in igneous, metamorphic and many sedimentary rocks. Structure Felsdpars are framework silicates where each silica tetrahedra share all corners with its four neighbouring tetrahedra. A portion of the tetrahedra contain Al +3 instead if Si +4. The charge is balanced by incorporating Na +, Ca +2 or K +. Plagioclase 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. Eg 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: Eg: An 0-3, Ab low albite An 3-22, Ab Peristerites An 22-73, Ab Intermediate Structure An Ab 0-27 Anorthite Structure 48

2 Structure cont. 49 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 Plag 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. Alkali Group [Orthoclase] [Albite] 1) Sanadine (K,Na)AlSi 3 O 8 - monoclinic - high temperature - Al randomly distributed - often see carlsbad twinning 2) Microcline (K,Na)AlSi 3 O 8 - triclinic - much more ordering of Al in the structure - any heating will randomly redistribute the Al - often see tartan twinning

3 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 (CaAl 2 Si 2 O 8 ) and Albite (NaAlSi 3 O 8 ). Alkali feldspars - solid solution between Albite and Orthoclase (KAlSi 3 O 8 ) is limited (dependent on temperature). 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 P H2O will change the crystallization range of plag Eg: P H2O of 5000 bars will drop the crystallization temperature of Anorthite from 1553 C to 1234 C. 50

4 Chemistry cont 51 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. Eg: Or 25 Or 60 on Fig 46. However, submicroscopically, every crystal has two components: Laths of plag 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 plag intergrowths in K rich feldspar host (Fig 34 handout). - low albite - Microcline -Or 20 Or 80 Perthite - most common Perthite is orthoclase or microcline host with albite lamellae. Called Antiperthite If you have Alkali feldspar in a plagioclase host.

5 Alteration 52 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. Optical and Physical Properties Plagioclase Colour: Colourless Form: Laths or euhedral xtls RI, Bire, 2Vand Optic Figure: All vary systematically with composition.

6 Optical and Physical Properties cont Plagioclase Cleavage: 3 directions (two good) Extinction: Varies with composition. Twinning: Albite twinning present 53 Alkali Feldspars Colour: Colourless Form: Phenocrysts or euhedral to anhedral crystals Relief: Low Birefrincence: Weak Cleavage: 3 directions (2 good) Interference figure: Biaxial negative

7 Optical and Physical Properties cont Alkali Feldspars 2VAngle: Varies with composition Extinction: Varies with composition and axial plane Twinning: Carlsbad, Albite and Pericline can be present 54

8 Phase Relationships First examine the Alkaki Feldspar Series (Or-Ab) 55 This diagram (Fig 6-4a) is for 1 atms and a dry melt (no H 2 O). 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.

9 Phase Relationships cont. 56 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. 2) Figure 6-4b and Figure 20. When a melt contains H 2 O the fields of the phase diagrams change drastically. i) The leucite field disappears with increasing H 2 O. Only occurs in volcanics (quenched). ii) Crystallization temperatures are reduced. iii) Composition changes continuously along liquidussolidus 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.

10 Phase Relationships cont 57 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. 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

11 Phase Relationships cont 58 Eskola Experiment Took Ca plag, added crushed SiO 2, H 2 O and soda (NaHCO 3 ) and heated to 300 C for three days. The result was that much of the plag 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 plag is the high temperature end member. Now lets look at the system NaAlSiO 4 -KAlSiO 4 -SiO 2 There is no solid solution between these end members. They are simple binary eutectic systems.

12 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: 59 KAlSi 2 O 6 + SiO 2 KAlSi 3 O 8 (leucite) (quartz) (k-feldspar) 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).

13 Phase Relationships cont 60 Now lets look at the 3-D version of this system (NaAlSiO 4 -KAlSiO 4 -Si 2 O) Examine Figure 8-13 and note some of its complications: 1) Structure is temperature dependent (ie 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).

14 Phase Relationships 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 P H2O 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. 61

15 Phase Relationships cont 62 If the P H2O 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).

16 Phase Relationships cont 63 In general: a) Silica saturated compositions end crystallization at the minimum along the SiO 2 -Feldspar join (Peralkaline, oversaturated rocks). b) Silica undersaturated compositions end crystallization at the Ternary Eutectic (syenites and trachyte) (Figure 8-12).