Biaxial Minerals Descriptions

Transcription

1 Biaxial Minerals Desriptions Olivine Pyroxenes Orthopyroxene Clinopyroxene Amphiole Atinolite Mias Biotite, musovite, hlorite Feldspars Plagiolase Miroline, ortholase, sanidine Amphioles Two groups to onsider: orthoamphioles linoamphioles Similar to pyroxenes (single hain) ut have a doule hain of silia tetrahedra elongated to axis Exhiit a range of ompositions with a orresponding range of optial properties. (p ompositional range) Compositions an not aurately e determined ased on optial properties. Classifiation of Amphioles Fe-Mg Amphioles Anthophyllite (O) (Mg,Fe) 7 Si 8 O 22 (O H) 2 Gedrite (O) (Mg,Fe) 5 Al 2 (Al 2 Si 6 )O 22 (O H) 2 Cummingtonite-grunerite (M) (Fe, Mg) 7 Si 8 O 22 (O H) 2 Cali Amphioles (M) Tremolite-atinolite Ca 2 (Mg,Fe 2+ ) 5 Si 8 O 22 (OH) 2 (Na,K) 0-1 Ca 2 (Mg,Fe 2+,Fe 3+,Al) 5 (Si,Al) 8 O 22 (OH) 2 Oxyhornlende (Na,K) 0-1 Ca 2 (Mg,Fe 2+,Fe 3+,Al) 5 (Si,Al) 8 O 22 (O,OH) 2 Kaersutite NaCa 2 (Mg,Fe 2+ ) 4 TiSi 6 Al 2 O 22 (OH) 2 Sodi-ali amphioles (M) Katophorite Na(Na,Ca)(Mg,Fe 2+,Fe 3+,Al) 5 (Si 7 AlO 22 (OH) 2 Rihertite Na(Na,Ca)(Mg,Fe 2+ ) 5 Si 8 O 22 (OH) 2 Sodi Amphioles (M) Glauophane Na 2 (Mg,Fe 2+ ) 3 Al 2 Si 8 O 22 (OH) 2 Rieekite Na 2 (Mg,Fe 2+ ) 3 Fe 3+ 2 Si 8 O 22 (OH) 2 Arfedsonite-ekermanite NaNa 2 (Mg,Fe 2+ ) 4 (Fe 3+,Al)Si 8 O 22 (OH) 2 1

2 Orthoamphiole Orthorhomi amphiole Anthophyllite (Mg,Fe) 7 Si 8 O 22 (O H) 2 Restrited to metamorphosed asalti roks Beause it is orthorhomi it exhiits extintion in elongate setions All remaining amphioles are monolini and exhiit inlined extintion in elongate setions Amphiole Monolioni Amphioles or Amphioles most ommon all monolini generally all optially -ve 2 major ones to e familiar with: Tremolite - Atinolite Ca 2 Mg 5 Si 8 O 22 (OH) 2 -Ca 2 Fe 5 Si 8 O 22 (OH) 2 Common Ca 2 (Mg,Fe,Al) 5 Si 8 O 22 (OH) 2 Variale ompositions = variale optial properties. Refrative Index n α = n β = n γ = Relief, Birefringene, Retardation Exhiits moderate to high relief Birefringene Interferene olours vary from 1st order orange to upper 2 nd order - lower 3 rd order olours Average 2 nd order lue-green interferene olours 2

3 Opti Sign Opti sign either +ve or ve 2V X angle varies from 35 to 130, depending on omposition Generally 2V X = therefore optially -ve Colour is distintly oloured and pleohroi in thin setion. Shades of green, yellow-green, lue-green and rown. Pleohroism X Z yellow-green olive green dark green pale rown greenish dark green greenish-rown reddish-rown red-rown variety of pleohroi olours. Pleohroism hl px hl px leavage 3

4 Exhiits good amphiole leavage on plane at 56 and 124 Grain shape is ontrolled y leavage and are usually elongated to axis leavages Form Ours as: slender prismati to laded rystals with a 4 or 6 sided ross setion whih exhiit amphiole leavage at 56 and 124 also as anhedral irregular grains a Monolini Opti orientation: X^a = +3 to -19 = Z^ = +12 to +39 P is to X 2V X = Z P 011 4

5 Basal setions exhiit symmetrial extintion, slow ray to long diagonal etween leavages Elongated setions are length slow, and the Z^ extintion angle is used to identify hornlende Basal Setion to axis 4 to 6 sided grain and/or 2 leavages at Symmetrial Extintion ~ Bxo Figure P Z' X' a Symmetrial Extintion EA 1 EA 2 leavages EA 1 = EA 2 5

6 Opti Normal Setion Z one leavage Inlined Extintion Maximum interferene olour Opti Normal Figure 2V X = X Z^ = Z^ = a a^ = 75 Inlined Extintion eg. Extintion Angle Setion to a axis Parallel Extintion ~ Bxa Figure Z' P a X' 6

7 Alteration may e altered to iotite, hlorite or other Fe-Mg siliates Ourrene is found in a variety of lithologies: Igneous (granites, through garos, syenites to ultramafis) metamorphi environments May e primary or seondary in origin Distinguishing Features 2 leavages grain shape inlined extintion pleohroism 2V X = Z P 011 X 7