Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Compact Course Microscopy of rock-forming Minerals Part 6: Olivine, Carbonate, volcanic glass

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Olivine Formula: (Mg,Fe) 2 [SiO 4 ] Symmetry: orthorhombic n: 1,636 – 1,879  n: 0,033 – 0,052 2Vz: 84° - 140° max. I. F. (30μm) : II. – III. order Observations: Colourless, high relief, high interference colours (esp. For Fe-rich variants), weak cleavage. Alteration to „Iddingsite“ or serpentinite. Composition can be easily determined measuring the angle between optical axes. Forsterit Fayalit 2V FoFax 90°

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Olivine 1 mm Observations: Idiomorphic crystals with red alteration at grain boundries (Iddingsite). Note: You can determine the optical properties and estimate the composition of olivines in thin section !

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Olivine, typical crystal shape Nr. 72 0,5 mm Observations: Idiomorphic crystals with iddingsite alteration at grain boundaries. Sometimes olivine is completely replaced by alteration minerals, but its typical shape is preserved and allows identification.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Olivine 1 mm Observations: Idiomorphic olivine with compositional zonation shown by increase of interference colours towards the outer rim : Fe-contents increases outward.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Olivine 1 mm Observations: Olivine as major phase in igneous cumulate, together with minor plagioclase and chromite spinel. Note the parallel orientation of crystals due to „sedimentary“ processes during crystal accumulation.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,2 mm Observations: Typical angular extinction between sub-grains of deformed olivines Olivine

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Observations: Olivine in gabbro. Alteration (serpentinization) along cracks. Serpentine is less Fe- richt than the original olivine. This results in the fomration of fine-grained Fe-oxides along altered zones. Olivine

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Observations: Olivine in gabbro completey replaced. Alteration products blackened by fine-grained Fe- oxides. Olivine At contact to (Al-rich) plagioclase: „kelephytic“ reaction rims with chlorite.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Olivine, strongly altered, detail

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Olivine Observations: Incipient serpentinisation. The nickel content is in relatively high concentrations in olivine which can be incorporated into newly- formed serpentine (Chrysotil), Ni is included in Garnierite (green-light-brown pleochroism) Biotite : deep brown Note: Most thin sections are thinner, thus colours will not be as intense as in this example. Compare crystal shapes of altered and fresh olivines.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Serpentine Observations: Almost complete serpentinisation with typical mesh-textures of chrysotile. Small patches of olivine remain (high birefringence). Fe from the olivine that can not be incorporated into the serpetines forms a separate Fe-oxide phase (opaque).

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Serpentine, Detail with relict olivine Observations: There are two types of chrysotile fibres : X- and Z. Note: Note: You can determine the different varieties by their optical properties. z x z-Faser x-Faser

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Carbonate (Calcite, Dolomite) Formula: Ca[CO 3 ], CaMg[CO 3 ] 2 Symmetry: trigonal n: 1,486 – 1,698  n: 0,17 – 0,18 2V: - max. I. F. (30μm) : very high (white) Observations: Easy to recognize by extremely high interference colours but much lower relief compared to titanite. Changing relief and chagrins in different directions : pseudo- pleochroism. Calcite and dolomite are distinguished by the position of cleavage and twin planes (or by colouring methods). nenenene nononono Calcite Dolomite

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Carbonate, Calcite and dolomite are distinguished by the position of cleavage Calcite Twin plane : {0112} long (parallel the long diagonale of cleavage planes) Dolomite Twin plane :{0221} short (parallel the short diagonale of cleavage planes)

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Carbonate (Calcite) Observations: Perfect cleavage in three directions (pressure twins at 105°). Strongly changing relief depending on the orientation of the grain: Pseudo-pleochroism. Uniaxial negative, nice image of optcal axis easy to obtain with many isochromatic fringes. Note: Note: In this example, the thin section is very thin. In thin sections of normal thickness, calcite would have a „white“ interference colour !

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,2 mm Calcite Observations: In position with high interference colour where n e ' is close to n e, the n e ‘ is below the refractive index of the resin. As a result, the „Becke-Linie“ moves outward (left image). In a position with n o parallel to the polarizer,the „Becke-Linie“ moves inward (right image). As a result, the chagrins chages when the crystal is turned in plane polarized light. This is reminsicent to (but optically not identical with) the effect of pleochroism. Direction of polarizing plane nononono ne'ne'ne'ne' nononono ne'ne'ne'ne'

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Carbonate (Dolomite) Observations: Typical twin lamellae due to pressure-induced deformation and twinning in dolomite. Strong birefringence of dolomite causes interference effects within the lamellae causing interference colours even without crossed polarizer.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Carbonate (Aragonite) Observations: Interference image: biaxial with small axial angle: Aragonite (CaCO 3, orthorhombic, 2V x =18°).

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei Glass Formula: - Symmetry: - n: 1,48 – 1,61  n: - 2V: - max. I. F. (30μm) : - Observations: Almost exclusively in volcanic rocks. Natureal glass is also, but only very rarely in fulgurite from strickening lights melting rock or soil on the ground gound and from friction at the base of giant gravitational collapse masses. Refractive index is a function of and inversely related to SiO 2 -content. Chemically unstable, devitrification and alteration is often observed.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,5 mm Volcanic Glass Observations: Clinopyroxene, plagioclase opaque Fe-oxide. In a fine- grained, paetly glassy matrix. 2. generation of smaller cpx crystals and plagioclase in a glassy matrix. Devitrifications starts at grain boundaries of the crystals. Note: Note: You can determine the refractive index of the glass relative to the resin and estimate the approximate composition of the melt that formed the glass.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,2 mm Volcanic Glass, Detail Observations: Clinopyroxene, plagioclase opaque Fe-oxide. In a fine- grained, paetly glassy matrix. 2. generation of smaller cpx crystals and plagioclase in a glassy matrix. Devitrifications starts at grain boundaries of the crystals. Note: Note: You can determine the refractive index of the glass relative to the resin and estimate the approximate composition of the melt that formed the glass.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,1 mm Volcanic Glass Observations: Sceletal opaque ores and devitrified areas in interstices of the fine-grained matrix between Ti-augite phenocrysts. Note: Note: You can determine the refractive index of the glass relative to the resin and estimate the approximate composition of the melt that formed the glass.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 1 mm Volcanic Glass Observations: Nice devitrification textures in partly glassy matrix between phenocrysts of high T-quartz.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,5 mm Volcanic Glass, Detail Observations: Nice devitrification textures in partly glassy matrix between phenocrysts of high T-quartz.

Source of course material Dr. Olaf Medenbach, Ruhr-Universität Bochum Optical microscopy of rock-forming minerals, G. Wörner, USTC Hefei 0,2 mm Volcanic Glass, Detail Observations: Nice devitrification textures in partly glassy matrix between phenocrysts of high T-quartz.