Common, Important Textures of Mafic Igneous Rocks Student Handout for Lab Manual ESS 405/505 - Petrology

Shapes of Individual Mineral grains in thin section – Euhedral to Subhedral Shapes Euhedral to subhedral – mineral grains have good crystal faces; outlines of grains look like geometric shapes. Each mineral has a specific euhedral shape depending on the Crystal System (symmetry). (analyzer out for all images except lower left, analyzer in) Euhedral olivine Similar shape to pyx. Crystal faces meet at acute angle (“pointy tips”). Olivine oxidized & altered to clay (lower photo). Euhedral pyroxene Blocky cross-sections, rectangular to almost square, 6- or 8-sided Euhedral plagioclase has rectangular shapes (called plagioclase laths)

Shapes of Individual Mineral grains in thin section – Skeletal Shapes Skeletal – mineral grains are anhedral (do not have good crystal faces) and have holes, now filled in by other minerals. Skeletal shapes indicate rapid growth of mineral (not necessary rapid cooling, just rapid growth). Experiments on Crystal Growth – skeletal shapes Skeletal plagioclase elongated rectangles with forked ends (swallowtail) Skeletal pyroxene Elongated grains form a chain with internal holes Skeletal Fe-Ti-oxides (magnetite and ilmenite) crystals have holes inside (analyzer out) Eight black-and-white photos on left from Lofgren, Gary, Experimental studies on the dynamic crystallization of silicate melts, in Hargraves, R.B., ed., Physics of Magmatic Processes, 1980, Princeton, N.J.: Princeton University Press, p. 487-552. Analyzer out: top right, bottom left. Analyzer in: bottom right

Shapes of Individual Mineral grains in thin section – Interstitial Shapes Interstitial – grains fill in the spaces between other minerals. Interstitial grains are subhedral to anhedral because their shape is controlled by the space between the other grains. Interstitial minerals typically finish crystallizing later than the other grains. Interstitial Pyroxene Pyroxene fills in spaces between euhedral plagioclase. Interpretation: pyroxene grew after or at the same time as plagioclase, but finished crystallizing after plagioclase. (analyzer out) Interstitial quartz and K-feldspar outlined in red Interpretation: quartz and K-feldspar crystallized after plagioclase (think Bowen’s Reaction Series). (analyzer in) Interstital intergrowth of quartz and K-feldspar (micrographic texture or granophyre) – Interpretation: intergrowth of quartz and k-spar crystallized together, after other minerals (think Bowen’s Reaction Series). (analyzer in)

Overall Textures of Mafic Igneous Rocks (Basalt, Diabase, Gabbro) Hypidiomorphic-Granular Ophitic to Subophitic Mineral grains are euhedral, subhedral, and anhedral in shape (some have good crystal faces and some don’t). Many pyroxene grains are euhedral to subhedral and do not enclose plagioclase grains. (analyzer out) Pyroxene grains are large and partly or completely enclose plagioclase grains. Plagioclase grains are euhedral to subhedral; pyroxene grains are more anhedral as they grow around and enclose the plagioclase. Poikilitic is the general term for a grain that encloses another mineral. Ophitic is a specific type of poikilitic texture in which pyroxene encloses plagioclase. (analyzer out) Intergranular texture (left top & bottom): plagioclase laths with interstitial pyroxene and opaques, in diabase . (bottom, analyzer in) Intersertal texture (right top) in basalt: plagioclase laths with interstitial volcanic glass (green and brown) and pyroxene (analyzer out)