Mineralogy of the Carrizozo Malpais

The Carrizozo basaltic lava flows contain phenocrysts of olivine, plagioclase and pyroxene. Green olivine (Fa20) are 1 mm or less in size, whitish plagioclase (An52-57) are less than 2 mm and pyroxene are not visible in hand sample (Faris, 1980). The groundmass of the lava is composed of plagioclase, olivine, augite, glass, magnetite and ilmenite (Faris, 1980). Some phenocrystic olivine and plagioclas has been partially resorbed, suggesting some degree of disequilibrium between the phenocrysts and lava (Faris, 1980).

The figure below is a backscattered electron image of a polished chip of rock from the upper Carrizozo lava flow. The horizontal field of view is almost 1 mm. The brighter areas on the image represent higher mean atomic number phases, and the darker represent lower.

The brightest areas on the image, which tend to have acicular shapes, are the mineral magnetite. The next brightest phase, which is blocky and in some cases shows a shade gradient from core to rim, representative of chemical zonation within the phase, is olivine (pts. 1-8 on the image). The medium grey phase is pyroxene (pts 9-12). The darkest grey, which makes up the rest of the imaged area, is plagioclase (pts 13-15). The chemical composition of these phases is listed in the table below, and close-up images of olivine and pyroxene are also available.