"Monzogabbro"

Presumably as a marketing ploy because the name sounds neat, NWA 4734 is being sold as a rock type known as monzogabbro. The rocks is a basalt, not a monzogabbro. There are no meteorites that are monzogabbros. A basalt is an extrusive volcanic rock - it cooled from a magma that was extruded onto the surface. A gabbro is is an intrusive volcanic rock, often called a plutonic rock, because the magma cooled more slowly below the surface. Consequently, the grain size of the minerals in a gabbro is larger than that of a basalt. Definitions vary, but gabbros usually have mineral grain greater than 2 mm in size. The two main minerals in a gabbro are clinopyroxene ("cpx") and plagioclase (calcic) feldspar.

A monzogabbro is a plutonic rock. The exact definition is a bit complicated, but it is explained well here. In a gabbro, 90-100% of the feldspar is plagioclase and <10% is alkali (potassium) feldspar. In a monzogabbro, 65-90% of the feldspar is plagioclase and the remaining 10-35% of the feldspar is alkali feldspar. Even if NWA 4734 were sufficiently coarse-grained to be considered a gabbro, it does not contain enough alkali feldspar (alias, "K-feldspar" or "Kspar") to be a monzogabbro. The report below states (cryptically) that plagioclase and "Kspar" together total 32% and that "Interstitial glass is high in silica (75 wt%) and contains microcrysts of K-feldspar with a significant celsian component." [Celsian is another alkali feldspar, one containing barium.] For a proper classification, a complete "modal analysis" would have to be done in order to tell us the exact percentages of plagioclase and alkali feldspar. We are unaware of any published modal analysis for NWA 4734, but there are modal analyses for the LAP look-alikes. For the LAP meteorites, Anand et al. (2004) report 0.79% "K-glass" and Day et al. (2005) report a mean of 0.70% "K-Gls," which is the closest thing to alkali feldspar. Collins et al. (2005) state "Plagioclase laths are a dominant component in the LAP meteorites..." and don't mention alkali feldspar at all. In a monzogabbro, the alkali feldspar would be coarse grained. In NWA 4734, it is very fine grained and occurs only in interstitial glass, so it is difficult to determine the exact amount modally.

The total concentration of K (as K₂O) in NWA 4734 is 0.12% (our own unpublished data), almost the same as for the LAP meteorites (0.11%, Anand et al., 2005; 0.10%, Day et al., 2005; 0.115%, Joy et al., 2006). If all the K and Ba is contained in alkali feldspar, then NWA 4734 cannot contain more than 0.8% alkali feldspar (normative). Thus, NWA 4734 is not a monzogabbro (10-35%).