GLASS-RICH CHONDRULES IN ORDINARY CHONDRITES

There are two types of glass-rich chondrules in unequilibrated ordinar y chondrites (OC): (1) porphyritic chondrules containing 55-85 vol% gl ass or microcrystalline mesostasis and (2) nonporphyritic chondrules, containing 90-99 vol% glass. These two types are similar in mineralogy and bulk composition to previously described Al-rich chondrules in OC . In addition to Si-, Al- and Na-rich glass or Ca-Al-rich microcrystal line mesostasis, glass-rich chondrules contain dendritic and skeletal crystals of olivine, Al2O3-rich low-Ca pyroxene and fassaite. Some cho ndrules contain relict grains of forsterite +/- Mg-Al spinel. We sugge st that glass-rich chondrules were formed early in nebular history by melting fine-grained precursor materials rich in refractory (Ca, Al, T i) and moderately volatile (Na, K) components (possibly related to Ca- Al-rich inclusions) admixed with coarse relict forsterite and spinel g rains derived from previously disrupted type-I chondrules.