A SURVEY OF CLASTS AND LARGE CHONDRULES IN ORDINARY CHONDRITES

We report the results of a survey of clasts and large (>5 mm) chondrul es (macrochondrules) within the 833 ordinary chondrites of the Natural History Museum collection. Thirty-six macrochondrules and 24 clasts w ere identified and studied. Macrochondrules have textures and mineral assemblages like normal chondrules and so share a common origin. Clast s show evidence for fracturing from larger bodies and can be classifie d as either: (1) chemically fractionated if they have major and trace- element compositions differing substantially from most chondrules and clasts; (2) impact melt clasts if they have microporphyritic textures and signs that indicate they are derived from shock-melted chondritic material; (3) microporhyritic clasts if they are similar to the last c ategory but lack evidence for derivation through shock melting; or (4) indeterminate clasts forming a diverse class that includes all those clasts that do not fit into the other categories.