I-type Cosmic Spherules
I-types are by far the most common cosmic spherules found in sedimentary rocks, but rare in rooftop collections. They are dominated by iron-oxide minerals called wüstite and magnetite with minor amounts of Ni and platinum group elements concentrated into small spherical “nuggets”. They are currently split into three broad categories based on the presence of a “metal bead” - a rounded interior section which has not been oxidised during entry.
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OX I-type - metal bead-free, oxide-dominated spherules consisting of mixtures of magnetite and wustite dendrites (DEN). The magnetite content they have can influence the shape of the dendrites: magnetite-poor varieties have thin "re-entrant" dendrites, whilst magnetite-rich have coarse-grained "barred" or "equant" magnetite crystals.
MET I-type - metal-bead bearing spherules have iron-oxide shells dominated by wustite. They can be homogeneous (no observable crystal boundaries) or dendritic (composed of Christmas-tree patterned crystals). The often have evidence of metal beads on their surface such as pits and thick magnetite rims.
OXMET (transitional) I-type - spherules which share textural and mineralogical features of both MET and OX spherules. They commonly have wustite mantles (WUSM) surrounded metal beads which could be weathered into ferrihydrite (FER, an oxyhydroxide mineral), and magnetite rimes (MAGR).