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We describe Spitzer IRS spectroscopic observations of the 10 Myr-old star, EFCha. Compositional modeling of the spectra from 5 {\mu}m to 35 {\mu}m confirmsthat it is surrounded by a luminous debris disk with LD/L\star ~ 10-3,containing dust with temperatures between 225 K and 430 K characteristic of theterrestrial zone. The EF Cha spectrum shows evidence for many solid-statefeatures, unlike most cold, low-luminosity debris disks but like some other10-20 Myr-old luminous, warm debris disks (e.g. HD 113766A). The EF Cha debrisdisk is unusually rich in a species or combination of species whoseemissivities resemble that of finely powdered, laboratory-measuredphyllosilicate species (talc, saponite, and smectite), which are likelyproduced by aqueous alteration of primordial anhydrous rocky materials. Thedust and, by inference, the parent bodies of the debris also contain abundantamorphous silicates and metal sulfides, and possibly water ice. The dust'stotal olivine to pyroxene ratio of ~ 2 also provides evidence of aqueousalteration. The large mass volume of grains with sizes comparable to or belowthe radiation blow-out limit implies that planetesimals may be colliding at arate high enough to yield the emitting dust but not so high as to devolatizethe planetesimals via impact processing. Because phyllosilicates are producedby the interactions between anhydrous rock and warm, reactive water, EF Cha'sdisk is a likely signpost for water delivery to the terrestrial zone of a youngplanetary system.

Spitzer IRS Spectroscopy of the 10 Myr-old EF Cha Debris Disk: Evidence for Phyllosilicate-Rich Dust in the Terrestrial Zone