We present IRS spectra and revised MIPS photometry for the 18 members of the \eta Chamaeleontis cluster .
Aged 8 Myr , the \eta Cha cluster is one of the few nearby regions within the 5-10 Myr age range , during which the disk fraction decreases dramatically and giant planet formation must come to an end .
For the 15 low-mass members , we measure a disk fraction \sim 50 % , high for their 8 Myr age , and 4 of the 8 disks lack near-IR excesses , consistent with the empirical definition of “ transition ” disks .
Most of the disks are comparable to geometrically flat disks .
The comparison with regions of different ages suggests that at least some of the “ transition ” disks may represent the normal type of disk around low-mass stars .
Therefore , their flattened structure and inner holes may be related to other factors ( initial masses of the disk and the star , environment , binarity ) , rather than to pure time evolution .
We analyze the silicate dust in the disk atmosphere , finding moderate crystalline fractions ( \sim 10-30 % ) and typical grain sizes \sim 1-3Â \mu m , without any characteristic trend in the composition .
These results are common to other regions of different ages , suggesting that the initial grain processing occurs very early in the disk lifetime ( < 1 Myr ) .
Large grain sizes in the disk atmosphere can not be used as a proxy for age , but are likely related to higher disk turbulence .
The dust mineralogy varies between the 8-12Â \mu m and the 20-30Â \mu m features , suggesting high temperature dust processing and little radial mixing .
Finally , the analysis of IR and optical data on the B9 star \eta Cha reveals that it is probably surrounded by a young debris disk with a large inner hole , instead of being a classical Be star .