Mineralogical studies of silicate features emitted by dust grains in protoplanetary disks and Solar System bodies can shed light on the progress of planet formation .
The significant fraction of crystalline material in comets , chondritic meteorites and interplanetary dust particles indicates a modification of the almost completely amorphous interstellar medium ( ISM ) dust from which they formed .
The production of crystalline silicates , thus , must happen in protoplanetary disks , where dust evolves to build planets and planetesimals .
Different scenarios have been proposed , but it is still unclear how and when this happens .
This paper presents dust grain mineralogy ( composition , crystallinity and grain size distribution ) of a complete sample of protoplanetary disks in the young Serpens cluster .
These results are compared to those in the young Taurus region and to sources that have retained their protoplanetary disks in the older Upper Scorpius and \eta Chamaeleontis stellar clusters , using the same analysis technique for all samples .
This comparison allows an investigation of the grain mineralogy evolution with time for a total sample of 139 disks .
The mean cluster age and disk fraction are used as indicators of the evolutionary stage of the different populations .
Our results show that the disks in the different regions have similar distributions of mean grain sizes and crystallinity fractions ( \sim 10 – 20 % ) despite the spread in mean ages .
Furthermore , there is no evidence of preferential grain sizes for any given disk geometry , nor for the mean cluster crystallinity fraction to increase with mean age in the 1 – 8 Myr range .
The main implication is that a modest level of crystallinity is established in the disk surface early on ( \leq 1 Myr ) , reaching an equilibrium that is independent of what may be happening in the disk midplane .
These results are discussed in the context of planet formation , in comparison with mineralogical results from small bodies in our own Solar System .