Forsterite is one of the crystalline dust species that is often observed in
protoplanetary disks and solar system comets. Being absent in the interstellar
medium, it must be produced during the disk lifetime. It can therefore serve as
a tracer of dust processing and disk evolution, which can lead to a better
understanding of the physical processes occurring in the disk, and possibly
planet formation. However, the connection of these processes with the overall
disk crystallinity remains unclear. We aim to characterize the forsterite
abundance and spatial distribution in the disk of the Herbig Be star HD 100546,
to investigate if a connection exists with the large disk gap. We use a 2D
radiative transfer code, MCMax, to model the circumstellar dust around HD
100546. We use VISIR Q-band imaging to probe the outer disk geometry and
mid-infrared features to model the spatial distribution of forsterite. The
temperature-dependent shape of the 69 micron feature observed with Herschel
PACS is used as a critical tool to constrain this distribution. We find a
crystalline mass fraction of 40 - 60 %, located close to the disk wall between
13 and 20 AU, and possibly farther out at the disk surface. The forsterite is
in thermal contact with the other dust species. We put an upper limit on the
iron content of forsterite of 0.3 %. Optical depth effects play a key role in
explaining the observed forsterite features, hiding warm forsterite from view
at short wavelengths. The disk wall acts as a showcase: it displays a localized
high abundance of forsterite, which gives rise to a high observed
crystallinity, while the overall mass fraction of forsterite is a factor of ten
lower.Comment: A&A accepted, 17 pages, 14 figure