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We present a detection of polarised dust emission in the Crab pulsar wind nebula. We constrain the bulk composition of the dust with new SOFIA/HAWC+ polarimetric data in band C 89 µm and band D 154 µm. After correcting for synchrotron polarisation, we report dust polarisation fractions ranging between 3.7-9.6% and 2.7-7.6% in three individual dusty filaments at 89 and 154 µm, respectively. The detected polarised signal suggests the presence of large (> 0.05-0.1 µm) grains in the Crab Nebula. With the observed polarisation, and polarised and total fluxes, we constrain the temperatures and masses of carbonaceous and silicate grains. We find that the carbon-rich grain mass fraction varies between 12 and 70%, demonstrating that carbonaceous and silicate grains co-exist in this SNR. Temperatures range from ~ 40 to ~ 70 K and ~ 30 to ~ 50 K for carbonaceous and silicate grains, respectively. Dust masses range from ~ 0.0001 to ~ 0.01 Solar masses for carbonaceous grains and to 0.1 Solar masses for silicate grains, in three individual regions.