Heavy liquid density separation is commonly utilised in isolating specific minerals from bulk sediment for luminescence dating. K-feldspars are commonly separated from other minerals by isolating the 2.58 g cm−3 fraction which yields infrared stimulated luminescence (IRSL) signals conducive to dating. However, initial measurements of the IRSL and thermoluminescence (TL) signals from the 2.58 g cm−3 fraction of alluvial fan sediments from the Hajar Mountains, northern Oman, were atypical of K-feldspar luminescence signals reported in the literature. In this study, three different density separates (2.58 g cm−3, 2.58–2.54 g cm−3 and 2.565 g cm−3) were trialled to attempt to isolate pure K-feldspar samples for dating, to measure K-feldspar IRSL signals which are not contaminated with inputs from other feldspar, quartz and/or heavy mineral signals. The mineralogy of these separates was explored using microscopy, X-ray diffraction and mass spectrometry. We found that despite more refined heavy liquid density separation, the proportions of K-feldspar in these separates had a maximum of 40% by weight, remaining significantly contaminated by other minerals. IRSL and TL signals were atypical of K-feldspar for all density separates and it is likely that there is a contribution of signal from IR-sensitive contaminants, such as muscovite. This highlights the importance of investigating the mineralogy of samples for luminescence dating, especially in regions where little prior work has been undertaken.