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Optical and near-infrared spectroscopy of the newly discovered peculiar Ldwarf 2MASS J11263991-5003550 are presented. Folkes et al. identified thissource as a high proper motion L9+/-1 dwarf based on its strong H2O absorptionat 1.4 micron. We find that the optical spectrum of 2MASS J1126-5003 is in factconsistent with that of a normal L4.5 dwarf with notably enhanced FeHabsorption at 9896 A. However, its near-infrared spectrum is unusually blue,with strong H2O and weak CO bands similar in character to several recentlyidentified ``blue L dwarfs''. Using 2MASS J1126-5003 as a case study, andguided by trends in the condensate cloud models of Burrows et al. and Marley etal., we find that the observed spectral peculiarities of these sources can beadequately explained by the presence of thin and/or large-grained condensateclouds as compared to normal field L dwarfs. Atypical surface gravities ormetallicities alone cannot reproduce the observed peculiarities, although theymay be partly responsible for the unusual condensate properties. We also ruleout unresolved multiplicity as a cause for the spectral peculiarities of 2MASSJ1126-5003. Our analysis is supported by examination of Spitzer mid-infraredspectral data from Cushing et al. which show that bluer L dwarfs tend to haveweaker 10 micron absorption, a feature tentatively associated with silicateoxide grains. With their unique spectral properties, blue L dwarfs like 2MASSJ1126-5003 should prove useful in studying the formation and properties ofcondensates and condensate clouds in low temperature atmospheres.

Clouds, Gravity and Metallicity in Blue L dwarfs: The Case of 2MASS J11263991-5003550