A silica-based highly acoustically-anti-guiding optical fiber, fabricated using a molten core approach employing spinel (MgAl2O4) for the first time, is presented. To our knowledge, this is the first truly single mode optical fiber fabricated from a precursor crystal. It is shown that MgO increases the acoustic velocity when added to silica (some physical parameters of MgO are identified) and that the Brillouin gain in the core is less than one third that in the cladding in one of the fibers. This results from a massive acoustic waveguide attenuation term that broadens the spectrum to well over 200 MHz. For the first time, to the best of our knowledge, this also enabled the validation of the intrinsic Brillouin line-width (~20 MHz) of pure silica in fiber form via a direct measurement.