The crystal structure of the borosilicate MCM-70 (siliceous framework formula Si12O24) was determined from synchrotron powder diffraction data with the program FOCUS. The framework crystallizes in space group Pmn2(1), where a = 13.663, b = 4.779, c = 8.723 angstrom, and forms ID ellipsoidal 10-ring channels with the following dimensions: 5.0 x 3.1 angstrom. Rietveld refinement of the model against synchrotron powder data from solvated material gives Rwp = 0.15, R(F-2) = 0.11. In addition to the four tetrahedral sites and seven framework oxygens, one potassium position is found during this refinement. Because of the unreasonable geometry of five putative extraframework oxygen sites, another synchrotron pattern was obtained from a dehydrated specimen. A refinement in space group P1n1 (removing the mirror operation of Pmn2(1)), where a = 13.670, b = 4.781, c = 8.687 angstrom, and beta = 90.24 degrees, verified that the previous framework geometry is preserved as well as the potassium position. One extraframework oxygen was located that would yield a reasonable K-O distance. The existence of potassium is verified by electron energy dispersive spectroscopic measurements as well as quantitative elemental analysis. (There are approximately 2.35 K sites per 12 Si in the unit cell.) It is likely that the constricted channels occlude KOH. B-11 and Si-29 MAS NMR measurements indicate a framework SiO2/B2O3 of approximately 40:1, which is consistent with a wavelength dispersive spectroscopic measurement. The silicate framework density is 2.10 gm/cm(3). The resulting framework density for T sites, 21.1, is unusually high for a zeolite, just below the value for paracelsian (21.4) or scapolite (21.8), each of which also has a smallest four-ring loop. The H-1 -> Si-29 CP MAS measurements suggest sample heterogeneity, that is, a portion of the sample that is strongly coupled to hydrogen and efficiently cross polarizes and another portion that does not.