A new sodium lithium silicate (as mineral silinaite) is synthesized from dispersions of silica in aqueous solutions of NaOH and LiOH at 90-150 degrees C. Synthetic samples of silinaite are characterized by X-ray powder diagrams, IR and MAS-NMR spectra, thermal dehydration, and gas adsorption measurements. Thermal dehydration produces an anhydrous lithium sodium silicate which melts at about 650 degrees C. The lithium and sodium ions can be exchanged by protons and alkylammonium ions. Exchange of the alkali metal inns by protons transforms silinaite into the H+ form, into alpha-H2Si2O5, or into a mixture of both acids. Formation of the acids largely increases the specific surface area. The H+ form of silinaite contains considerable amounts of micropores. Water is continuously desorbed between room temperature and 1000 degrees C. The influence of kinetic factors (trapping of water molecules between the collapsing layers of the acid) is stronger than in silinaite. The acid only intercalates a few types of guest molecules, e.g. alkyl- and arylamines and basic heterocyclic compounds. The intracrystalline reactivity is as low as for other monophyllosilicic acids such as alpha- and beta-H2Si2O5 and the H+ forms of kanemite and makatite.