Silicic acid, lithium magnesium sodium salt

Endpoint summary

Description of key information

Terrestrial toxicity testing has been waived based on the results of the chemical safety assessment not indicating the need to further investigate the effect of the substance on terrestrial toxicity.  If released to the environment, Laponite is expected to combine indistinguishably with the soil or sediment due to its similarity with inorganic soils/sediments and will be subjected to natural processes under environmental conditions (cation exchange, dissolution, sedimentation).  The amount of soluble free Na, Mg, Li and Si that would be made available will not contribute to the background levels of these ions already present. Terrestrial toxicity will not occur in excess of exposure to naturally occurring sediments.  

Additional information

Furthermore Laponite is a synthetically manufactured layered silicate similar to a naturally occurring smectite clay: Hectorite. Laponite, a Silicate(2-), disodium, reaction products with lithium magnesium sodium silicate, (EC no 258-476-2) is a monoconstituent substance and is an inorganic layered silicate structure with a unit cell of the following composition: Na_0.7[Li_0.3Mg_5.5Si₈O₂₀(OH)₄]. Hectorite is a trioctahedral, magnesium based smectite clay with a unit cell of the following composition: H2LiMgNaO12Si4-2. The unique properties of the naturally occurring hectorite active mineral are very small platelet size, an elongated platelet structure with an inherent negative charge counterbalanced by exchangeable sodium (Na+) ions, light color with low iron content and high viscosifying ability in water. Any nanoform exposures from laponite would be comparable to background exposure from the nanoform compartment of hectorite and other naturally occurring clays (Kahn , 1957).  The constituents of this silicate are Na, Mg, Si and Li. The substance does not exhibit hydrolysis or dissociation to its component ions unless the pH of the aqueous medium is < 4.0, which is lower than most environmental pH conditions.  If laponite release results in dissolution in the environment no significant increase in exposure is expected to aquatic, sediment, or terrestrial species is expected to occur. Na, Mg, Li and Si are already widely found in the environment both in waters and soils / sediments. The following rock types will gradually erode and generate soils/sediments through geological degradation.   Sodium is a major element in all igneous rock types except ultramafic rocks.  Stueber and Goles (1967) report an average Na concentration of 105 mg kg-1, although ultramafic inclusions were found to contain up to 1,490 mg kg-1 Na.  Felsic and mafic plutonic rock types, such as granite and gabbro, contain percentage level concentrations of Na (around 2-3%); other felsic rocks, e.g., nepheline syenite, have been reported to contain up to 6.6% Na (Wedepohl 1978).  Basalt, andesite and other effusive rocks contain similar concentrations of Na to the plutonic series, with an average of 2–3%. Magnesium is a significant component of phyllosilicate minerals, such as chlorite, montmorillonite and glauconite, although it is low in illite, so shale (1.5% Mg) has higher concentrations than sandstone (0.7% Mg).  Magnesium carbonate minerals, commonly dolomite, are generally more soluble than Ca carbonate and form around 30% of all carbonates.  Magnesium occurs in sediments either as a solute in porewater or as an important component in the formation of late stage diagenetic chlorite and dolomite (Wedepohl 1978). The average concentration of Mg in loess is 0.68% (McLennan and Murray 1999).  Lithium is a lithophile metallic element, occurring predominantly in silicate minerals, and is widely present as an accessory element in Kfeldspar, biotite mica, amphibole and clay minerals, such as illite, where it can substitute for K, Na and Mg.  It also forms several rather rare minerals occurring mostly in pegmatite, including spodumene LiAlSi2O6 and lepidolite K2Li3Al4Si7O21. The median Li content in stream sediment is 20.8 mg kg-1, with a range from 0.28 to 271 mg kg-1.  Silica occurs as amorphous to crystalline form in many types of igneous, metamorphic, and sedimentary rocks, but in sediments and sedimentary rocks much of the silica is detrital material. The chief forms of silica are hydrous opal, crytocrystalline chalcedony, and crystalline quartz. Sedimentary silica enters into the development of authigenic quartz and silicates, into clay minerals, and into other secondary compounds as a result of geochemical action.