Sepiolite zeolitic water

Finally, the 29Si CP/MAS-NMR spectrum of a partially dehydrated sepiolite that was subsequently exposed to acetone vapor is presented in Fig. 2c, and is strikingly similar to the spectrum of the original, untreated sepiolite (Fig. 2a). Since zeolitic water molecules are not present in this sample, and in light of the discussion of the partially dehydrated sepiolite sample, it appears that the acetone molecules have penetrated inside the microporous channels and reversed the structural changes that were caused by partial dehydration. Thus Fig. 2c confirms that acetone molecules enter the microporous channels of sepiolite, and are not simply adsorbed on the crystallite exterior surfaces. 

HCOs, Cr, F, and N03 The initial pore water chemistry is based on a sample that was ultracentrifuged from the proposed repository host rock (Tptpmn). Minerals considered include silica phases (a-cristobalite, quartz, tridymite, amorphous silica, and opal-CT), calcite, feldspars, smectites, illite, kaolinite, sepiolite, zeolites, fluorite, hematite, and gypsum. Treatment of CO includes gas-water equilibration, diffusion, and advection. 

The water adsorbed on external surfaces and the zeolitic water from the nanoporous tunnels is removed at relatively low temperatures. The elimination of coordinated, structural water starts when the zeolitic water is lost and is completed when dehydroxylation begins. Folding of the sepiolite crystals occurs when some structural water has been removed. This process, reversible for temperatures below 350°C, becomes irreversible once all the structural water molecules are removed and partial dehydroxylation has occurred, forming an anhydride form. Finally, the remaining Mg-OH hydroxyl groups are released at 850°C. 

Sepiolite is a hydrated magnesium silicate having an internal structure of channels which can accomodate zeolitic water and other molecules. The magnesium ion in sepiolite crystal is exchangeable with various metal ions(l). Loughlinite has a similar structure to sepiolite and is known as natural Na-sepiolite(2), Sepiolite is an effective catalyst for hydrorefining of hydrocarbons, fuel oils(3) and conversion of ethanol into ethylene or bute-l,3-diene(l). 

Sepiolite clay (<100 mesh) was heated in air at 120°C in order to remove the zeolitic and surface bound water molecules. The partially dehydrated clay mineral was subsequently exposed to acetone vapor at room temperature for a period of four days. H and 29Si CP MAS-NMR experiments revealed that the acetone molecules penetrated into the microporous channels of the sepiolite structure. Broad line 2H NMR studies using acetone-d6 revealed that, in addition to fast methyl group rotations, the guest acetone-d6 molecules were also undergoing 2-fold re-orientations about the carbonyl bond. The presence of acetone-d6 molecules adsorbed on the exterior surfaces of the sepiolite crystals was also detected at room temperature. 

The conditions necessary for artificial synthesis of smectites suggest the kind of natural environment in which these minerals will be found. This environment is alkaline as a result of restricted drainage and/or evaporative salt accumulation (see Chapter 8), so that the normally mobile alkaline and alkaline earth ions (Na, K, Ca, Mg) accumulate, along with silica. Neoformation of smectites from Na-rich saline water, and possibly illites from more K-rich water, is favored under these conditions. Other very silica-rich silicate minerals, such as attapulgites, sepiolites, and zeolites, are also known to fomi under these conditions. Less alkaline conditions may be necessary for Fe and Al-rich (dioctahedral) smectite formation, and a reducing environment may assist crystallization. 

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