Sepiolite origin

Fig. 24.1. Volumes of minerals (amorphous silica, calcite, and sepiolite) precipitated during a reaction model simulating at 25 °C the evaporation of Sierra Nevada spring water in equilibrium with atmospheric C02, plotted against the concentration factor. For example, a concentration factor of x 100 means that of the original 1 kg of water, 10 grams remain.

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. 

The hydrothermal stability of the SA1 sample has been compared with that of the original SMg in Figure 6. The X-Ray diffraction pattern of an A1 sepiolite before (a) and after (b) steaming at 700aC, 100% steam and a natural sepiolite (SMg) also steamed at 7000C (c). The results indicate that after this thermal treatment an important fraction of the aluminic sepiolite remains unalterated, while the natural sample is completely folded. The folding of the sepiolite is shown by the disappearance of the (1,10) reflexion at 7.2, 20, and the appearance of a new reflexion at 8.9, 20 (16). Moreover, the adsorption of pyridine shows the presence of a residual Bronsted acidity in the SA1 after the hydrothermal treatment which is approximately 30% of the untreated sample. 

Table II.—Original And Recomputed Data For The Free Energy Of Formation Of Kaolinite And Sepiolite...

Stoessell R. K. and Hay R. L. (1978) The geochemical origin of sepiolite and kerolite at Amboseli, Kenya. Contrib. Mineral. Petrol. 65, 255-267. 

Palygorskite and sepiolite are magnesium-rich fibrous aluminosilicates that have been identified in basal deep-sea sediments (Table 4 e.g., Hathaway and Sachs, 1965 Bowles et al., 1971 Bonatti and Joensuu, 1968 Church and Velde, 1979 Jones and Galan, 1988 Velde, 1985). These phases are commonly associated with smectite, and it has been suggested that they originate by alteration of montmorillonite by low-temperature, magnesium-rich, hydrothermal solutions (Bonatti and Joensuu, 1968), e.g.. 

Fuller s earth contains montmorillonite and sometimes palygorskite-sepiolite. Some of the other minerals that may be present in Fuller s earth are calcite, dolomite, and quartz. It has sedimentary, residual and hydrothermal origin. It is a non-plastic, thixotropic material. It has long been used to clean raw wool from grease and dirt. It is also used for decolourizing, filtering, and purifying oils and animal fat. 

Of note was the low total pore volume of the pure titania monolith after treatment at 800°C in comparison to the materials containing sepiolite which maintained high total pore volumes after treatment at this temperature. After treatment at 1000°C the pore volumes were severely reduced due to the phase changes of anatase to rutile above 1000°C [11] and sepiolite to enstatite above 830°C [12], which caused a collapse of the original pore structure and a substancial shrinking of the monolith. After treatment at 1000°C the pore volumes of the mixed composition monoliths were all much lower than expected, especially for the 20 80 wt% material. 

Figure 5.48 includes the adsorption isotherms of N2 at 77 K for sepiolite S and the two original carbons (P and C). The shape of the isotherm for sepiolite is near to Type II, characteristic of this material, whereas the isotherms for the activated carbons are of Type I, as expected. Micropore volumes show differences in microporosity and micropore size distribution of the adsorbents. The similarity of Vq(N2 77 K) and Vo(C02 273 K) values for sepiolite indicates the existence of both narrow and homogeneous microporosity due... 

The adsorption of HDP on the surface of the original sepiolite gives a high-affinity adsorption isotherm. The amount of HDP adsorbed in the region of ion exchange (i.e., for the immeasurably low equilibrium concentrations) is 0.2 mmol/g, which is in... 

F. 15 SAXS experiments on sepiolite minerals (1) original sample, (2) treated 12 h with 2 M HCl, (3) treated 36 h with 2 M HCl. arious layer silicates (1) kaolinite, The intensities are moved by one (2) and two (3) units related to... 

For water, the monolayer capacity calculated by the B.E.T. method was converted in equivalent surface area using a cross sectional area of 14.8 for the water molecule. Results plotted in table 3 (column 4) show that regarding rehydration after heating at different temperatures, the attapulgite can rehydrate after the structure is folded up to a final outgassing temperature of 225°C. This value is the same that observed for sepiolite (ref. 4). Beyond this temperature, the new bonds originating in the anhydrous structure resist rehydration. 

It should be noted that this mineral was found in a basalt in Australia, associated with anorthite in Norway, i.e., in condition of genesis much more varied than for sepiolites On the other hand, the interpretation of the authors who have studied these deposits often assumed hydrothermal origin. If we can still consider this mineral as formed at low temperature, it seems that it can appear in a larger range of conditions than sepiolites, and in particular it would be less dependent on the chemical composition of the original rocks. 

It also occurs in Utah and Colorado. The deposits concerned are always limestone, often giobertite of lacustral origin, but there is also sepiolite in the marine deposits of Keuper and Permotrias. 

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