Sepiolite is a valid mineral with the following chemical formula: $8MgO\, \cdot \,12Si{O_2}\, \cdot \,n{H_2}O.$$

Its unit cell characteristics are: $a\, = \,13.4{\AA};\,b\, = \,26.8{\AA};\,c\, = \,5.28{\AA};Z\, = \,2;\,D_{{2^h}}^6\, - \,P\frac{{{2_1}}}{n}\,\frac{2}{c}\,\frac{2}{n}.$

They were determined on the sepiolite from Ampandrandava (Madagascar). Its structure was determined by means of the “trial and error” method and the observed and calculated intensities rendered a good agreement.

The formula of the structure is: $\mathop 1\limits_\infty {[{H_2}O]_n} \cdot \mathop 1\limits_\infty \{ {(Mg,F{e^{3 + }},F{e^{2 + }},\,Al)_8}{({H_2}O)_4}{(O,OH)_4}[{(Si,Al)_{10}}{O_{30}}]\} \cdot n\, \leqslant \,8.$

The mineral contains zeolitic, crystal- and constitution-water. A good agreement was achieved between the chemical, thermal, and morphological qualities and the x-ray data. The mineral remains stable up to about 350°C and begins to change into an “anhydride” above this temperature.

The formula of the structure of sepiolite. “anhydride” is: $\mathop 1\limits_\infty \{ {(Mg,F{e^{3 + }},F{e^{2 + }},\,Al)_8}{(O,\,OH)_4}[{(Si,\,Al)_{10}}{O_{30}}]\}.$

Its unit coll characteristics are: $a\, = \,10.9{\AA};\,b\, = \,23.3{\AA};\,c = 5.28{\AA};\,a \approx \,{90^ \circ };\,Z = \,2;\,C_{{2^h}}^5\, - \,P_n^{{2_1}}\,11.$

The minerals xylotile, mountain wood and gunnbjarnite are isotypical with sepiolite.