[Chemical composition] Fe 2+ can be replaced by Mg 2+ (≤4%), occasionally by Co (≤85%), Zn (≤74%); Al 3+ can be Fe 3+ (≤5% )instead. [Crystal structure] orthorhombic system; Figure G-14 The rule of cross stone and kyanite (quoted from Pan Zhaoyu et al., 1993) y and z are crystal axes, and subscripts s and k represent cross stone and kyanite, respectively. [Form] The crystal form is short column (Fig. G-15(a), (b)). The twin crystal is extremely characteristic, often interspersed with twin crystals (Fig. G-15(c), (d), (e)), twin planes or along (031), and the two bodies are nearly orthogonal to cross (c); or According to (231), the two individuals are obliquely nearly 60° into oblique crosses (d) and (e). Some are also irregularly granulated (Figure G-16). Figure G-15 Crystal and twin crystal of the cross stone (quoted from Pan Zhaoyu et al., 1993) (a), (b) single crystal; (c), (d), (e) double crystal oblique column: m{110}, r{101}; parallel double-sided: c{001}, b{010}  Figure G-16 Irregular granular cross stone crystal aggregate [Physical properties] dark brown, reddish brown, yellowish brown; glass luster, but after the change (or impure) often dim dull or earthy. Cleavage {010} medium. Hardness 7.5. Relative density: 3.74 ~ 3.83. [genesis and production] is mainly a product of regional metamorphism and a few contact metamorphism. [Identification features] short columnar shape, rhombic cross section, especially double crystal shape, dark brown, reddish brown, high hardness, which can be distinguished from andalusite . [Main use] has the meaning of mineralogy and petrology.
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a 0 = 0.781 nm, b 0 = 1.662 nm, c 0 = 0.565 nm; Z = 2. The crystal structure can be regarded as a parallel (010) plane kyanite structure layer (ie, a parallel (100) layer formed by the [AlO 6 ] octahedral chain in the kyanite structure) and the iron hydroxide layer are alternately stacked. This makes the (100) plane of the kyanite often form a regular connection according to the (010) plane of the cross stone (Fig. G-14). 
FeAl 4 [SiO 4] 2 O 2 (OH) 2 or written as Fe (OH) 2+ 2 Al 2 [SiO 4] O, which is equivalent to two kyanite hydroxide plus iron.