The general formula (as given by Sokolova and Hawthorne, 2016) for the astrophyllite group minerals is A_2pB_rC₇D₂(T₄O₁₂)₂IX^OD2X^OA4X^PDnW_A2 where C represents cations at the M(1-4) sites in the O sheet and are commonly Fe²⁺, Mn, Na, Mg, Zn, Fe³⁺, Ca, Zr, Li; D represents cations in the H sheet and are either in 6 or 5 coordination and are Ti, Nb, Zr, Sn⁴⁺, ⁵Fe³⁺, Mg, Al; T = Si, Al; A_2pB_rW_A2 (I block) with p =1, 2; r = 1, 2; A = K, Rb, Cs, Ba, H₂O, Li, Pb²⁺, Na, ▫ where ▫ = vacancy; B = Na, Ca, Ba, H₂O, ▫; X^o refers to anions in the O sheet not bonded to T sites, X^OD = oxygen anions in common at the 3M and D vertices; X^OA = OH, F anions at the common vertices of 3M polyhedra; X^PD = F, O, OH, H₂O, ▫, apical anions of D cations at the edges of the HOH block; W_A = H₂O, ▫; and for X^PDn, n = 0. 1, 2. The astrophyllite group minerals form 2:1 phyllosilicate-type structures with portions of the structure described as HOH (analogous to TOT in 2:1 phyllosilicates) with T₄O₁₂ ribbons comprising the H (heterogeneous, hetero- meaning “extra”) sheet. Alternating with HOH blocks are intermediate (I) blocks along the c axis. Sokolova and Hawthorne (2016) described the astrophyllite group as a “supergroup” with three divisions (groups): the astrophyllite group, the kupletskite group and the devitoite group. HOH blocks may link directly (as in astrophyllite group, with Fe²⁺ dominant) or do not link (as in devitoite group) or direct linkage with Mn²⁺ dominant (as in kupletskite group). The linkages involve “bridges” of D-X^pD-D. These titanosilicates have similar a axial lengths to phyllosilicates (both near 5.4 Å) and d(001) values (~10.9 Å , although somewhat variable vs 10.0 Å in 2:1 phyllosilicates). The supergroup divisions are:
Astrophyllite Group, Fe²⁺ dominant, direct HOH linkage
astrophyllite K₂NaFe²⁺⁷Ti₂(Si₄O₁₂)₂O₂(OH)₄F
iobophyllite K₂NaFe²⁺⁷(Nb,Ti)(Si₄O₁₂)₂O₂(OH)₄(F,O)
zircophyllite K₂NaFe²⁺⁷Zr₂(Si₄O₁₂)₂O₂(OH)₄F
bulgakite Li₂(Ca,Na)Fe²⁺⁷Ti₂(Si₄O₁₂)₂O₂(OH)₄(F,O)(H₂O)₂
nalivkinite Li₂NaFe²⁺⁷Ti₂(Si₄O₁₂)₂O₂(OH)₄F(H₂O)₂
tarbagataite (K ▫)CaFe²⁺⁷Ti₂(Si₄O₁₂)₂O₂(OH)₅
Kupletskite Group, Mn²⁺ dominant, direct HOH linkage
kupletskite-1A K₂NaMn₇Ti₂(Si₄O₁₂)₂O₂(OH)₄F
kupletskite-2M K₂NaMn₇Ti₂(Si₄O₁₂)₂O₂(OH)₄F
kupletskite-(Cs) Cs₂NaMn₇Ti₂(Si₄O₁₂)₂O₂(OH)₄F
niobokupletskite K₂NaMn₇(Nb,Ti)₂(Si₄O₁₂)₂O₂(OH)₄(O,F)
Devitoite group
devitoite Ba₆Fe²⁺⁷Fe³⁺²(Si₄O₁₂)2(PO4 )2 (CO3 )O2 (OH)4
sveinbergeite (H₂O)₂[Ca(H₂O)](Fe²⁺⁶Fe³⁺)Ti²(Si₄O¹²)₂O₂(OH)₄(OH,H₂O)
lobanovite K₂Na(Fe²⁺⁴Mg₂Na)Ti₂(Si₄O₁₂)₂O₂(OH)₄
HOH blocks are found in other (heterophyllosilicate) titanosilicates, and these minerals have been described by Ferraris and co-workers (e.g., for a partial summary, see Ferraris, 1997,
Sokolova, 2006, Jin et al., 2018). These include:
nafertisite [Na,K, ▫)₄(Fe²⁺,Fe³⁺, ▫)₁₀(Ti₂O₃Si₁₂O₃₄)(O,OH)₆],
bafertisite [(Ba₂(Fe,Mn)₄Ti₂(Si₂O₇)₂O₂(OH)₂F₂,
jinshajiangite (Na,Ca)(Ba,K)Fe₄Ti₂(Si₂O₇)₂O₂(OH)₂F,
perraultite (Na,Ca)(Ba,K)Mn₄Ti₂(Si₂O₇)₂O₂(OH)₂F,
lamprophyllite Na₂(Sr,Ti,Na,Fe)₄(Ti₂O₂Si₄O₁₄)(O,F)₂,
seidozerite Na_1.6Ca_0.275Mn_0.425Ti_0.575Zr_0.925(Si₂O₇)OF,
and many others. The titanosilicates are found in hyperagpaitic (highly peralkaline nepheline syenites) rocks.