Structure deformations induced by pressure and temperature in synthetic "Cs-tetra-ferri-annite" 1M [Cs (sub 1.78) (Fe (super 2+) (sub 5.93) Fe (super 3+) (sub 0.07) )(Si (sub 6.15) Fe (super 3+) (sub 1.80) Al (sub 0.05) )O 20 (OH) 4 ], space group C2/m, were analyzed to investigate the capability of the mica structure to store the radiogenic isotopes 135 Cs and 137 Cs. "Cs-tetra-ferri-annite" is not a mineral name, but for the sake of brevity is used here to designate a synthetic analog of the mineral tetra-ferri-annite. The bulk modulus and its pressure derivative determined by fitting the unit-cell volumes between 0 and 47 kbar to a third-order Birch-Murnaghan equation of state are K 0 = 257(8) kbar and K' 0 = 21(1), respectively. Between 23 degrees C and 582 degrees C, the a and b lattice parameters remain essentially unchanged, but the thermal expansion coefficient of the c axis is alpha c = 3.12(9) X 10 (super -5) degrees C (super -1) . High pressure (P) and high temperature (T) produce limited internal strain in the structure. The tetrahedral rotation angle, alpha , is very small and does not change significantly throughout the P and T range investigated. Above 450 degrees C in air, "Cs-tetra-ferri-annite" underwent an oxidation of octahedral iron in the M2cis site, balanced by the loss of H and shown by a decrease of the unit-cell volume. Independent isobaric data on thermal expansion and isothermal compressibility data define the "geometric" equation of state for "Cs-tetra-ferri-annite": V/V 0 = 1+3.0(1) 10 (super -5) T - 2.68(9) 10 (super -3) P+2.0(2) X 10 (super -5) P 2 where T is in degrees Celsius, P is in kilobars. The alpha /beta ratio of about 12 bar/ degrees C indicate that the cell volume of "Cs-tetra-ferri-annite" remains unchanged under geothermal gradients of approximately 23 degrees C/km. On the whole, the data confirm that the structure of "Cs-tetra-ferri-annite" may be a suitable candidate for the storage of large ions, such as Cs in the interlayer and should be considered as a potential Synroc component.