Ca-dachiardite is present in Cretaceous volcanic breccias affected by very low grade metamorphism. The cement of the breccia shows quartz, Ca-dachiardite, mordenite, cristobalite, calcite and siderite, while in the clasts there are pectolite, prehnite, yugawaralite, Ca-dachiardite, albite, adularia, interestratified smectite/chlorite (S/C), quartz and cristobalite. Most Ca-dachiardites are found in the cement of the breccia. It occurs principally as fibrous tubes of parallel or lightly divergent bladed and tabular crystals, arranged in concentrical layers with length along 'b' and the fibers; the crystals are flattened on {001} and rarely on {100}. It shows cyclic twins in cross section normal to the length and multiple twins parallel to {001}, but single crystals are also present. The optical character shows: a=b; g:c= 43°; 2Va=55°/60°; strong dispersion r>v. The structural formulae is: Ca1.54-1.72 K0.82-0.98 Na0 (Al3.65-3.98 Si19.95-20.25 O48) 13H2O ; TSi= 0.83-0.85; R2+/(R+ + R2+)=0.61/0.68; DEC=Ca and K. The strong diffraction lines are: 3.460 Å; 3.190 Å and 1.870 Å; a0 =18.679Å; b0=7.488 Å; c0 10.267 Å; b=107°86'. The secondary mineralogical succession deposited according with the decrease in temperature and Xco2, could be: calcite (step I); pectolite, prehnite, quartz, yugawaralite, Ca-dachiardite, S/C, adularia, albite (step II, zeolite facies); mordenite and cristobalite (step III) and then, siderite and calcite (step IV). The metamorphic assemblages suggests that the crystallization conditions of Ca-dachiardite would be 200-220°C at pressures smaller than 0.5 kb, from high aSi and aCa fluids.Se hallo dachiardita calcica en brechas de rocas volcanicas cretacicas afectadas por metamorfismo de muy bajo grado. El RESUMEN:cemento de la brecha presenta cuarzo, dachiardita calcica, mordenita, cristobalita, calcita y siderita, mientras que en los clastos hay pectolita, prehnita, yugawaralita, dachiardita calcica, albita, adularia, interestratificados de esmectita clorita (S/C), cuarzo y cristobalita. La mayor parte de dachiardita se halla en el cemento de la brecha, donde forma estructuras tubulares que contienen cristales hojosos a tabulares, paralelos o levemente radiados dispuestos en capas concentricas y elongados a lo largo del eje cristalografico 'b' y de los tubos; los cristales estan achatados segun {001} o raramente {100}. Muestra maclas ciclicas en la seccion perpendicular al alargamiento y maclas multiples paralelas a {001}, aunque tambien hay individuos sin maclar. Las caracteristicas opticas son: a=b; g:c= 43°; 2Va= 55°/60°; dispersion fuerte r>v. La formula estructural es: Ca1,54-1.72 K0,82-0,98 Na0 (Al3,65-3,98 Si19,95-20,25 O48) 13H2O ; TSi= 0,83-0,85; R2+/ (R+ + R2+) = 0,61/0,68; DEC = Ca y K. Las lineas de difraccion mas fuertes son: 3,460 Å; 3,190 Å y 1,870 Å; a0 =18,679 Å; b0=7,488 Å; c0=10,267 Å; b=107°86'. La sucesion mineralogica secundaria depositada con el descenso de la temperatura y cambios en la Xco2 seria: calcita (etapa I); pectolita, prehnita, yugawaralita, cuarzo, dachiardita calcica, interestratificados S/C, adularia, albita (etapa II, facies de ceolita); mordenita y cristobalita (etapa III) y por ultimo, calcita y siderita (etapa IV). Las paragenesis metamorficas indican temperaturas de formacion para la dachiardita calcica de 200 a 220°C a presiones menores que 0,5 kb, a partir de fluidos enriquecidos en aSi y aCa