Dendrites

Two categories of dendrites exist: those in which the shape is mainly determined by crystal structure, and those where this is not the case, the material being amorphous or ultra-finely crystalline. Only dendrites of the second category are dealt with in this paper, and especially those found in the Solnhofen Limestone of southern Germany. These dendrites consist either of Mn- or Fe-(hydr)oxides. Two classes occur: (1) fissure dendrites, limited to joints and bedding plane fissures, etc.; (2) internal dendrites, branching in three dimensions, and penetrating from dendrites or other Mn-or Fe-accumulations on joint surfaces or bedding planes into the non-fissured rock. Both types form by deposition in pores, not wider than a few μm, thus constituting ramified systems of host rock, cemented by Mn- or Fe-material. The first generation of Mn-dendrites in the Solnhofen Limestone evolves in two steps: (1) manganiferous solutions, derived from residual loams and alluvial deposits overlying the limestone, penetrate into joints, and from there into bedding plane fissures and unfissured rock; (2) from these solutions solid dendrite material is deposited. The precipitation starts from older deposits of the same composition: specks, lumps, crusts etc., or from newly generated nuclei. In this way either 'tree'- or 'star'-shaped dendrites are produced. 'Tree'-shaped dendrites on bedding planes, starting from joints, grow in directions away from these joints, the material being supplied from the opposite directions. Once formed, renewed supply of Mn-material results in thickening and broadening of stems and branches, the ‘texture’ thereby becoming coarser, but the total size remaining unchanged.The first formation of Fe-dendrites takes place in the same manner, but their further growth is more complicated. The branching of these dendrites is due to the circumstance that diffusion of the components of the solution is limited to a system of micropores. This inhibits the production of ‘massive’, round patches around the points of origin, because the average distances to be covered by the ions, molecules or sol particles before they are fixed on the growing deposits, are much smaller for branched shapes. Most dendrites are the result of repeated deposition. The transportation of Mn and Fe to the fissures or other sites of precipitation probably occurs chiefly as dissolved chelates. In most cases the precipitation itself may be due to a rise in pH as a consequence of dissolution of CaCO3.