Three thermal springs, at Lýsuhóll on Snæfellnes Peninsula on the west coast of Iceland, with vent water temperatures of 20 to 46°C are characterized by thriving microbial mats that mediate the precipitation of opal-A, Fe-rich opal-A, Fe-rich reticulate coatings, and calcite that are intermixed with each other at micro- to macro-scales. Although many of the microbes are preserved by silicification, there is no evidence of preservation by calcification. The calcite crystals that grew in the confines of the microbial mats, despite not being perfectly formed and < 1 mm long, can be divided into prismatic forms (types I to III), trigonal prisms, dodecahedrons, plates, rhombic forms (types I to III), and dendrites. Apart from the dendrites, all of the other crystals are mesocrystals formed of nanocrystals that are aligned in perfect crystallographic registers. All of these crystals grew through the non-classical crystal growth model, which is a particle-based system. In this case, the growth of the nanocrystals and the development of the mesocrystals took place in the microdomains that developed in the biofilms. Recognition of this type of crystal growth in older spring deposits may be difficult because (1) the mesocrystals may be transformed to monocrystals by fusion of the nanocrystals, (2) evidence of the microbes and biofilms may be missing because they are commonly lost to decay before preservation by calcification can take place, and/or (3) these mesocrystals may become the main nucleation centers for larger calcite crystals that grow via the classical crystal growth model. Irrespective of these nuances, the possibility that calcite crystals initially developed as mesocrystals should be a critical consideration in the evaluation of any spring system where calcite is being precipitated.