Paper No. 9
Presentation Time: 9:00 AM-6:30 PM


FLETCHER, Raymond C., Earth & Environmental Systems Institute, Pennsylvania State University, University Park, PA 16802 and MOZLEY, Peter S., Earth and Environmental Science, New Mexico Tech, Socorro, NM 87801,

Displacive carbonate crystal growth is recognized in a wide variety of settings, including calcretes, within phyllosilicates in sandstones, and in and around carbonate concretions. Displacive cement requires a mechanical response, but little attention to this has been given. Its incorporation in a model assuming constant calcite supersaturation satisfies observations for alternating D- and ND-shells in an elongate concretion composed of calcite-cemented sand.

The set of radii at which D-shells and ND-shells initiated and their intergranular cement volumes (IGV) were measured for a single concretion. All D-shells have IGV ~ 70, and are treated as veins in which sand is incorporated; the IGV of ND-shells decreases outward from 56 to 20.

Observed sharp transitions between shells are modeled by conditions on the kinetics of radial growth in D- and ND-shells. For a D-shell, in which the host sand is deformed, (dre/dt)D=K[RTlnW+srr(re)V0]; for an ND-shell, in which growth is by filling of porosity, (dre/dt)ND=K’RTlnW. re is the external surface of the concretion where growth takes place, K and K’<K are kinetics constants, R is the gas constant, T is temperature, W is saturation state, srr is radial compressive normal stress, and V0 is calcite specific volume. The D-ND transition takes place when the displacive growth rate drops to the non-displacive rate. The ND-D transition occurs when the D-rate that would be operative at the current srr(re) equals 1+m times the ND rate. The assumption is that nucleation of a D-shell requires overstep, while the transition to an ND shell does not. To compute srr(re), the sand is treated as elastic. The Young’s Modulus (E) of sand is 10-70 MPa << 1–70 GPa of the cemented concretion, so the concretion is treated as rigid.

Model dimensionless parameters are: S = Eo(1-IGV*/100) (de/a)/[(1+n)RTlnW], k = K’/K and m; de is an effective vein thickness, in which all displacive calcite is put into a single external vein with inner surface r = a, and IGV* is for the prior ND-shell, The model requires that (1-IGV*/100)(de/a) for all D-shells be equal, and ratios of all pairs of successive radii at which D-shells are initiated be equal; both are satisfied by the data. Two parameters can be fixed for an arbitrary value of the 3rd – e.g., m; the range in S may be estimated from concretion and field data.