『Abstract
In agate structures three substructures can be discerned: common
agate banding, infiltration channel banding, and horizontal layering.
Infiltration channel banding has so many specific features that
it cannot be created by a process involving a high degree of randomness,
such as deformation. Infiltration channels are indeed inflow structures
that formed on the interior of the very first chalcedony layer.
Both, the common agate banding and the infiltration channel banding
are generated by two different processes of silica transport,
that depend upon the same general mechanisms: A silica concentration
gradient is generated and maintained, directed from the intergranular
solution of the cavity-surrounding rock into the cavity, in which
the agate structure is building up. The driving force is the osmotic
pressure difference caused by the semipermeability of the currently
youngest, innermost silica gel membrane. The very first gel layer
(membrane) that immediately lines the inner surface of the cavity
has a special role. (i) If the very first gel layer remains intact,
common agate banding is formed: silica is transported by diffusion
down the concentration gradient directly into the cavity (direct
process). (ii) If the very first gel layer is penetrated by one
or more capillary fissures, infiltration channel banding is formed:
silica is transported by injection of solution through capillary
fissures into the cavity (injection process).
Both these processes contain a relaxation mechanism (‘internal
rhythm’ reflected by the agate banding), caused by the incommensurable
time scales of deposition and ageing of the silica gel. The depleted
solvent is recycled back to the intergranular space of the surrounding
rock through the total outer surface f the cavity (continuity
condition).
To test the hydrodynamic plausibility of the model, flow patterns
were computed, (A) based on Schlichting's model of the ‘plane
laminar jet’ and (B) for Hagen-Poiseuille flow supposed to be
fully developed in the fissure. In the vicinity of the entrance
point the strongly accelerated flow diminishes the probability
of accretion continuously towards the entrance point of the jet,
where it is zero, reflected by the striking elegance with which
the agate bands generally thin out in this direction.
The combined model predicts entrance velocities, which are so
low that the flow is dissipated to Brownian motion within an order
of magnitude of the cavity dimensions. The calculated transport
rate of silica into the cavity yields durations of formation between
decades and thousands of years, in accordance with geological
field experience.
As full agate (completely silicified) is the normal case and
porous biscuit agate occurs rarely, the model proposed contains
the idea that during the transition from biscuit agate to full
agate the densification of the structure is caused by the incorporation
of moganite. The densification of each single layer is finished,
before the deposition of the next layer starts.』
Preface
1. Introduction
2. Infiltration channels of agate structures I: Transport model
2.1. Additional observations of agate structures
2.2. A new model of agate formation
2.3. General pre-requisites of agate formation
2.4. The basic model of agate formation - part 1
2.4.1. Process 1: Common agate banding - the direct process
2.4.2. Process 2: Infiltration channel banding - the injection
process
2.4.3. Interaction between the two processes
2.4.4. Apposition fabric of agates: control by internal and
external rhythmical processes
3. Infiltration channels of agate structures II: Hydrodynamic
model
3.1. Specific model: Process of shaping the infiltration
channels
3.2. A new approach: Water molecule cluster size, Brownian motion
and limiting velocity zL
3.3. Model A: Laminar plane free jet, fluid into fluid, non-buoyant
3.4. Model B: Hagen-Poiseuille flow for a plane fissure
3.4.1. Drainage depth of the compensation current along the
wall
3.4.2. Penetration depth of the jet and kinematic momentum K
3.5. The combined model (A + B): Axial penetration depth and
estimation of the duration of the filling processes
3.5.1. Estimation of the axial penetration depth
3.5.2. Estimation of the duration of the filling process
3.6. The basic model of agate formation - part 2
4. Future tasks and final remarks
Acknowledgement
References