Biomedical Engineering Reference
In-Depth Information
hot. The sign of the difference in (electro-)chemical potential determines the sense
a physical process will take although it does not say anything about the rate of the
reaction (reaction kinetics).
The history of earth's minerals started by accretion of cosmic dust, meteorites
and subsequently those formed in earth's magma. That, let us say, happened the
first
day
paraphrasing Genesis 1 of the Bible:
the earth was barren with no form of life
followed by the command to the light to shine in the first day
. At the typical high
temperatures of the magma, a relatively homogeneous mineral soup was produced.
Olivine, pyroxene and feldspars were the main minerals in a repertoire containing an
estimated number of 250 different minerals at the end of a geological period some
4.55 Gya (i.e.,
G
iga
y
ears
a
go).
On cooling, say during
day 2
, the elements of the archean soup became more
selective in choosing their reaction partners, not dictated by butterflies in the
stomach but by principles of minimizing free energy. Fractional crystallization (con-
ditioned by reaction kinetics, concentration, temperature, pressure, solubility, etc.),
the formation of continents, hydrothermal processes, metamorphism, weathering
and so on led to further diversification and the repertoire was growing to about 1,500
different minerals. This was the situation at the end of the Neoarchean earth with, as
in Genesis 1 the
second day
,
a dome to separate the water above it from the water
below
.
1
The atmosphere was anoxic but life (microorganisms) was emerging and
had a definite effect (major or minor is not well established) on the mineral compo-
sition of sedimentary rocks. Anyway, two things seem to be clear: (1) according to
some origin-of-life scenarios the
second day
of chemical processing was a prerequi-
site for life and (2) once life started it co-conditioned the earth's mineral evolution.
We will come back on this issue in Chap.
14
.
The dawn of
day 3
(
2.5 Gya) is characterized by the
The Great Oxidation
Event
. As just said, for the past 2.5 billion years earth's minerals co-evolved with
life. In Genesis1:11 God said
I command the earth to produce all kinds of plants,
including fruit trees and grain
and so it was. The oxygenation of the atmosphere
was principally a consequence of oxygenic photosynthesis (cyanobacteria). Living
organisms found it worthwhile to frustrate thermodynamics. They expended part of
their energy reservoir to synthesize minerals that otherwise would not form under
the given conditions of temperature and pressure. Gypsum is here the obvious exam-
ple. Sulfate is at present ocean's second most abundant anion. CaSO
4
was formed
by the presence of SO
2
4
, partly provided through oxidation of sulfur by bacterial
activity, the companion source being photooxidation of sulfur. Another example of
bioactivated mineral formation are the highly organized aragonite crystals, a poly-
morph of CaCO
3
, in the pearl shells of some mollusks [
233
,
234
]. The iridescent
<
1
The inspiring review by Hazen and colleagues was the trigger to refer to Genesis [232
]. The
sequence of geological happenings as elaborated in the Modern Era parallels the logic thinking of
Genesis authors some two and a half millennia ago. Realistic time scales, however, are a nineteenth
century performance when geology emerged as a new scientific discipline. An indirect product
of this emerging science was Darwin (born 1809) and its masterpiece
On the Origin of Species
(published 1859).
