Andradite,
ideal end-member formula Ca3Fe2Si3O12,
is one of the common rock-forming silicate garnets found in Earth’s
crust. Its P-T stability and
thermodynamic
properties have been investigated several times using phase-equilibrium
results. The heat-capacity behavior, Cp,
of a synthetic sample has been studied via calorimetry including a low
temperature adiabatic investigation. In addition, various physical
properties of
andradite have been studied by diffraction and different spectroscopic
methods.
There are still, however, outstanding questions regarding andradite’s
precise thermodynamic
behavior. Three issues are: i) Could there be differences in the
thermodynamic
properties, namely heat capacity, Cp,
between synthetic and natural andradite crystals, as observed in the
Ca-garnet grossular,
Ca3Al2Si3O12?
ii) What is the precise
thermal nature of the low-temperature magnetic-phase-transition
behavior of
andradite? and iii) How quantitative are the first and older published
calorimetric
(i.e., adiabatic and DSC) heat capacity results? In this work, four
natural,
nearly end-member single crystals and two
synthetic polycrystalline andradite samples were carefully
characterized,
depending on the sample, by optical microscope examination, X-ray
powder
diffraction, microprobe analysis, and IR and UV/VIS single-crystal
spectroscopy. The IR spectra of the different samples often show a main, intense OH-
stretching band located
at 3563 cm-1, but other OH-
bands can sometimes be
observed as well. Structural OH- concentrations,
calculated from the
IR spectra, vary from about 0.006 to 0.240 wt % H2O
using a
calibration based on grossular. UV/VIS spectra indicate that there can
be
slight, but not fully understood, differences in the electronic state,
probably
involving Fe, between synthetic versus natural andradite crystals. Cp
behavior, employing the same andradite samples that were used for the
other
measurements, was determined by relaxation micro-calorimetry between 2
and 300
K and by DSC methods between 150/300 and 700/950 K. The low-temperature
Cp
results show a
magnetic
phase transition with a Néel temperature of 11.3 ± 0.2 K,
which could be
slightly affected by the precise electronic state of the crystals. The
published
adiabatic calorimetry results on andradite made down to 8 K (Robie et
al., 1987)
do not provide a full and correct thermal description of this magnetic
transition,
because it extends to even lower temperatures. The calorimetry Cp
measurements for the different samples give a best estimate for the
standard
third-law entropy at 298.15 K for andradite of So
≈ 324 ±
2 J/mol∙K vs the value of 316.4
±
2.0 J/mol∙K, as determined from the adiabatic investigation
(Robie et al., 1987). A synthetic sample, which may best represent
end-member
andradite gives So
= 324 ±
2.3 J/mol∙K.
The published adiabatic data are generally slightly higher in value.
Both
natural and synthetic crystals give similar So
values within experimental uncertainty of about 1.0 %, but one natural
andradite, richer in OH,
may have a very
slightly higher value around So
»
326 J/mol∙K. Low temperature DSC measurements made below 298 K agree
excellently with those from relaxation calorimetry. The DSC
measurements at 298
K show a similarity in Cp
behavior among natural and
synthetic andradites. A Cp
polynomial for use between 300 to about 1000 K was calculated from the
data on
synthetic andradite (SD23) giving: