Chemistry Reference
In-Depth Information
Piemontite
is a manganesiferous epidote where manganese is in the Mn
3+
form: Ca
2
(Mn
3+
, Fe
3+
, Al) Al
2
Si
3
O
12
(OH). Piemontite form a continuous series
from pistachites to an end member of formula Ca
2
Mn
3+
Al
2
Si
3
O
12
(OH). Alu-
minum in M position M may be partially replaced by Mn
3+
, defining an end
member of formula Ca
2
Mn
3+
2
AlSi
3
O
12
(OH).
Rare earth elements (REE) enter in the lattice of epidote by
substitution:
Ca
2+
Fe
3+
REE
3+
Fe
2+
In addition, Th, U, Mn
2+
may enter in the X site and the beryllium may
enter as a substitution of Si.
Allanite
(also known as
orthite
) is a REE epi-
dote of formula:
(Ca, Ce, La, Y, Th, U, Mn
2+
)
2
(Al, Fe
3+
, Fe
2+
)Al
2
Si
3
O
12
(OH)
Pumpellyite
has a structure similar to that of epidote: chains of octahe-
dra, linked by SiO
4
tetrahedra and Si
2
O
7
double tetrahedra. This gives place
to 7-fold coordinated sites, occupied by Ca in the chains of octahedra, four
sites are occupied by only Al; two sites contain (Al, Fe, Mg). OH ions par-
tially replace oxygen. Hence the formula is:
Ca
2
(Al, Mg, Fe
2+
, Fe
3+
)Al
2
(Si
2
(O, OH)
7
) (SiO
4
) (O, OH)
3
The composition of pumpellyite varies by substitutions Al Fe
3+
and
Mg Fe
2+
and, rarely, by the presence of chromium and manganese.
The stability range is limited by temperature: the pumpellyite is replaced
at lower temperatures (below 200-250°C) by the association chlorite
+
preh-
nite and towards higher temperatures (above 350-400°C) by zoisite
+
chlo-
rite
calcic garnet. These reactions define the prehnite-pumpellyite facies
between zeolite and greenschist facies. Pumpellyite is also found at higher
pressures in blueschist facies.
Lawsonite
is a mineral close to epidote in its structure: chains of alumi-
num octahedra, linked by Si
2
O
7
groups This gives place for sites for Ca and
OH. Chemically lawsonite varies little: CaAl
2
Si
2
O
7
(OH)
2
,H
2
O; this formula
is similar to that of anorthite except for the presence of H
2
O and a more
compact structure that makes lawsonite a mineral of high-pressure. At lower
pressures and higher temperatures it is replaced by zoisite
+
quartz
(Figures 1.9 and 3.4) and at "low" pressure (up to 10kb) and at high tem-
perature, by anorthite.
+
kyanite
+
Prehnite
Prehnite Ca
2
Al
2
Si
3
O
10
(OH)
2
is a phyllosilicate, made of Si
3
AlO
10
sheets, where
alumina is in the tetrahedral position, bound by chains Al(in octahedral
