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Suomen Geologinen Seura. N: 0 35. Geologiska Sällskapet i Finland. 35
PIEMONTITE 1)
The mineral occurs as well formed short prisms, about 10 mm in length,
together with calcite, plagioclase; spessartite gamet, phlogopite, quartz and
oxide ore. The mineral was chemically analysed by Mr. Väänänen. The
analysis is given in Table 8. The table includes the unit cell dimensions, the
calculated atomic content of the cell and the optical properties of the mineral.
The d-spacings of the mineral are given in Table 3.
Table 8. Chemical composition and the unit cell of piemontite from Goldongri, India.
Analyst Paavo Väänänen, M. Sc., of the Geological Survey of Finland.
wt % I
Cation
%
Unit cell content
Si0 2 36.40 35.0 Si 5.91} 5.96 The uni t cell:
Al 2 0 5 15.89 18.0 Al 3.05 a = 8.88 A
Fe 2 0 5
11.06 8.0 Fe 3 +
b = 5.66 »
FeO c = 10.16 »
Mn 2 0 5 11.99 8.8 Mn ,."\2."
3 + 1.48 392 ß = 115. 5°
MnO 0.55 0.4 Mn 2 + 0.08 . Vo = 464.67
MgO 0.00 Z =4
Ti0 2
0.11 0.1 Ti 0.01 Space group P 2 1 1m
CaO 22.25 23.0 Ca 3.88}
D4 20
= 3.522
Na 2 0 0.41 0.8 Na 0.13 4.02
K 2 0 0.04 K 0.01 The optical properties
H 2 O+ 0.93 5.9 OH 0.97 (Sodium light)
H 2 O- 0.00
25.24 a = 1.768 yellow
CO 2 n.d. °
ß = 1.803 orange
PbO 0.04 Y = 1.843 carmine
CuO n.d. 2Va=68°
P 2 0 5 0.13 0.1 P 0.02 c:a = 38°
99.80 100.0
The unit cell content readily shows that there is just enough calcium
and sodium in the mineral to fill the eight co-ordination sites in the structure
(3.88 +0.13 = 4.01). On the other hand, there is just enough silicium to
occupy the tetrahedral positions (5.96). As usual in epidote minerals, Al
evidently occupies the two octahedral sites in the cell which belong to tlie
chain structure of the mineral. The remaining cations per unit cell, e. (J.,
1.0 Al, 1.4 Fe and 1.5 Mn, divide among themselves the four remaining
octahedral positions. This corresponds to the solid solution mixture: 27 mole
% clinozoisite, 35 % pistasite and 38 % piemontite (Ca 2 Mn 2 AISi 3 0 12 (OH)).
The piemontite now analysed is thus unusually rich in iron and manganese.
In fact, the sum of the pistasite and piemontite is one of the highest ever
analysed. This is clearly seen in the refractive indices of the mineral, which
1) The term piemontite - corresponding to the original Italian place name - is used instead
of »piedmontite» for the reason given by Deer, Howie and Zussman (1962).