COMMISSION GEOLOGIOUE - Arkisto.gsf.fi

36 Bulletin de la Commissioll geologique dc Finlande N: 0 212.
are higher than any values reported earlier. The same holds also for the
specific gravity of the mineral. The piemontite from Längban, Sweden
(Malmqvist 1929), and that from Chikla, India (Bilgrami 1956), have high er
contents of Fe 2
0 3
+Mn 2
0 3
but the indeces are lower. The refractivity
measured on those minerals must be associated with the considerably higher
Mn/Fe ratio. The optical properties measured from the now analysed material
do not fit the curves drawn by Deer, Howie and Zussman (1962) because
the curves do not take into account the variations in the Mn /Fe ratio. The
pistasite (Ca 2
FeAl 2 Si 3
0 12 (OH)) - piemontite (Ca 2 MnAI 2 Si 3 0 12 (OH)) diagram
of Marmo, Neuvonen and Ojanperä (1959) cannot be used for comparison
since their curves take into ac count only those piemontites in which the
ratio Al to (Fe+Mn) is about 2. In the mineral now analysed this ratio is
as low as l.05.
The unit cell dimensions determined agree weIl with those reported by
Marmo, Neuvonen and Ojanperä (1959). They differ greatly, however, from
the values usually listed for piemontite. The deviation cannot be explained
by the difference in the choice of the monoclinic unit cello The calculated
density for the pure piemontite, Ca 2
Mn 2 AISi 3 0 12 (OH), is as high as 4.2 if
computed according to the cell dimensions given in the hand books a = 8.05,
b = 5.70, C = 9.41 and x = 115°42'). The density calculated from the
dimensions now measured is 3.57 and thus agrees weIl with the density
value obtained with the direct pycnometer measurements 3.5 22 . It is most
likely, consequently, that there is an error in the earlier cell size determinations.
ACKNOWLEDGMENTS
The authors thank Sund ar Mineral (P) Ltd. for the facilities offered
during the field work, the Emil Aaltonen Foundation for financial aid and
Prof. W. D. West of Saugar University for encouragement. Thanks are
also due to Mr. Paavo Väänänen of the Geological Survey of Finland for
the chemical analyses and to Mr. A. Törn of the Computing Centre in Turku
for his co-operation. The authors wish also to thank Messrs Erkki Marttila
and Osmo Inkinen for their help in carrying out the optical and X-ray
measurements. The English of the manuscript was corrected by Mr. Paul
Sjöblom, M. A.