Сorrelations between fluorine, iron and titanium contents in magnesium members of the humite group
Gerasimova E.I *,**, Pekov I.V *,**, Kononkova N.N. **
*Lomonosov Moscow State University, Moscow, Russia
**Vernadsky Institute of Geochemistry and Analytical Chemistry of Russian Academy of Sciences
e-mail: ekaterina-gera@rambler.ru
Minerals of the humite group are monoclinic and orthorhombic orthosilicates with additional anions, the members of a morphotropic series with general formula nA2SiO4žA(R)2 with A = Mg, Mn2+, Ca (+ subordinate Fe, Ti, Zn, etc.) and R = F, OH (+ subordinate O). The group includes magnesium, manganese and calcium members. Five mineral species form the magnesium subgroup, namely norbergite, Mg3[SiO4]F2, chondrodite, Mg5[SiO4]2F2, humite, Mg7[SiO4]3F2, clinohumite, Mg9[SiO4]4F2, and hydroxylclinohumite, Mg9[SiO4]4(OH)2. The structures of humite-group minerals consist of Mg(F,OH)2 layers and nMg2[SiO4] blocks which alternate along the a axis. The silicate blocks have the same structure as olivine; their thickness increases with increase of n from norbergite to clinohumite.
Magnesium humite-group minerals are widespread in nature. They occur in skarn and skarn-greisen formations, calciphyres, rhodingites, alkaline-ultrabasic complexes and some other types of rocks. They can be not only accessory but also rock-forming minerals.
We have obtained 277 electron-microprobe analyses of magnesium members of the humite group: 23 for norbergite, 167 for chondrodite, 46 for humite and 51for clinohumite and hydroxylclinohumite. The empirical formulae were calculated on the basis of 4 cations (A+Si) for norberhite, 7 for chondrodite, 9 for humite, and 13 for clinohumites. From these data, the average values and ranges of contents of fluorine (wt.% and apfu), iron [Fe/(Mg+Fe+Ti+Mn+Zn), in atom proportions] and titanium [Ti/(Mg+Fe+Ti+Mn+Zn), in atom proportions] were calculated (Tables 1 and 2).
Table 1. Fluorine, iron and titanium contents (wt.%: average values and ranges) in the studied minerals
|
|
F |
TiO2 |
FeO |
|
Norbergite |
15.7 |
0.25 |
0.96 |
|
13.5-18.6 |
0.01-0.68 |
0.07-3.12 |
|
|
Chondrodite |
6.8 |
0.40 |
4.41 |
|
0.0-10.9 |
0.00-7.91 |
0.02-11.78 |
|
|
Humite |
2.5 |
1.58 |
3.85 |
|
0.0-5.7 |
0.00-5.48 |
0.12-11.84 |
|
|
Clinohumite |
1.9 |
1.39 |
3.74 |
|
0.0-4.4 |
0.00-5.34 |
0.09-12.49 |
Table 2. Contents of fluorine (apfu), iron (Fe/ΣMe) and titanium (Ti/ΣMe) and total value of Fe+Ti (Ti+Fe)/ΣMe: atom proportions, average values and ranges
|
|
F |
Fe/ΣMe |
Ti/ΣMe |
(Ti+Fe)/ΣMe |
|
Norbergite |
1.8 |
0.009 |
0.002 |
0.011 |
|
1.5-2.4 |
0.000-0.030 |
0.000-0.004 |
0.003-0.034 |
|
|
Chondrodite |
1.3 |
0.043 |
0.004 |
0.047 |
|
0.0-2.2 |
0.000-0.118 |
0.000-0.070 |
0.000-0.121 |
|
|
Humite |
0.6 |
0.039 |
0.015 |
0.054 |
|
0.0-1.5 |
0.001-0.125 |
0.000-0.052 |
0.008-0.176 |
|
|
Clinohumite |
0.6 |
0.038 |
0.013 |
0.050 |
|
0.0-1.5 |
0.001-0.129 |
0.000-0.049 |
0.004-0.178 |
Note: ΣMe=(Mg+Fe+Ti+Mn+Zn)
Not only maximum F amount but also minimal values of Fe/ΣMe and Ti/ΣMe ratios are typical for norbergite. Chondrodite is characterized by intermediate value for F, maximum for Fe and, like norbergite, very low amount of Ti. Humite and clinohumite can be united as minerals with very close to each other, medium values of all discussed factors (Table 2). Thus, we can distinguish three subgroups: 1) norbergite, 2) chondrodite, and 3) humite and clinohumite, with individual features of Fe and Ti distribution.
Diagrams below show correlations between contents of Fe (Fig. 1a, c, d), Ti (Fig. 1b, d, f) and F in chondrodite, humite and clinohumite (no reason to make such diagrams for norbergite with very low contents of Fe and Ti). Fe content in humite and clinohumite increases with F increase while the content of Ti decrease. There are the anomalous points (in the oval) for both minerals. In one case (arrow I) the content of F increases with decrease of Ti content in clinohumite (Fig. 1d); in the other case (arrow II) increase of F does not correlate with the Ti/ΣМе ratio. Some values for humite and clinohumite (in the oval) are not in common trends: the highest Fe content were found in F-poor samples. Decrease of Ti in chondrodite also correlates with increase of F content. The graph of Fe:F correlation for chondrodite is quite different from ones for other studied minerals. The Fe/ΣМе ratio increases with F content increase: a field with the most concentration of points is in the range of 1.0 – 1.8 apfu F (Fig. 1i)
The conclusions from the above-discussed empirical data are as follows:
1) The negative correlation between Ti and F in the humite minerals is not caused by the F → OH substitution. It is the most probably described by the following scheme: Mg2+ + 2F- → Ti4+ + 2O2-.
2) The Fe/ΣМе ratio in humite and clinohumite increases with increase of F content. Is could not be explained basing on simple schemes of substitutions and requires a study of local situations in the structures of the minerals. It could be caused by ordered distribution of Mg and Fe and differences in their anion coordination.
3) In chondrodite, a “competition” of Fe and Ti takes place (Fig. 2a). The inverted situation (Fig. 2b, c) is, in general, observed in humite and clinohumite: Ti and Fe correlate positively. Causes of these differences are still not clear.
Thus, the statistic investigation of the correlations between fluorine, iron and titanium in magnesium members of the humite group raised the more questions, than the answers were given. To clarify the nature of these empirically found regularities, a structural study has been initiated.
Fig. 1. Correlations between Fe, Ti and F contents in humite, clinohumite and chondrodite
a b
c d
i f
Fig. 2. Correlations between Fe and Ti contents in humite, clinohumite and chondrodite
a b c
