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The composition and paragenesis of the hornblendes of the Glen Tilt complex, Perthshire

Published online by Cambridge University Press:  14 March 2018

W. A. Deer
W. A. Deer*
Affiliation:
Department of Geology, University of Manchester
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Extract

The south-eastern margin of the Glen Tilt complex consists of a long strip of diorites with a number of small associated patches of rocks of appinitic type. Xenoliths, both of hornblendite and hornblendeschist, the latter belonging to the Perthshire series of the Dalradian, are found enclosed within the dioritic rocks, which range petrographically from diorite to quartz-mica-diorite. In all these rocks hornblende is the most important fcrromagnesian constituent always predominating over pyroxene in the basic members and frequently persisting to the exclusion of biotite in the intermediate rocks of the intrusion. The hornblendes were examined chemically because of the difficulty of estimating their composition even from a thorough optical investigation. As they play such an important role in the complex it is considered that they may give some indication of the history and mode of formation of the rocks in which they occur. Such a chemical investigation of a series of related hornblendes will also indicate the possible range of composition of common amphiboles within the diorites and related rocks of a single complex.

Type
Research Article
Information
Mineralogical magazine and journal of the Mineralogical Society , Volume 25 , Issue 161 , June 1938 , pp. 56 - 74
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1938

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References

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Average hornblendite: SiO2 42·80, TiO2 1·62, Al2O3 10·55, Fe2O3 6·62, FeO 9·16, MnO 0·24, MgO 12·48, CaO 11·67, Na2O 1·89, K2O 1·00, H20 1·73, p2O5 0·24.

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  1. 1.

    1. (OH,F)0·6 (Ca,Na)2·4 (Mg,Fe″,Fe‴,Mn,Ti,Al)5·25[(Si,Al)8 O22].

  2. 2.

    2. (OH,F)1·2 (Ca,Na)2·4 (Mg,Fe″,Fe‴,Mn,Ti,Al)5·25[(Si,Al)8 O22].

  3. 3.

    3. (OH)0·8 (Ca,Na)2·5 (Mg,Fe″,Fe‴,Mn,Ti,Al)5·4[(Si,Al)8 O22].

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Analysis of basaltic hornblende:

SiO2; 38·30, TiO2 6·06, Al2O3 12·87, Fe2O3 7·98, FeO 6·96, MnO 0·12, MgO 11·79, CaO 10·47, Na2O 3·11, K2O 1·30, H2O+ 1·10, F 0·05, giving the formula: (OH,F)l·1 (Ca,Na,K)2·8 (Mg,Fe″,Fe‴,Mn,Ti)5·1 [(Si5·7 Al2·3)O22].

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