AMPH: A program for calculating formulae and for assigning names to the amphibole group of minerals

Computers & GeosciencesVol. 22, No. 8, pp. 931-933, 1996 Copyright Q 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0098-3004/96 $15.00 + 0.00 PII: S009&3004(96)00018-0

SHORT NOTE AMPH: A PROGRAM FOR CALCULATING FORMULAE AND FOR ASSIGNING NAMES TO THE AMPHIBOLE GROUP OF MINERALS. D. RAMESHWAR

RAO’ and T. V. SUBBA RA02

Wadia Institute of Himalayan Geology, 33 General Mahadeo Singh Road Dehradun-248 001, India and *Advanced Data Processing Research Institute, Manovikasnagar, Hyderabad-500 009, India (Receiaed I September 199.5; revised 16 January 1996)

INTRODUCTION

Amphiboles have been described as a mineralogical “garbage-can” or “waste-basket” that does not discriminate among the constituents tossed into it (Robinson and others, 1982). The members of the amphibole group of minerals occur in a wide range of P-T environments ranging from greenschists to lower granulite facies conditions, and are also common constituents of intermediate talc-alkaline igneous rocks. Amphibole minerals provide information about the history and conditions of rock formation. Because of the complex nature of amphibole structure, effective comparisons of amphibole compositions can be made only after casting the analyses into reasonable structural formulae. This paper provides an “AMPH” program written in simple BASIC language to calculate chemical formulae and assign names to members of the amphibole group. A few such programs are quoted in the literature (cf. Rock and Leake, 1984; Rock, 1987; Mogessie and Tessadri, 1982; Mogessie, Tessadri, and Veltman, 1990; Spear and Kimball, 1984); some of these programs are not accessible to users, some can be used only on Macintosh computers, whereas others deal only with iron partitioning. The advantage of our program is that it is written in BASIC, its structure is easy to understand, it is simple to use, and can be employed by PC users. Interested users can obtain a copy of the program, by sending the first author a MS-DOS formatted 5.25” or 3.5” HD/DD floppy diskette, or by anonymous FTP from the server IAMG.ORG. “AMPH”

The “AMPH” into three steps:

PROGRAM

program

can be divided broadly

(A) iron partitioning; (B) site occupancies of the cations; CAGE0

22/8--D

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(C) assigning a proper name to the amphibole mineral. Please note that, in the example where Fe0 and Fe,Oj weight percentages are available, the program skips Step (A). However, one precaution should be taken if the Fe,O, value is zero. In such situations a value of 0.002 or so (and not zero) should be input against FezO,, so that the program skips Step (A). This low value, neither affects the cation distribution nor the classification of the amphibole mineral. Step (A). Iron partitioning In the initial stages the program partitions the total iron into Fe+’ and Fe + ‘. There are different possible procedures to solve the problem partially. Calculation on the basis of 23(O) and then adjustment of the total cations by differing Fe +*/Fe +j content is the most widely used. This method using an iteration technique is followed in the present program. Four different recalculation procedures are employed and are based on the sum of cations of the amphibole formula. Total cations to 15 exclusive of Na and K. The cation sum is recalculated to 15 excluding all the Na and K. This recalculation provides a minimal amount of Fe +3. According to Robinson and others (1982) this often works well for Fe-Mn-Mg amphiboles. Total cations to 16. For amphiboles which have a cation sum greater than 16, the analyses are recalculated to 16. This recalculation also provides the minimum amount of Fe + ‘. Total cations to 13 exclusive of Ca, Na, and K. If the cation sum is less than 16 and more than 13, then the analysis is recalculated to 13 cations excluding Ca, Na, and K. This formulation gives the maximum amount of Fe + ‘. Total cations to 15 exclusive of K. In some amphibole analyses the cation sum excluding

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Short Note

Ca, Na, and K is less than 13. In this instance recalculation of the analyses to 15 cations excluding K is adopted. Note that this reformulation mate Fe+’ in Ca-amphiboles 1982).

can seriously overesti(Robinson and others,

Step (B). Site occupancies of the cations The structural formulae are recalculated after determining the general cation values and partitioning the total iron into Fe0 and Fe,O, in Step (A). As it is not possible to determine H,O with EPMA and it is also probable that the analysis may have unreported F or Cl contents the standard formula calculation is made based on 23(O), instead of 24(O). The site occupancies of the amphiboles are calculated as follows: 8.00 cations are assigned to T using Si, then Al, then Cr + ‘, then Fe + 3, then Ti +4. 5.00 cations are assigned to C using excess Al, Cre3, FeC3,Ti+4 from Step (l), then Mg, then Fe+‘, then Mn, then Li. Sum B is adjusted to 2.000 using excess Mg, Fe+’ Mn, Li from Step (2), then Ca, then Na. Excels Na from Step (3) and all K are adjusted to A site. Step (C). Assigning proper name to the amphibole mineral The amphiboles are classified into divisions based on B-site occupancy:

four major

Fe-Mn-Mg amphiboles: if (Ca + Na) in B-site is less than 1.34. Calcic amphiboles: if (Ca + Na) in B-site is equal to or greater than 1.34, and Na in B-site is less than 0.67. Sodic-calcic amphiboles: if (Ca + Na) in B-site is equal to or greater than 1.34, and Na in B-site is equal to or greater than 0.67 and less than 1.34. Alkali amphiboles: if Na in B-site is equal to or greater than 1.34. Once the major groups are identified, the specific names in each group are assigned on the basis of (Na + K) in the A-site, Mn in the octahedral site, Li content, Mg/(Mg + Fe +‘) ratio, Fe +‘/(Mg + Fe +‘) ratio and Fe + ‘/(Fe + ’ + Al”) ratio. More than 80 mineral names have been proposed in Leake’s scheme of classification (cf. Leake, 1978). A total of 73 amphibole members are classified by this program, 14 alkali amphiboles, 10 sodic-calcic amphiboles, 32 calcic amphiboles, and 17 Fe-MnMg amphiboles. This program does not classify subsilicic amphiboles with Si less than 5.75, however, the program will perform iron partitioning and cation distribution and prints a message: “THE PRO-

GRAM DOES NOT CLASSIFY THE SUBSILICIC AMPHIBOLES”. The important thing to be remembered for the Fe-Mn-Mg amphiboles is that, there are orthorhombit and monoclinic members of this group which cannot be distinguished chemically. As performed by some earlier workers (Mogessie, Tessadri, and Veltman, 1990), the present program also gives two names corresponding to the orthorhombic and monoclinic systems. One has to study the mineral optically to distinguish between the two alternatives. PROGRAM OPERATION

The program initially prompts for sample identity, followed by input of weight percentage of the oxides from the chemical analysis. For microprobe data, all Fe is entered as FeO, and Fe203 is entered as zero. The program then prompts: “IF INPUT DATA IS O.K. TYPE Y (OR) ELSE N”. If “N” the program goes back to the initial prompt for the input of weight percentages of the various oxides. If “Y” the program gives five options for partitioning total iron. Option 1 can be used if one is not sure which cation sum should be used for recalculation. In this instance the program checks the mineral data and selects the appropriate number of cations. Option 2 can be used when partitioning of iron is not required and options 3,4, and 5 are used, respectively, for iron partitioning using 16 cations, 13 cations, and 15 cations. Once an option has been selected, the program starts calculating with a message: “WAIT THE PROGRAM IS BEING EXECUTED”. This is followed by printing of the results and a prompt: “DO YOU WISH TO CALCULATE MORE DATA (Y OR N)“. If “Y” the program will run again for the new sample; “N” quits the program. The output contains sample identity, weight percentages of oxides, the partitioning of FeOT into Fe0 and Fe,O,, Fe cations into Fe + ’ and Fe + 3, the cation distribution, and the specific name of the amphibole mineral against Leake’s classification. Four worked examples, one from each major group are given in the Appendix. Acknowledgment-The first author is grateful to the Director, Wadia Institute of Himalayan Geology, Dehradun, India, for providing necessary facilities in the institute to carry out the work. REFERENCES

Deer, W. A., Howie, R. A., and Zussman, J., 1992, An introduction to the rock forming minerals (2nd ed.): Longman Scientific and Technical Publications, London, 696 p. Henderson, C. M. B., Pendlebury, K., and Foland, K. A., 1989, Mineralogy and petrology of the Red Hill Alkaline Igenous Compex New Hampshire. U.S.A. Jour. Petrology, v. 30, no. 3, p. 627666. Leake, B. E., 1978, Nomenclature of amphiboles: Mineral Mag., v. 42, no. 324, p. 533-563.

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Short Note Mogessie, A., and Tessadri, R., 1982, A BASIC computer program to determine the name of an amphibole from an electron microprobe analysis: Geol. Palaeont. Mitt. Innsbruck, v. 11, no. 7, p. 259-289. Mogessie, A., Tessadri, R., and Veltman, C. B., 1990, EMP-AMPH-a hypercard program to determine the name of an amphibole from electron microprobe analysis according to the international mineralogical association scheme: Computers & Geosciences, v. 16, no. 3, p. 309-330. Robinson, P., Spear, F. S., Schumacher, J. C., Laird, J., Klein, C., Evans, B. W., and Doolan, B. L., 1982, Phase relations of metamorphic amphiboles-natural occurrence and theory, in Veblen, D. R., and Ribbe, P.

H., eds., Amphiboles-Petrology and Experimental Phase Relations: Reviews of Mineralogy, v. 9B, p. l-211. Rock, N. M. S., 1987, A FORTRAN program for tabulating and naming amphibole analyses according to the International Mineralogical Association Scheme: Mineral. Petrol., v. 37, no. 1, p. 79-88. Rock, N. M. S., and Leake, B. E., 1984, The International Mineralogical Association amphibole nomenclature scheme: computerization and its consequences: Mineral. Mag., v. 48, no. 347, p. 211-227. Spear, F. 8, and Kimball, K. L., 1984, RECAMP-a FORTRAN IV program for estimating Fe’ + contents in amphiboles: Computers & Geosciences, v. IO, no. 2-3, p. 317-325.

APPENDIX Table 3. Example no. 3. Sample identity: 13

Table 1. Example no. 1. Sample identity: OSS-B 75/37 11 Partitioning of feot and classification of amphibole mineral Oxide

Wt%

Cations

a. f. u

SiO, AllO, FeO(T)

43.92 5.18 29.63

Si Al”

6.922 0.962

T-Site

7.884

Al” Ti Fe+’ Fe+’ Mn Mg Cr Li Ott-Site Ca Na K B-Site

0.000 0.141 2.919 0.926 0.234 0.717 0.000 0.000 4.996 1.106 1.696 0.318 3.120 0.234 0.894 2.000 1.120

TiO? Fe0 Fe,O, MnO MgO crzo, L&O

1.19 22.60 7.81 1.75 3.05 0.00 0.00

CaO Na,O KXO

6.55 5.55 1.58

Mn (act-site) Na (B-site) Ca + Na (B-site) Na + K (A-site) Leake’s classification:

Katophorite

Data from microprobe analysis (cf. Henderson, Pendlebury, and Foland, 1989). In Step (A) recalculation to 16 cation scheme is followed. Amphibole belongs to sodic-calcic amphibole group.

Partitioning of feat and classification of amohibole mineral Oxide

WP/a

Cations

a. f. u.

SiO, A&O, FeO(T)

58.04 10.31 6.12

TiO, Fe0 Fe,@ MnO MgO Cr*O3 L&O

0.66 6.12 2.89 0.07 11.71 0.00 0.00

CaO Na10 KzO

1.37 6.97 0.02

Si Al” T-site Al” Ti Fe” Fe+’ Mn Mg Cr Li Ott-site Ca Na K B-site

7.879 0.121 8.000 1.528 0.067 0.695 0.295 0.008 2.370 0.000 0.000 4.963 0.199 1.834 0.003 2.037 0.008 1.801 2.000 0.037

Mn (act-site) Na (B-site) Ca + Na (B-site) Na + K (A-site) Leake’s classification:

Glaucoohane

Wet chemical analysis data (cf. Deer, Howie, and Zussman, 1992). Step (A) is skipped as both Fe0 and Fe>O, are available. Amphibole belongs to alkali amphibole group.

Table 2. Example no. 2. Sample identity: 2 Table 4. Example no. 4. Sample identity: OCS-A 77114 9

Partitioning of feot and classification of amphibole mineral Oxide

Wt%

Cations

a. f. u.

SiO? A&O, FeO(T)

40.75 19.81 19.29

TiOl Fe0 FelO, MnO MgO Cr*O, L&O

0.25 19.29 1.22 0.25 13.81 0.01 0.03

CaO Na20 K:O

0.27 I .92 0.04

Si Al” T-site Al”’ Ti Fe+* Fe+’ Mn Mg Cr Li Ott-site Ca Na K B-site

5.953 2.047 8ooa 1.363 0.027 2.356 0.134 0.03 1 3.007 0.001 0.018 6.938 0.042 0.544 0.007 0.593 0.000 0.544 0.586 0.007

Mn (act-site) Na (B-site) Ca + Na (B-site) Na + K (A-site) Leake’s classification:

Cumminatonite Gedhte

Wet chemical analysis data (cf. Deer, Howie, and Zussman, 1992). Step (A) is skipped as both Fe0 and FeJOlare available. Amphibole belongs to FtMn-Mg amphibole group. Two names are printed corresponding to orthorhombic and monoclinic members.

Partitioning of feot and classification of amahibole mineral Oxide

Wt%

Cations

a. f. u.

SiOl

Si

FeO(T)

41.27 8.32 26.86

TiOz Fe0 Fez01 MnO MgO Cr~Oj Liz0

2.73 25.17 1.88 0.78 4.29 0.00 0.00

CaO Na>O K,O

10.37 2.67 1.48

6.49 I 1.509 8.000 0.034 0.323 3.311 0.223 0.104 I .006 0.000 0.000 5.000 1.748 0.814 0.297 2.859 0.104 0.252 2.o 0.859

AMA

Mn (act-site) Na (B-site) Ca + Na (B-site) Na + K (A-site) Leake’s classification:

T-site Al” Ti Fe+? Fe” Mn Mg Cr Li Ott-site Ca Na K B-site

Hastingsite hornblende

Data from microprobe analysis (cf. Henderson, Pendlebury, and Foland, 1989). In Step (A) recalculation to 13 excluding Ca, Na, and K scheme is followed. Amphibole belongs to calcic amphibole group.