CHEMICAL PETROLOGY AND MINERALOGY OF … · CHEMICAL PETROLOGY AND MINERALOGY OF HORNBLENDES IN N...
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CHEMICAL PETROLOGY AND MINERALOGY OFHORNBLENDES IN N
GRANIRTHWEST ADIRONDACKIC ROCKS*
A. F. BullrNGroN, Princelon (/ niversi,ty, Princeton, N ew Jersey
AND
B. F. LnoNann, gical Surt:ey, Dentter, Colorado
TS
Abstract .Introduction. .Petrologic Notes.
flornblende from granitic rocks ofHornblende from younger granitic
Stark complex.
Oot ical and Chemical Dala. . .Optical properties of some Adi hornblendes.
Indices oI re l ract ion. .
Birefringence ..
Optic axial angleDispersion. .Extinction angle.Pleochroism and absorption.
General relations of the Adirondack
Relerences.
891891892892893897897897898898899899900900901
belong to the femaghasl ingsi te and ingsite varieties and carry chlorine as well as
fluorine in significant amounts. The varia ions in composition of the hornblendes correlate
with the deEree of difierentiation of the of which thev are a part, or with the degree of
l. The methods used in determining the optical
discussed. A satisfactory, detailed correlationmodification of incorpotated foreignproperties are fully described and criticaof chemical and optical properties has yet been achieved, even for the limited part of
the hastingsite group investigated herg.
INr
Chemical analyses and optical prope
the northwest Adirondack Mountains,have been recalculated in terms of a t
Granitic rocks with hornbmasses and are a major elemen
northwest Adirondack X,Iountain
blendes. .
of seven hornblendes from granitic rocks of
York, are presented. The chemical analyses
ical amphibole formula. All the hornblendes
ODUCTION
as a varietal mineral form very large
in the pre-Cambrian bedrock of the
of New York (Buddington, 1939 and
ion on the chemical composition and
ndes, together with a brief discussionBuddington initiated the study of the
1948). This paperoptical propertiesof their petrologic
is a contribuof the hornblenvironment.
U. S. Geologicai Survey.x Publication authorized by the
891
892 A. F. BUDDINGTON AND B. F. LEONARD
hornblendes and prepared the section on chemical petrology. Leonardcontributed the optical data.
The chemical analyses were made at the Rock Analysis Laboratory ofthe University of l,Iinnesota. The cost of the analyses was borne by agrant of funds from the Eugene Higgins Trust allocated to PrincetonUniversity.
The hornblendes were concentrated for analysis by use of the Frantzmagnetic separator and heavy liquids. This work was done by H. L.James and R. J. Smith. The hornblende concentrates submitted forchemical analysis were better than 99.5 per cent pure. lVlost of themwere about 99.9 per cent pure. The contaminants were chiefly minutegrains of qtartz and potash feldspar that had not broken free from thehornblende, even after crushing to -100, *200 mesh.
There is no reason to doubt that the hornblendes in the fractionstudied optically were identical with those in the concentrate submittedfor chemical analysis.
The writers have followed Bil l ings' subdivision (1928) of the hastings-ite group, according to the molecular proportions of FeO and MgO;Jerrohasti.ngsite, FeO/MgO)2; femaghastingsite, FeO/MgO 12 bfi )!;and magnesiohastingsite, FeO/MgO < j.
PBtnorocrc NorBs
The granitic rocks of the northwest Adirondacks are of two difierentages. The older granitic rocks, part of the Diana-Stark complex, havebeen intensely deformed, recrystallized, and intruded by a younger seriesof granitic rocks. These younger granites in large part have a sheetlike,phacolithic, and domical relationship to all older rocks.
Chemical analyses of rocks from which the hornblendes 1, 3, 4, and 6(Table 1) and pyroxene no. 8 (Table 1) were concentrated are to appearin a forthcoming publication of the U. S. Geological Survey entit led"Geology and mineral deposits of the St. Lawrence County magnetitedistrict, New York," by Buddington and Leonard.
Hornblende Jrom Granitic Rocks of Diana-.Stork Complex
The older granitic rocks are facies of the Diana-Stark complex. Thiscomplex has been described and interpreted (Buddington, 1939, pp. 73-109; Buddington, 1948, pp.2a-29) as a very great sheet of igneous rockthat has been intensely deformed and recrystall ized. The sheet has astratiform character resulting from difierentiation primarily by frac-tional crystallization. Pyroxene syenite gneiss with lenses of shonkiniteand feldspathic ultramafic (ferroaugite, ilmenite, and magnetite) gneissoccurs in the lower part. This grades upward through pyroxene qlf,artz
HORNBLENDE IN NORTHWES:I ADIRONDACK GRANITIC ROCKS 893
syenite gneiss and hornblendic quartz-bearing syenite gneiss into horn-
blende granite gneiss, which forms the upper facies. The pyroxenic facies
of this sheet are green in color, whereas the hornblendic facies are pink.
Hornblende 5 occurs as porphyroblasts in a pink granoblastic quartz-
bearing syenite gneiss, which grades into a pyroxene quartz-bearing
syenite gneiss stratigraphically below and a hornblende granite gneiss
stratigraphically above. Hornblendes 6 and 7 are from wholly recrystal-
lized granite gneiss characteristic of the upper facies of the Diana-Stark
sheet. The variation of the mafic minerals from ferroaugite (nos. 8 and 9,
Table 1) in the lower part of the sheet through femaghastingsite (no. 5)
to ferrohastingsite (nos. 6 and 7) in the upper part of the sheet is con-
sistent with what might be expected as a product of fractional crystalliza-
tion and gravity sorting of successive crystal fractions. The ratio of mol
per cent (FeO plus MnO)/MgO increases systematically from that in the
ferroaugite (nos. 8 and 9:0.86 and 0.85, respectively) through femag-
hast ingsi te (no.5:1.18) to ferrohast ingsi te (nos.6 and 7:3 '20 and
4.09, respectively). This increase in ratio of Feo to MgO is correlated
with a higher percentage ol quaftz and a higher ratio of normative ortho-
clase to albite in the ferrohastingsite-bearing gneisses.
Hornblende Jrom Younger Gron'itic Rocks
The predominant member of the younger granitic rocks is a horn-
blende-microperthite granite commonly with 20 to 30 per cent quartz'
Hornblende 2 is from this type of rock. The hornblende granite is in part
strongly deformed and recrystallized to a gneiss. Hornblende 3 is from
the hornblend,e to be just in the border zone between femaghastingsite
and ferrohastingsite.There is also a facies of the hornblende granite in which the quartz
content is commonly greater than 27 per cent and the ratio Kzo/NazO is
slightly higher than in the normal granite. The hornblende (no. 4) of this
type of rock is a ferrohastingsite with a relatively high ratio of ferrous
iron to magnesia. (The particular rock from which the analyzed horn-
blende was taken has a lower percentage of quaftz than is normal for
this facies of the granites.) All the chemical and mineralogical character'
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HORNBLENDE IN NORTHWEST ADIRONDACK GRANITIC ROCKS 897
istics of this granite, as well as the field evidence, are consistent with the
hypothesis that the more quartzose facies represents a slightly younger
d.ifferentiate of the same magma that yielded the normal granite.
Microcline-rich granite gneiss occurs as sheetlike masses intimately
associated with metasedimentary rocks and amphibolite. The microcline
granite gneiss is interpreted as in large part the product of modification,
granitization, and migmatitization of metasediments and amphibolite by
potash-rich solutions. The hornblendic facies of the microcline granite
gneisses are always associated with included layers, schlieren, and dis-
integrated relics of amphibolite. The hornblende ranges from femaghast-
ingsite to ferrohastingsite. The variation in composition probably de-
pends in part upon the degree of modification by granitizing solutions of
the hornblende derived by incorporation from amphibolite.
OPrrcnr AND CHEMrcar Dera
Optical Properties oJ Sorne Ad,irondack Hornblend'es*
Indices of Refraction
The indices of refraction were determined in sodium light by the im-
mersion method, using freshly standardized liquids varying by intervals
of 0.002 in the range from 1.660 to 1.700, and by slightly less than 0.005
in the range from 1.700 to L725. The temperature at which the measure-
ments were made was closely controlled.The indices a and .y were determined on cleavage flakes parallel to the
{010} parting. The index B was determined on rare grains perpendicular
to an optic axis, or on very rare grains perpendicular to Bxu'
Five separate determinations of a, B, and 7 were made for most horn-
blendes, though the number of determinations ranged from 4 to 11' For
hornblende 4, a was determined with exceptional precision; total and
partial birefringences were measured. by H. H. Hess, using a technique
developed for the study of pyroxenes (Hess, 1949); and the values for
B and,7 were calculated from a and the partial birefringences'
The reported values for indices of refraction are thought to be the best
approximations possible at this time. Two factors seriousiy affect the ac-
curacy of the determinations:1. Pure hornblende concentrates from a single hand specimen are
slightly variable in composition. Because of weak absorption parallel to
X, o ca.t usually be determined with greater accuracy than B or 7; and
on a given grain the value of a can be determined in sodium light with an
x To keep within the page limits imposed for this issue of the journal, it was necessary
to delete a section on crystil form anJ cleavage of the hornblendes and to condense the
sections on measurement of indices of refraction, 2V, and extinction angle'
898 A. F. BUDDINGTON AND B. F. LEONARD
error of 0.001 or less. However, the range in a for several grains in thesame mount may be as m.uch as 0.003. A similar range was observedfor the values of B and ,y. (But see factor 2, just below.)
2. Absorption parallel to Y and Z is so strong that the determinativeerror in B and 7 may be slightly greater than 0.001.
Birefringence
The values for total birefringence reported just below the indices of re-fraction in Table 2 were determined, as usual, by dif ierence. Because ofthe range in a and 7, reflecting variable composition, it was feared thatthese values for total birefringence might be slightry in error. To checkthis possibility, separate birefringence determinations were made bymeans of a Berek compensator for all hornblendes except nos. 2 and 3.The resuits are given at the bottom of rable 2. For hornblendes, exceptno. 4, described in the preceding section, the measurements were made onordinary thin sections. The thickness of a quartz grain adjacent to, or onlyone grain removed from, a suitable hornblende was determined by meansof the Berek compensator. The total or partial birefringence of the horn-blende was then determined. The measurements for hornblendes 5 and6 were made on the microscope stage, using X-Z sections of quartz andsuitably oriented sections of hornblende. The measurements for horn-blendes 1 and 7 were made on a universal stage according to the techniquedescribed by Emmons (1943, pp. 172 IB2).
The agreement between birefringences determined by immersion andby measurement of retardation in ordinary thin sections is generallygood' (See table 2.) At least the direct determination of birefringenceserves as a check against serious errors in the relative values of theindices of refraction of a given hornblende. For example, 7 minus a forhornblende 5 seemed very low relative to the total birefringence of theother hornblendes. However, direct measurements of the partial bire-fringences of this hornblende confirmed the results of the immersionwork.
Opl ic Axia l Angle
The optic axial angle was measured in thin section on the universalstage, using a number of grains normal or nearly normal to Bx". Thisangle is reported in Table 2 as 2Y^.o,. under the best of conditions, theprecision of measurement on these hornblendes ranges from f" to 2". Therange of values reported for 2v^"u". in Table 2 is due in part to slightvariation in composition of the hornblende, from grain to grain, and inpart to unavoidable errors in measurement. strong absorption parallelto Y and z makes proper orientation of sections subnormal to
-Bxu ex-
HORNBLENDE IN NORTHWEST ADIRONDACK GRANITIC ROCKS 8q9
ceedingly difficult on the universal stage, and the resulting determination
of 2V is therefore suspect. In addition, total reflection from the surface
of the grain may occasionally be confused with one extinction position.
Dispersion
The dispersion of the optic axes is inclined. It is always distinct or
strong for one optic axis; it is less pronounced for the other, even when
apparent dispersion is reported as strong for both axes (e.g., hornblende
1, Table 2) . Dispersion was determined qualitatively by examining many
optic axis figures for each hornblende. It was not possible to assign the
relative intensity of dispersion to optic axes A or B, nor was it possible
to measure 2V in red light and in blue light. A correct statement of the
true optic axial dispersion for hornblende 1, and perhaps for hornblendes
5 and 6, would of course depend on such measurements'
Extinction Angle
The extinction angle Z /r,c was measured on the microscope stage, using
hornblende sections parallei to the optic plane. The values reported in
Table 2 are averages of repeated determinations made on a number of
suitably oriented grains. Thin sections, rather than immersed fragments,
were used. Because of strong absorption parallel to z, it is difficult to get
the correct extinction position ol Z, as well as to see the trace of the
cleavage whose zone axis is c. Moreover, the cleavage trace observed on
sections parallel to the optic plane is moderately to highly irregular.
Consequently, the extinction angle measured on a single grain may vary
by as much as 9o. A range of 3" to 4o is common. Probably relatively
Iittle of this variation is due to compositional variation in the hornblende.
Owing to the diffi.culty of determining extinction positions and orient-
ing cleavage planes on the universal stage, U-stage measurements of
zAc for hornblende 1 showed a greater range (* 7o) than microscope-
stage measurements made on the same grains cut parallel to the optic
plane. U-stage measurements of ZAc in re-oriented random sections of
hornblende 1 were even less reliable, showing a range of I 10". Measur-
ingZ/\cby observing the extinction position of X, and then rotating 90"
on the outer vertical axis, may have increased the accuracy of some de-
terminations by |o or 1".At present, there seems to be no reliable method for accurately de-
termining Z/x,c on the hastingsites described in this paper' Leonard pre-
fers to measure ZAc onX-Z sections on the microscope stage, if necessary
checking the orientation of the same grains on the universal stage and
checking to see that c is truly the zone axis for the observed cleavage
traces.
A. F, BUDDINGTON AND B. F. LEONARD
Pleochroism and Absorption
The colors reported for X, Y, and Z are those observed in thin sectionshaving an average thickness of 0.020 to 0.025 mm. (These thicknesseswere determined by measurements of retardation in suitably orientedqtartz grains.) In thin sections 0.030 mm. thick, X is pale but Y and Zfor most hornblendes are very dark or almost black, the body color ofhornblendes seen in hand specimen. Crushed hornblendes immersed inoil transmit almost no light parallel to Y and Z unless the fragments aresmaller than about 200 mesh. For most hornblendes, Y and Z rcmainvery dark, though appreciably colored, for fragments as small as -400,
*600 mesh.The determination of the color of Y is sometimes difficult if the horn-
blende has strong dispersion. A section a few degrees subnormal to anoptic axis will include more emergent red light or more emergent blue,depending on the orientation of the grain. A relatively small tjlt will thusproduce a substantial change in the observed color of Y, which is actuallyY'. The difficulty is obviated, of course, if the sections are truly normalto the optic axis or acute bisectrix.
The absorption indicatrix of the hornblendes coincides with the opticalindicatrix. This relation is quite difierent from that obtaining amongcertain common clinopyroxenes, especially among members rich in Fe"'(Hess, 1949).
General Relations of the Ad.irondack Hornblendes
All the Adirondack hornblendes in granitic rocks for which data areavailable show a noteworthy amount of chlorine. There are but fewanalyses of hornblendes in the literature that give the per cent of chlorine.If this is to be taken to indicate that chlorine is minor in amount, thenthe Adirondack hornblendes are exceptionally rich in this element. It ispossible, however, that chlorine occurs in significant amounts in somehornblendes in which it has not been determined. A ferrohastingsitefrom the rapakivi granites of Finland, described by Sahama (1947), has0.51 per cent Cl and 1.06 per cent F. A pargasite from the Tiree marbleof the Hebrides, described by Hallimond (1947), has 0.46 per cent Cl and0.16 per cent F. Hall imond (1947, pp.237-238) notes that the amphiboleapproaches hastingsite in composition. A ferrohastingsite described byKrutov (1936) and named by him dashkessanite,has 7.24 per cent CI. Afemaghastingsite from garnetiferous amphibolite of the central Adiron-dacks, New York, described by Buddington (1952, p. 42, no. 9), has0.63 per cent Cl and only 0.08 per cent F. A comparison of hornblendesfrom Adirondack amphibolites (Buddington, 1952, pp. 42 and 53) alsoshows that the total alkalis are higher in hornblendes having significant
HORNBLENDE IN NORTHWEST ADIRONDACR GRANITIC ROCKS 901
amounts of F or Cl than in those having only a slight amount of thehalogens.
Both in the granitic rocks of the Diana-Stark complex and in theyounger granitic rocks, the ferrohastingsite hornblendes carry a veryslightly higher percentage of total alkalis than the femaghastingsites.Also, in each of the two rock series there is a systematic decrease in theper cent of total OH, F, and Cl as the ratio MgO/FeO decreases.
Hornblendes of similar chemical composition occur both in primaryigneous rock and in the gneissic equivalents.
A pargasitic variety of hornblende is found in a thin layer of thoroughlyrecrystallized amphibolite enclosed in the granite from which hornblende2 was concentrated. Pargasite from the amphibolite contains 0.91 percent F and 0.03 per cent Cl (Buddington, 1952,p.42, no. 1). This con-trasts with 1.50 per cent F and 0.37 per cent Cl found in the femaghast-ingsite (no. 2) of the granite. The femaghastingsite (no. 2) of the granitehas a much higher ratio of
FeO * MnO_==- : 1 .89
than the pargasite of the amphibolite, which has
F e o * M n o : 0 . 7 8 .
Msoas one would expect.
The main purpose of this investigation has been to define the chemicalcomposition and optical properties of hornblendes from granitic rocks ofthe northwest Adirondacks, and to discuss their petrologic significance.A secondary goal was to correlate these data and the previously pub-lished data on similar amphiboles, in order to increase our understandingof the hastingsite group. So far, a reasonably precise correlation has notbeen achieved, owing mainly to the extreme complexity of the hastingsitegroup and to the lack of reliable and essentially complete chemical andoptical data on its members. The lines of attack recently employed byFoslie (1945) and Sundius (1946) have been especially fruitful, but theneed for additional data is great.
Rnlnnpxces
Brr,t-tNcs, Manuxo (1928), The chemistry, optics, and genesis of the hastingsite group ofamphiboles : Arn. M ineral., 13, 287 -296.
BunotNctoN, A. F. (1939), Adirondack igneous rocks and their metamorphism: Geol. Soc.Amer. , Mem.7, 354 pp.
-=- (1948), Origin of granitic rocks of the northwest Adirondacks, in Gilluly, James,chm., Origin of granite: Geol. Soc. Amer., Mem.28, 2l-43.
902 A. F. BUDDINGTON AND B, F. LEONARD
- (1952), Chemical petrology of some metamorphosed Adirondack gabbroic, syeniticand quartz syenitic rocks: Bowen volume, Am. Jour. Sci..,37-84.
Euuoxs, R. C. (1943), The universal stage: Geol'. Soc. Amer., Mem.8,205 pp.Foslrr, SrrrNan (1945), Hastingsites, and amphiboles from the epidote-amphibolite
facies : N orsk geol. liilsshr., 25, 7 4-98.HallruoNn, A. F. (1947), Pyroxenes, amphibole, and mica from the Tiree marble (with
chemical analyses by C. O. Harvey): Mineralog. Mag.,28,230-243.Hnss, H. H., (1949), Chemical composition and optical properties of common clinopy-
roxenes, Part I : Am. Mineral,., 34, 621466.Knurov, G. A. (1936), Dashkessanite-a new chlorine amphibole of the hastingsite group:
Acad.. Sci.U.R.S.S. Btill,., ser. geol.,34l-373. (Cited in M'inerol'og. Abs., 1937,6' 438.)Senaur, T. G. (1947), Rapakivi amphibole from Uuksunjoki, Salmi area: Comm. geol.
Finlande Bull. l4O, 159-162.Suxorus, Nrr.s (1946), The classification of the hornblendes and the solid solution relations
in the amphibole group: Soeri.ges geol. Untlersi)hning, ser . C , no . 480, 36 pp.