Composition and structure of metamorphosed

International Geology Review

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Composition and structure of metamorphosed ferromanganese nodules, new vein formations of manganese hydroxides, and the surrounding pelagic sediments in the Southern Basin of the Pacific Ocean floor N.F. Andrushchenko , B.P. Gradusov , V.A. Yeroshchev-Shak , R.S. Yanshina & S.Ye. Borisovskiy To cite this article: N.F. Andrushchenko , B.P. Gradusov , V.A. Yeroshchev-Shak , R.S. Yanshina & S.Ye. Borisovskiy (1975) Composition and structure of metamorphosed ferromanganese nodules, new vein formations of manganese hydroxides, and the surrounding pelagic sediments in the Southern Basin of the Pacific Ocean floor, International Geology Review, 17:12, 1375-1392, DOI: 10.1080/00206817509471540 To link to this article: http://dx.doi.org/10.1080/00206817509471540

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Date: 15 June 2017, At: 04:49

Composition and structure of metamorphosed ferromanganese nodules, new vein formations of manganese hydroxides, a n d the surrounding pelagic sediments in the Southern Basin of the Pacific O c e a n f l o o r N.F. AndVushchenfco,

INTRODUCTION The object of the detailed investigations was an unusual m a t e r i a l collected in the region of the Southern Basin of the Pacific Ocean floor, with f e a t u r e s of intense manifestation of v o l canic p r o c e s s e s and subsequent h y d r o t h e r m a l a l t e r a t i o n s . T h e s e p r o c e s s e s to a significant degree t r a n s f o r m e d the f e r r o m a n g a n e s e nodules and the pelagic s e d i m e n t s , causing the development of a new type of oceanic manganese mineralization. The m a t e r i a l was collected during dredging on the 48th c r u i s e of the s c i e n t i f i c - r e s e a r c h v e s s e l "Vityaz' " ( 1970) in s e c t o r 2 (Stn. 6298) in the s o u t h w e s t e r n portion of the ocean (buoy c o o r d i n a t e s : lat 22°41* 0 9 " S. ; long 160°5O' 0 8 " W . ) . In addition, we a l s o examined m a t e r i a l collected in 1968 a t the time of the 43rd c r u i s e of the "Vityaz' " from Station 5965, located in the n o r t h e r n p a r t of the s e c t o r investigated (fig. 1). In the general geologic plan, the a r e a b e longs to the southern m a r g i n a l portion of the major island r i d g e of the Cook and Tubua'i islands on the A u s t r a l i a n s u b m a r i n e r i d g e . In tectonic r e s p e c t s , it is located on the e x t e n sion of one of the long sublatitudinal tectonic zones, known a s the F e r d i n a n d e s fault zone ( P u s h c h a r o v s k i y , 19721. The features of manifestation of h y d r o t h e r mal activity in this a r e a have been c o n s i d e r e d by Skornyakova et al. (1973) . The clay m i n e r a l s have been investigated by Gorbunova (1973) .

el al

SOME FEATURES OF THE FERROMANGANESE NODULES AND BOTTOM SEDIMENTS Trawling within the a r e a under c o n s i d e r a tion (Stations 2, 20, 24, 30, and 62) has brought up a large amount of fragments of a l t e r e d b r i c k r e d clays with b r e c c i a - l i k e s t r u c t u r e . They a r e c h a r a c t e r i z e d by a s t r e a k y uneven color, dependent on the s e g r e g a t i o n of s e c t o r s of r e d d i s h - b r o w n , whitish-yellow, and white clays. The bleached clays have m o s t commonly c r y stallized; they contain accumulations of minute c r y s t a l l i n e g r a i n s of a p a t i t e , phillipsite, and have to a m a r k e d d e g r e e been m i n e r a l i z e d . They have been cut by newly formed m a t e r i a l s of n u m e r o u s o r e d e n d r i t e s , and contain buds and mieronodules consisting mainly of c r y p t o melane, todorokite, v e r n a d i t e , and l e s s f r e quently, r a n c i e i t e . The l a r g e t h r e e - d i m e n s i o n a l d e n d r i t e s r e a c h 1-2 cm in s i z e with a thickness of 0 . 5 cm (fig. 2 : 4 ) .

The s u r f a c e of the pelagic s e d i m e n t s over a l m o s t the whole a r e a of the s e c t o r investigated is covered with a c c u m u l a t i o n s of f e r r o m a n g a n e s e nodules. They have v a r i e d f o r m s : s p h e r i c a l , oval, intergrowth, and platy. The most common a r e the s p h e r i c a l nodules with c o a r s e l y s t r a t i fied construction, frequently completely m i n e r a l i z e d (fig. 3 : 1 and 2) . Their c o r e s a r e usually formed of s m a l l concretions of e x t r e m e l y fine, concentrically s t r a t i f i e d s t r u c t u r e . They have been enriched to a m a r k e d d e g r e e in s m a l l fragments of fish bones and teeth, and also g r a i n s of c l a s t i c m a t e r i a l . L e s s frequently, fragments of palagonitized and chloritized glass occur in the c o r e s of the nodules. Still less common a r e f r a g m e n t s of l a r g e fish teeth ( fig. 3: 3 ) . The s i z e of the nodules v a r i e s from 3 to 9 cm in d i a m e t e r , and they a r e most c o m monly 5 to 7 cm a c r o s s .

Translated from Sostav i stroyeniye metamorfizovannykh zhelezomargantsevykh konkretsiy, zhil'nykh novoobrazovaniy gidrookislov margantsa i vmeshchayushehikh ikh pelagicheskikhosadkov v Yushnoy kotlovine dna Tikhogo okeana, AN SSSR Izvestiya, ser. geol., 1975, no. 1, p. 91-111. Co-authors with Andrushchenko are B. P. Gradusov, V.A. YeroshchevShak, R.S. Yanshina, and S. Ye. Borisovskiy. They are with the Institute of Geology of Ore Deposits, Petrography. Mineralogy, and Geochemistry of the Over the a r e a of nodule distribution a r e USSR Academy of Sciences (IGEM); the V. V. Dokuchayev Soil Institute; and the Geological Institute v a r i o u s blocks of t u f f - b r e c c i a s , covered with of the USSR Academy of Sciences. i r o n - m a n g a n e s e c r u s t s . One such l a r g e block, Internat. Geology Rev., v. 17, no. 12 1375

INTERNATIONAL GEOLOGY REVIEW

=2]ff

JE\'Q FIGURE 1. Map of distribution of bottom sediments in sector 2 (Stn. 6298), compiled from data of Skornyakova et al. (1974). 1 - eupelagic clays; 2 - zeolitic clays - silt-pelite muds; 3 - marly eupelagic clays (10-50% CaCC>3); 4 - areas with predominance of outcrops of volcanic rocks, and discontinuous sedimentary covers. Concentrations of nodules on surface of bottom (%): 5 - 0-40, 6 - 0-100, 7 - 40-100; 8 - lava sheets; 9 - areas of distribution of hydrothermally altered sediments; 10 - dredging points. Figures station numbers. measuring 72 x 60 x 50 cm, was dredged up at Stn. 53 from a depth of 4530 m. It consists of a breccia, made up of large angular fragments of basalt, enclosed in altered (pelitized and zeolitized) eupelagic clays, containing new formations of hydroxides of manganese and iron with reniform and dendritic structure (fig. 3: 4 ) . Normally in such blocks, the thickness of the upper ore crust, exposed to the water, varies from 3 to 5 cm. The crust is very compact, and has a coarsely stratified construction. The thickness of the lower ore crust, adjoining the sediment, does not exceed 1 cm. It has a dendritic construction and is quite friable. In places, the lower ore crust is absent.

their known viscosity, they readily crumble on impact. They are characterized by a relict concentrically stratified and banded structure. In the altered nodules, there are frequently extremely finely contorted and unusual globular structures (fig. 3: 5 and 6) , in which a con centric construction i s markedly displayed as a result of crystallization. The internal surfaces of the ore centers in this c a s e consist mainly of a crystalline pyrolusite, and on the outside they are surrounded by an aggregate of c r y s t a l line grains of the s a m e mineral. They are brown or pitch-black in color. Such nodules are most widely distributed in the vicinity of Stations 2, 24, 30, and 62.

Special interest attaches to the ferromanganese nodules which have been altered by postvolcanic hydrothermal p r o c e s s e s . They are comparatively soft, and as a result of

The individual centers and dendritic ore accumulations consist of aggregates of minute crystalline grains of todorokite and they are frequently surrounded by little brushes of its minute crystals. 1376

IN.F. ANDRUSHCHENKO. ET AL.

FIGURE 2. New ore formations in metamorphosed bottom sediments (Stn. 6298-30). 1 - eryptomelane and todorokite (black) in altered pelagic sediment (gray). White - cavities painted over. Thin section, x30; 2 - eryptomelane (white field) with dendritic inclusions of todorokite and rancieite with eryptocrystalline construction (gray). In rectangle, sector investigated with Microprobe MS-46. Polished section, x40; 3 - eryptomelane buds (white*, outlined with crystalline-granular todorokite (gray). Hand specimen, x8; 4 three-dimensional dendrite of todorokite-cryptomelane composition from leaching cavity in altered bottom sediment, x8; 5 - reniform segregations of eryptomelane with cryptocrystalline construction, with contraction cracks, among altered, and to a marked degree bleached, bottom sediment. Polished section. x90; 6 - todorokite of collomorphic construction, crystallized In varying degree along concentric zones. Polished section, x320.

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FIGURE 3. Ferromanganese nodules and their internal structure. Sector 2, Stn. 6298. 1 - spherical nodules, reduced 7 times, Stn. 2; 2 - part of completely mineralized nodule with coarsely stratified construction. Polished section, x40, Stn. 2; 3 - ferromanganese nodules with cores consisting of large sharks' teeth, reduced 3 times; 4 - fragment of tuff-breccia, covered by ferromanganese crust of dendritic construction, reduced 10 times, Stn. 53; 5 - finely laminated texture of ore crust of altered nodule. Polished section, xlOD, Stn. 2; 6 - ore crust of altered ferromanganese nodule with globular construction. Polished section, x90, Stn. 24.

1378

N.F. A1MDRUSHCHEIVK0. ET AL.

FIGURE 4. Fringe of crystalline-granular todorokite around collomorphic segregations of vernadite (black). Polished section with analyzer, x90, Stn.30.

FIGURE 5. Reniform segregations of todorokite with radiate construction. Dark sectors - vernadite, light fringe - pyrolusite. Polished section, X165, Stn. 6333-13.

In the m o s t intensely a l t e r e d nodules, banded t e x t u r e s develop, typical of the m e t a m o r p h o s e d m a n g a n e s e o r e s , until now unknown among the i r o n - m a n g a n e s e nodules of the ocean. T h e s e very fine bands have been formed by discontinuous chains of s m a l l c r y s t a l l i n e g r a i n s of b r a u n i t e , which frequently consist of typical s k e l e t a l c r y s t a l s and impregnations of m o n o c r y s t a l s which contain minute poikilitic i n growths of a s i l i c e o u s - c l a y m a t e r i a l . The bands of analogous construction f r e quently consist of c r y s t a l l i n e - g r a n u l a r h e m a t i t e .

Thus, we have identified v e r n a d i t e , t o d o r o kite, cryptomelane, braunite, rancieite, p y r o l u s i t e , and h e m a t i t e . The d i s c o v e r y of braunite and h e m a t i t e in the m e t a m o r p h o s e d nodules is of g r e a t genetic i m p o r t a n c e . They c h a r a c t e r i z e the p r o c e s s e s of r e c r y s t a l l i z a t i o n of the h y d r o x ides of m a n g a n e s e and iron and the development of their anhydrous oxide compounds, so far u n known among the o r e a c c u m u l a t i o n s on the ocean floor. We shall dwell on the c h a r a c t e r i s t i c s of the p r i n c i p a l o r e - f o r m i n g m i n e r a l s .

In polished sections at high magnifications ( x 680), t h e i r o r i g i n a l collomorphic c o n s t r u c tion, indicating the m e t a m o r p h i c t r a n s f o r m a t i o n of p r i m a r y hydroxides of m a n g a n e s e and i r o n , may be t r a c e d among such finely banded new o r e formations in p l a c e s . In the r e c r y s t a l l i z e d s i l i c e o u s - c l a y g r o u n d m a s s of the nodules, r e mains of f o r a m i n i f e r a l t e s t s a r e s o m e t i m e s p r e s e r v e d , being p a r t l y r e p l a c e d by finely c r y stalline quartz.

Vernadite i s a n a t u r a l hydrate of manganese dioxide. In the r e g i o n investigated, it is one of the p r i n c i p a l o r e - f o r m i n g m i n e r a l s in the u n a l t e r e d f e r r o m a n g a n e s e nodules. It o c c u r s in m i n o r amounts in the a l t e r e d nodules in the form of a sooty v a r i e t y in a s s o c i a t i o n with p y r o l u s i t e and todorokite. In the compact m a s s e s , v e r n a d i t e has a pitch-black c o l o r , and a c o n choidal f r a c t u r e . Its specific gravity is 2. 2. ' During the p r o c e s s of dehydration a l m o s t a l l the water is eliminated by heating to 120°.

MINERAL COMPOSITION O F METAMORPHOSED NODULES AND NEW FORMATIONS O F HYDROXIDES O F MANGANESE IN HYDROTHERMALLY A L T E R E D SEDIMENTS

In the nodules and among the new formations of m a n g a n e s e hydroxides v e r n a d i t e is X - r a y a m o r p h o u s , and only in the initial p h a s e of d e hydration and c r y s t a l l i z a t i o n do diffuse diffraction lines with d / n = 2 . 4 0 - 2 . 4 3 and 1.40-1.44 A appear against the g e n e r a l intense background of darkening of the powder p a t t e r n s .

The m i n e r a l o g i c a l study of the v a s t amount of m a t e r i a l from the v a r i o u s s e c t o r s of the a r e a , with the aid of m o d e r n methods of i n vestigation, has enabled us to identify the s t r u c t u r a l f e a t u r e s of the a l t e r e d manganese o r e s and to compile a list of the o r e m i n e r a l s i n volved in the composition of the f e r r o m a n g a n e s e nodules and new vein formations. 1379

In the m e t a m o r p h o s e d nodules, v e r n a d i t e , which f o r m s concentrically a r r a n g e d zones, loses water and p a s s e s into an a g g r e g a t e of

'Specific gravities were determined by the hydrostatic method by V.S. Amelina (IGEM AN SSSR'.

IINTEKINATIONAL GEOLOCY REVIEW

TABLE 1. Chemical composition of ferromanganese nodules and new formations of manganese hydroxides in altered bottom sediments fwt $ * St at ion No. and depth, m

Components

6298-30. 4720-4780

5965-1. 4830 5965-3. 4840

0333-13/8. 3020-3740

Cuba

i

Sample No.

Si02 Ti02 A1 2 0 3 Fe203 FeO MgO CaO Na,0

K26

MnO Mn02

H«0~

H 2 0+ CO., NiO CGO CUO

PA

Cl BaO

Total

73-12

73-13

73 14

12.36 2.12 5.84 26.75

10.98 2.25 5.51 27.97

10.51 2.37 3.57 22.88

Nil

73-10

7.21 0.15 3.90 2.39

73 I)

Nil

0.08

4.24

0.98 1.25

Nil

Nil

Nil

Nil

4.98 1.01 2.46 1.46 1.98 G8.90 6.21 8.41 0.13

1.51 5.99 0.19

24.23 10.02 8.25 0.12 0.37 0.65

5.55 1.72 2.14 1.23 3.77 56,24 7.48 7. U9

7.98 67.98 0.23 8.08 0.24

Nil

Not d e t .

Not det.

Not det.

0.G1 0.87

0.56 0.90

Not d e l .

Not det.

99.77 -U.2

99.13 —U.2

99.57

98.93

26.56 15.55 7.38 0.48 0.41 0.59 O.U87 0.26

Nil

0.33 0.025 0.122 tr. Not det.

Not det.

0.27

Nil Nil

2.39 2.91 2.79 0.63 Nil

1.47

0.20

Nil

Nil

Nil

2

1.67

2.30 2.22 2.10 0.63 1.14 22.74 9.78 8.08 0.37 0.27 0.70

2.30 2.22 2.0U 0.G5

1 7J-15

O 19

0.01

0.21

Not det. Not det.

Not d e t . Not d e t .

»

»

0.45

£

» »

;) 0.50

Not d e t .

Not d e t .

99.36

99.34

99.90

99.29

1.05 3.23 0.89 0.78 9.58 C8.80 0.7(1 9.22

— , — — — — —

tr. tr.

3.25 2.30 1.14 0.35 11.65 69.57 10.5

_ — .

0.05 O.o5 O.ol

— —

0.61

0.19

—

—

100.50

99.23

73-12 - average sample from two completely mineralized nodules of diagenetic origin; 73-13 average sample from upper ore zone of nodule, in contact with water; 73-14 - upper zone of nodule, in contact with sediment; 73-10 - thin veinlet of todorokite-cryptomelane composition in bleached red clay; 73-11 - reniform and dendritic formations of todorokite in bleached red clay; 73-15 todorokite of radiate construction, lining cavities in zeolitized hyaloclastic rock. 1 - chemical analytical data for todorokite from Charco-Redondo deposit (after Sokolova, Stepanov et al., 19711. 2 - chemical analytical data for todorokite from Charco-Redondo deposit (after Frondel et al.. 1960). Analyses by L.S. Tsimlyanskaya (samples 73-12 and 73-131, Y.M. Stepashkina (sample 73-10), andR.S. Yashina (samples 73-11, 73-14, and 73-15).

minute c r y s t a l l i t e s of p y r o l u s i t e , noted a t high magnifications. Todorokite in the f e r r o m a n g a n e s e nodules of the Pacific Ocean, as i s well known, is one of the widely d i s t r i b u t e d o r e - f o r m i n g m i n e r a l s (Andrushchenko and Skornyakova, 1969; S k o r nyakova and Andrushchenko, 1970; Buser and G r u t t e r , 1956; Glasby, 1972; Cronan and T o o m s , 1967, 1968, 1969). However, it is a l s o l e s s frequently found and in considerably s m a l l e r amounts a s c o m p a r e d with the a c c u m u lations of a m o r p h o u s and weakly c r y s t a l l i z e d vernadite. Among the new formations of m a n g a n e s e hydroxides in the a r e a of volcanic p r o c e s s e s , accumulations of a comparatively w e l l - c r y s t a l lized todorokite a r e exceptionally widely d i s tributed. In the s e c t o r s of intensely bleached

1380

r e d c l a y s , todorokite i s the p r i n c i p a l o r e forming m i n e r a l . It o c c u r s in a s s o c i a t i o n with c r y p t o m e l a n e , v e r n a d i t e , and r a n c i e i t e , f o r m ing botryoidal and r e n i f o r m s h a p e s . F r e q u e n t l y in the a l t e r e d pelagic s e d i m e n t s , todorokite forms a g g r e g a t e s of t h r e e - d i m e n s i o n a l d e n d r i t e s , filling the leaching c a v i t i e s , and is developed in t h e f o r m o f veinlets a l o n g s u b c a p i l l a r y c r a c k s . In the major c a v i t i e s , a g g r e g a t e s of p r i s m a t i c todorokite c r y s t a l s s u r r o u n d their walls and sequentially invest the n u m e r o u s r e n i f o r m formations of c r y p t o m e l a n e . In a freshly broken nodule, the c r y s t a l l i n e - g r a n u l a r t o d o rokite is readily distinguishable from c r y p t o m e l a n e by its comparatively dark r e d d i s h brown color and radially columnar s t r u c t u r e (fig. 2: 3 ) . Under reflected light, it may be s e e n that the reniform todorokite s o m e t i m e s has a

IN.F. AINDKUSH( HENkO, ET AL. TABLE 2. X-ray characteristics of todorokite

1

2

4

3

/

"a

/

*«

I

4a

/

10

9.58

10 7 10

9.47 7.02 4.73

10 4 1(1

9.47 7.02 4.73

7

1

3.52

1

3.52

1

3.17

6

3.19

7 7 3

2.44 2.38 2.19

8

2.45

1

1.958

1 lw

1.736 1.673

10

4.87

1 7

3.30 3.19

/

2.47

7

1.426

8w

5 8w

1.409

1.674 1.412

7

5

4 4 1

9.75 7.18 4.98 4.16 3.47

10 4 1

3.18 2.98 2.40

1 1 4 2 1 1 1 2 . .

/

da

1.982 1.84U 1.778 1.677 1.515 1.483 1.437 1.371

10 2P

6 2w l'2d 2 6 Id 7 2d Id 3d 1 d 1d 0

2d 1/2 d 4d 2d

ia 9.6 7.1 4.75 4.4 3.4 3.18 3.07 2.44 2.37 2 20 2.14 1.98 1.92 1.92 1.03 1.54 1.48 1.42 1.38

1 - todorokite forming main ore portion of hydrothermally altered nodule, Stn. 6298-2; 2 - from monomineralic ore segregations in intensely altered, porous nodule, reddish brown, Stn. 6298-30; 3 - todorokite forming fringes around reniform accumulations of oryptomelane in bleached bottom sediment, Stn. 6298-30; 4 - todorokite of radiate construction from geodes of intensely zeolitized and phosphatized hyaloclastic tuff, Stn. 6333-13; 5 - todorokite from Los Chivos deposit, Cuba (Sokolova et al., 1971).

concentrically banded or finely collomorphic structure, especially clearly manifested when weakly etched with H2S04. The outer shells of such formations are also characterized by a pinnate structure (fig. 2: 6). In transparent thin sections, todorokite is most commonly brownish red, with marked pleochroism in shades of brown, and is some times opaque. In reflected light, it is greenish white, and somewhat darker than cryptomelane. Birefringence and effects of anisotropy in the fine -grained and cryptocrystalline varieties are almost nonexistent, but are distinguishable in immersion liquids. Its coarsely crystalline varieties, have clear birefringence, and an anisotropy with color effects (from light gray to greyish brown) (fig. 4). ; Reddish-brown internal reflections a r e readily seen in immersion liquids. During the investigation of todorokite under the scanning electron microscope, the typical monoclinic habit of the crystalline grains of the mineral was observed (see fig. 10: 3 and 4).

which forms veinlets and reniform segregations among the bleached red clays (Stn. 30) and monomineralic accumulations of todorokite of radiate construction (Stn. 6333-13), are p r e sented in Table 1. For comparison, we have also presented chemical analytical data for the composition of ore films of ferromanganese nodules of diagenetic origin ( Stn. 5965-1, 59653) and of todorokite from the Charco-Redondo deposit ( Cuba) (Sokolova et al. , 1971). Attention is drawn to the relatively large amount of titanium in the ferromanganese nodules and its almost complete absence in the richest new vein formations of manganese hydroxides. T h e ( F e + Mn)/Ti ratio in the nodules is 22.23-26.8, whereas in the hydrothermal ore formations, this value increases to 477 and more.

The specific gravity of the mineral is 2. 39, and with correction for the presence of water (8.41°; ) , 2.98.

In the oceanic pre formations, as is well known, titanium is actually associated with terrigenous material, and its concentrations markedly decrease in the hydrothermal ore formations. A s N . M . Strakhov has noted, low and high values of the above ratio are objective indices of ore material of essentially lithogenic and exhalative origin (Strakhov, 1974a and b) .

The chemical analytical data of the ore material of essentially todorokitic composition,

The selection of samples for X-ray analysis of todorokite was achieved by drilling with a

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FIGURE 6. Banded s e g r e g a t i o n s of b r a u n i t e (white) in a s i l i c e o u s - c l a y r o c k . (Polished s e c t i o n , x320, Stn. 3 0 ) .

FIGURE 7.

Magnified p i c t u r e s of distribution of e l e m e n t s in c r y p t o m e l a n e - t o d o r o k i t e o r e . Stn. 3 0 .

1 - photo in a b s o r b e d e l e c t r o n s ; 2-7 - in X - r a y s of M n n , K a . A 1 Q , C a n , M g c , and Na2 and Mn(OH)2- Under definite physicochemical conditions of mineral formation which cause rapid precipitation of mineral material from solution, such a structure is

1384

particularly favorable for the entry of a significant number of ions with different ionic radii. Some deficiency of oxygen ions in the formulae ( 11. 7 and 11. 93, instead of 12) probably indicates the presence of an OH - ion in the minerals examined, equal in size to the oxygen ion. Recalculation of the chemical analytical data to the crystallochemical formula for todorokite reveals some excess of Mn02 and H20, dependent on the presence of a mechanical additive (vernadite) in the sample. On the X-ray powder photographs of these samples, diffusion reflections have been noted which indicate the presence of an amorphous hydrate of manganese dioxide (vernadite). lnoaddition, the lines 3. 17, 2. 38, 1. 673, and 1. 50 A, etc., have been identified, typical of pyrolusite (table 2). Insignificant amounts of vernadite in the thin, clearly anisotropic films of pyrolusite composition may sometimes be observed in the polished sections among the comparatively large radiate aggregates of todorokite (fig. 5).

IN.F. AMJRLSIICHEINKO. ET AL.

A small amount of iron oxide ( 1. 25; 0. 08 % ) is due to a mechanical addition of hematite. In paragenesis with todorokite, segregations of minute flakes of hematite with typical ruby-red internal reflections have frequently been identified in the polished sections. From the microscope and X-ray data, r e calculation of Sample 73-11 into mineral composition reveals an amount of about 84 % todorokite, 2.8% pyrolusite, 3.8% vernadite, 9.4% silica, clay particles, etc., in it. In Sample 73-15, there are todorokite ( 84 % ), pyrolusite ( 5 % ) , vernadite ( 4 % ) , and additives (7%). Cryptomelane. Among the manganese oxides of hydrothermal origin in the area investigated, cryptomelane, together with todorokite, is one of the principal ore-forming minerals. Aggregates of three-dimensional dendrites of this mineral have been developed along the fine cracks in the clay material of the altered bottom sediments. Small reniform segregations of cryptomelane have also been formed in the leaching cavities among the chloritized sediments and are frequently covered with a film of finely crystalline todorokite (fig. 2: 3). The reniform segregations of cryptomelane are usually dispersed through the network of fine contraction cracks and are readily separated into fragments of isometric shape. Cracks have been observed on the surface of the nodules and in their internal parts (fig. 2: 3 and 5). The dendritic aggregates and reniform accumulations of cryptomelane are characterized by a zoned-conchoidal internal construction, dependent on the rhythmic deposition of colloidal material. The cryptocrystalline cryptomelane is readily distinguishable from todorokite, with which it occurs in paragenesis, by the characteristic steel-gray color and matte conchoidal fracture surface. The specific gravity, determined by pycnometer and hydrostatic methods, is 4. 1. The infrared spectrum of cryptomelane, investigated by Ye. S. Rudnitskaya (IGEM AN SSSR), is characterized by two weak absorption bands at about 520 and 575 cm-1. These values are similar to the absorption bands (532 and 568 cm" 1 ) of the infrared spectrum identified for cryptomelane from Sainakami in Japan (Omori, 1964).

FIGURE 8. X-ray diffraction patterns of clay minerals (oriented preparations). a - air-dried preparations; b - with ethylene glycol; c - heated at 500c; d - heated at 600°; e - treated with 10% HC1 and the decomposition products removed with 0.5 HC1 and NaOH.

On the X-ray patterns for the cryptocrystalline variety of cryptomelane, all the typical lines for this mineral have been identified (after Rostov, 1971) . The most typical values for the interplanar distances and intensity of lines for this cryptomelane a r e : 6. 90( io);

1385


III ||||l%|||«-

1

73-1

\ 1 ■ *\

■ 1 1

\

73-2

7.7-/,

73-3

73-5

FIGURE 9. Infrared spectra (a) and DTA (b) patterns of clay minerals.

4 . 8 4 ( g ) ; 3 . 1 0 ( 1 0 ) ; 2. 43( 4 ) ; 2.144(5); 1.982(1); 1-526(6); 1.4l0( 8 ). Scanning of the reniform accumulations of cryptomelane in absorbed electrons and in the characteristic X -radiation of various elements involved in its composition was carried out over an area of 200 x 200 microns. 2 The photographs reflect the distribution of concentrations of elements according to intens ity of K f f l of the line of Mn, Ca, Al, Mg, K, and Na radiation (fig. 7). The results of the investigations indicate the variation in composition of the reniform accumulations of cryptomelane. The microdendritic collomorphic segregations of todorokite, which occur among the cryptomelane, have been significantly enriched in Al, Mg, and Ca, at the same time as

the cryptomelane, which fills the inter dendritic spaces and contains Na as well as K, and in this case, Mn is evenly distributed in the area examined (fig. 7). Thus, the data obtained have made the composition of the todorokite, which forms the dendritic segregations, and the cryptomelane, which fills the interdendritic spaces, more precise. Braunite. In the altered nodules, the aggregates of minute grains of braunite form dis continuous bands in the recrystallized siliceous clay mass, alternating with the centers of e s sentially hematitic composition. Such bands represent the original centers of hydroxides of manganese and iron, dehydrated and altered under the influence of erupted lavas and accompanying submarine gaseous emanations. In the polished sections, at only a comparatively small magnification, it may be seen that

determinations by S.Te. Borisovskiy (IGEM ANSSSR) on microprobe MS-46 ("Cameca").

1386

i \ . F . A N D R U S H C H E M K O , F T AL.

TABLE 6. X-ray characteristics of hydromicas (samples 73-1 and 73-2) and chlorite-montmorillonite mixed-layer formations (samples 73-3, 73-4, and 73-5' Powder preparations 73-1

O r i e n t e d p r e p a r a t i o n s , s a t u r a t e d with ethylene glycol 73-2

73-4

73-5

73-3

din

/

din

/

din

1

din

I

din

I

4.551 4.46/ 4.29 3.6

5 1 1/2

4.551 4.45/ 4.29 3.6

4 2 1

29.2 15.3 7.32

82 165 48

29.4 15.1 7.24

48 77 35

29.4 14.7 7.15

53

Hi

5.08

27

4.79

18

4.78

51

4.51 3.48

22 53

3.51

32

3.53

65

2.83

19

2.82

7

2.83

11

V. W.

—

—

3.35)

—

—

3.32J 3.0

3.33 3.0

10

1/2

V. W.

2.61 2.49 2.4

3 3 2 1

10 1

.—

—

—

—

V. W .

2.61 2.51 2.4

V. W.

2.17

73

V. W.

2 3 2 V. W.

2.14

2 v.w.

2.081 1.991 1.84 1.667 1.529 1.498

3 1/2 3w. 3 1

2.081 1.98/ 1.84 1.667 1.530 1.500

3 1

3w. 3 1

the c r y s t a l l i n e g r a i n s of b r a u n i t e , which form the discontinuous bands, a r e c h a r a c t e r i z e d by very sinuous outlines (fig. 6) . The braunite grains a r e usually intensely p o r o u s , and contain poikilitic ingrowths of s i l i c e o u s - c l a y p a r t i c l e s . Braunite a l s o f o r m s minute sinuous chains of idiomorphic c r y s t a l l i n e g r a i n s of s k e l e t a l construction. Minute s e g r e gations of idiomorphic c r y s t a l l i n e g r a i n s of braunite a r e a l s o o b s e r v e d , which in e s s e n c e r e p r e s e n t o r e p o r p h y r o b l a s t s , in very finely crystallized siliceous-clay sediment. Braunite is weakly a n i s o t r o p i c , and is c h a r a c t e r i z e d by brown i n t e r n a l reflections and m o d e r a t e reflectivity.

rounding r e c r y s t a l l i z e d s e d i m e n t s . In p l a c e s , h e m a t i t e has also been observed in the form of minute p o r p h y r o b l a s t s . The X - r a y p a t t e r n s of individual hematite grains p o s s e s s lines c h a r a c t e r i s t i c of this m i n e r a l , s i m i l a r in d / n and I values to those given by Mikheyev ( 1 9 5 7 ) . The p r i n c i p a l lines of hematite a r e v e r y c l e a r , which indicates a w e l l - c r y s t a l l i z e d m i n e r a l (table 4). MINERAL COMPOSITION O F HYDROTHERMALLY ALTERED BOTTOM SEDIMENTS During a study d e e p - w a t e r bottom thermal processes, from v a r i o u s p a r t s

The X - r a y data for individual b r a u n i t e g r a i n s from an a l t e r e d concretion at Station 6298-30 ( t a b l e 3) coincide with the a p p r o p r i a t e data given by Mikheyev ( 1957).

of the a l t e r e d n a t u r e of the s e d i m e n t s owing to h y d r o we investigated eight s a m p l e s of s e c t o r 2 in detail.

In the n o r t h e r n part of the s e c t o r , Sample 73 -6 was collected from a horizon, 0-3 cm thick, of weakly a l t e r e d r e d c l a y s , and a l s o bottom s e d i m e n t s , in c o n s i d e r a b l e d e g r e e a l t e r e d and converted to a bleached clay m a s s ( s a m p l e s 7 3 - 1 and 7 3 - 2 ) .

Hematite. In the a l t e r e d f e r r o m a n g a n e s e nodules, c r y s t a l l i n e grains of h e m a t i t e , like those of b r a u n i t e , form individual o r e bands. The hematite grains a r e usually intensely porous and frequently have a s k e l e t a l c o n s t r u c tion. They contain poikilitic ingrowths of minute s i l i c e o u s and clay p a r t i c l e s of the s u r 1387

In the c e n t r a l p a r t of the s e c t o r , five s a m p l e s were examined, involving different d e g r e e s of a l t e r a t i o n of r e d c l a y s , and m i n e r a l i z e d in varying d e g r e e ( s a m p l e s 7 3 - 3 , 73-4. 7 3 - 5 , 7 3 - 8 , and 7 3 - 9 ) .


FIGURE 10. Electron-microscope pictures, scanning microscope. 1 -hydromica with large amount of montmorillonite, Sample 73-1, x 2000; 2 - mixed layer chlorite-montmorillonite with small amount of hydromica, Sample 73-5, x 5000; 3-todorokite among clay matter, Sample 73-5, x 3000; 4 - aggregate of crystalline grains of todorokite, Sample 73-5, * 10,000.

F o r c o m p a r i s o n , we have examined Sample 73-7 from Station 6275-2, in outward a p p e a r a n c e r e p r e s e n t i n g u n a l t e r e d , typically d e e p - w a t e r red clays. The c h e m i c a l analytical data a r e given in Table 5. We s h a l l dwell on the c h e m i c a l - m i n e r a l o g i cal c h a r a c t e r i s t i c s of these s a m p l e s . Sample 73-6. Collected in the n o r t h w e s t e r n p a r t of the d e p r e s s i o n at a depth of 4380 m from a horizon, 0. 3 cm thick (Station 5 9 6 5 - 1 ) . It c o n s i s t s of a pelitic mud with n u m e r o u s f r a g m e n t s of foraminiferal t e s t s of s i l t and sand s i z e . Under the m i c r o s c o p e , l a r g e fragments of volcanic g l a s s , palagonitized in varying d e g r e e ( c o r a s e pelite) , have been observed among the weakly polarizing m a t e r i a l of the g r o u n d m a s s . T h e r e a r e also s e c t o r s consisting mainly of h y d r o m i c a s . In the o v e r a l l pelitic t e x t u r e of the sediment, t h e r e a r e individual s e c t o r s of organogenic construction. 1388

The investigations have shown that the < 0 . 0 0 1 mm fraction, s e p a r a t e d from this s a m p l e , c o n s i s t s mainly of h y d r o m i c a s ( d / n = 10. 1 A) and montmorillonite ( d / n = 14. 5 A ) , with a s m a l l quantity of c h l o r i t e and kaolinite (fig. 8 ) . On the X - r a y diffraction p a t t e r n s , m o n t m o r i l l o n i t e is c h a r a c t e r i z e d by a b r o a d r e f l e c tion of the f i r s t o r d e r , which indicates its weak o r d e r i n g . The hydromica reflection is c l e a r e r . On the b a s i s of the r a t i o of clay m i n e r a l s in the < 0 . 0 0 1 mm fraction, d e t e r m i n e d by B i s c a y e ' s method ( 1964), m o n t m o r i l l o n i t e c o m p r i s e s 36 c r-, h y d r o m i c a 59 r( , and c h l o r i t e and k a o l i n ite up to 5 % . Infrared s p e c t r o s c o p e data i n dicate that m o n t m o r i l l o n i t e and the h y d r o m i c a s a r e d i o c t a h e d r a l , and the 1450 c m - 1 band i n d i c a t e s the p r e s e n c e of c a r b o n a t e . Under the m i c r o s c o p e , a s m a l l amount of a s h m a t e r i a l has a l s o been o b s e r v e d . The clay fraction apparently contains organic m a t t e r of humic composition, as identified by a c l e a r l y defined 1370 c m - 1 band on the i n f r a r e d s p e c t r u m

N.F. 4NDRUSHCHENK0, ET \L. spheruUtes are present in considerable numbers. The texture of the sediment is pelitic, and in places, relict organogenic. The < 0. 001 mm fraction of the sediment consists mainly of montmorillonite ( 68 % ) , hydromica ( 30 % ) , and trioctahedral chlorite ( 2 %). In contrast to Sample 73 -6, kaolin is absent here, there is no humic matter or c a r bonates, and the montmorillonite is characterized by a greater degree of ordering. The last feature is indicated by the X-ray diffraction data and the results of infrared spectroscopy (figs. 8 and 9a). It is evident that the sediment, represented by Sample 72-7, was analogous to that of Sample 73-6, but underwent alterations, which were reflected in the formation of montmorillonite after volcanic glass and the greater structural ordering of the montmorillonite, in the decomposition of the kaolinite and the humic acids, and also in the replacement of carbonates by phosphates. Similar changes have been described by Kharin ( 1974). All these changes are postsedimentational, and in all probability are the result of the effect of hydrothermal solutions on the sediment. Samples 73-1 and 73-2 were taken from a depth of 4860 m at the foot of a rise in the northern part of the depression in sector 2 (Stn. 6298-20). For the investigation, samples were selected from the bleached clay with ashlike inclusions of ore material (73-1) and sectors colored bluish-gray ( 73-2).

FIGURE 11. Graphs of