CHAPTER V

~IINEHALOGY OF PHOSPHORITES

INTRODUCTION

Although near about 300 phosphate mineral species have

been recognized \Nriagu and Moore, 1984) 1 however out of

these, carbonate fluorapatite is the only major mineral of

marine phosphorites. Due to its widespread occurrence and

economic value, much work has been carried out to understand

its structure and composition, yet many ambiguities

exist in the literature on its structure, composition and

properties.

carbonate fluorapatite is a variety of apatite

that crystallizes in hexagonal system with P6 3/m space group

atomic structure. The unit cell contains 2 formula composition

yielding theoretical fluorapatite ca10 (P04 ) 6F2

. Thus 20

•

93

positive charges of calcium atoms are balanced by 20 negative

charges of 6 phosphate anion group plus 2 fluorine.

Previous l"lork :

Various review articles on the mineralogy of marine

apatite \ francol.i te) are available , Brasseur et al. J (1946).

Altschuler et al. 1 (1952), Ames (1959), Elliott (1964),

NcConnell (1952, 1965 J J Gulbrandsen ( 1970) 1

McClellan (1980)

have made significant contributions in this field.

Sedimentary apatite differs considerably from

fluorapatite. due to extensive substitution in its lattice.

Two types of sedimentary apatite recognized are carbonate

apatite and noncarbonate apatite. Further on the basis of

fluorine. the carbonate apatites are grouped into francolite

(containing> l% FJ and dahlite ( < l% F) (McConnell, 1938).

Carbonate fluorapatite (francolite) exhibits systematic

isomorphous substitution for both cations and anions (Table

VI). The most important and characteristic of these is the

carbonate substitution. Various views have earlier been

proposed regarc'.ing the mode of occurrence of carbonate in

apatite such as carbonate occurs as a separate phase (Thewlis

et al. 1939, Brasseur et al. 1946); as absorbed on the surface

of apatite crystallite (Neuman and Neuman. 1953); or carbonate

is a part of apatJ.te structure (Gruner and McConnell 1937).

However nm; Jl has been unanimously accepted that carbonate is

91

Table-VI Some proposed substitutions in the apatite structure

Constituent ion

Ca+ 2

-1 F

-2 0

Fluorapatite

Substituting ion

N +1 S +2 M +2 K+l 0+4 a,J:,n,,

#2 M +2 RE+2 1 +3 Ba 1 g 1

(lanthanons and yttrium)

c+ 41 s+6

1 si+41

As+S1

v+5

Cr+61

A1+ 3

-1 C1-ll co3-2 OH I

Pranco1ite

(After Nathan, 1984)

95

a part of apatite structure, and that co3

- 2 substitutes for -3

P04 • Borneman-Starynkevitch and Belov (1940. 1953) on

the basis of chemical analysis of natural francolites

suggested that a hydroxyl (or fluorine) ion accompanies

carbonate and that th (CO -3) ( -3 e groups 3 .oH or co3

.F) replace -3

the P04 group. Their work has been supported by Smith and Lehr

tl966), Gulbrandsen (1966), Elliott (1969) an~ Pacquet et al.

(1980).

Another frequent substitution in sedimentary apatites

2+ + 2+ is that of Ca by Na and Mg . Lehr et al .. (1967) have shown that

this substitution is directly connected with that of P0~ 3 by

substitution increase with the

increase of co3;Po4 molar ratio. It is therefore appears that

these two elements, in particular Na, which is more abundant

and also monovalent, combines with F to preserve the

electrical neutrality of the apatite crystal, when Po~3 is

replaced by co;2 ( Slansky. 1986<1). A correlation between Na

-3 -2. content and the degree of substitution of P04 by co3 has

already been noticed by Gulbrandsen (1960 , 1966).

The partial replacement of F by OH is also fairly

frequent in sedimentary apatites, and this replacement of F by

OH is probably responsible for the poor correlation between

variation in the molar content of F and the C03/P04 molar

rat~o (Slansky, 191:lua) Lehr et al. (1967) proposed a .general

formula for sedimentary apatites. considering the possible

9()

importance of substitution of Na, Mg. or 011 as (Ca, Na, Mg)lO (P04)6-x (C03 ) F ( F, OH) 2 in which varies X y y

between 0.33 and 0. 51

X generally lies between 0 and 1. 5. The

numbers of mules of Na, approximately x-y, is greater than the

number of moles of Mg. The total number of moles of Mg in most

marine sedimentary deposits is greater than 2.

Other substitutions of less importance allowed by

sedimentary apatite structure are Potassium, Strontium, -3 -4- _3 -"' Uranium, Thorium for Calcium

1and so

4, Si0

4, As0

4 or vo

4 for

-- ~ P04 .

Effects of carbonate substitution :

Carbonate substitution is considered one of the most

characteristic and important of all the substitutions in

apatite structure, as it produces distinct changes in the

physical and optical properties. McConnell (1973) proposed a

-2 direct relationship between amount of co

3 substitution and the

birefringence, which increases with an increase in co2

content. The carbonate substitution also affects the lattice

parameters a0 and c~ however studies reveal that effect on a 0

is more significant and regular than on c 0 (Lehr et al. 1967 ,

McClellan and Lehr, 1969). Parameter a0 is found to decrease

from about 9.370 l to about 9.320 ~ as the C0 3/P04 mole

ratio i ncrcascs from 0 to 0. 3, for example, francolites of

!lone Va11t·y Formation contain 4.3 to 3.33 wt. percent Co 2 have

97

aD values of 9.335to 9.345 ~ , and francolites from Hawthorn

Formation (C0 2-5.3%) have a0 value of 9.325 (McClellan, 1980),

while the co2 poor Kanpur francolites of Araval.li Formation

have aD values much higher, i.e. 9.366 " (Banerjee et al.

1980). Lehr et al (1967) and Banerjee et al. (1980), suggested

that there is a direct relation between size of apatite

crystallite and a0 parameter (as a•parameter is affected by co~

substitution), hence there is an indirect relation between the

co;2 substitution and crystallite size. The crystallite size

C.. o:>- . substl.. tutl.' "n. The carbonate decreases with increasing " v

substitution also affects the solubility of apatite in

citrates or acids, the solubility of apatite increases with

increasing substitution (Silverman et al., 1952). The

refractive index of apatite is also inversely correlated with

the amount of carbonate in apatite, which varies from 1.64 to

1.60 as the co3;Po 4 molar ratio increases from 0 to 0.3 (~ehr

et al., 1967).

Gulbrandsen (1970) developed an X-ray peak pair method

for estimating the amount of carbonate as percent co2 in

marine aiJatite.

Infra-red absorption spectrum is very sensitive to

carbonate substitutL" ·, even minute amount of carbonate can

be detected. Infra-red spectrum of carbonate ion in carbonate

apat 1 tc 1s ct1aracterized by a splitting of v 3 vibration

-1 6 between 1470 and 1410 em (Gulbrandsen et al., 196; Brophy and Nash,

98

1968) and a less prominent l"tt" f sp ~ ~ng o the v3 vibration

between 830 and 850 em -l (LeGeros et al. 1970). Lehr et al.

(1967) estimated the amount of co2 within the francolites

using infra-red spectrum, they proposed a co2

index for the

estimation of CO amount. 2

Weathering, alteration, metamorphism or heating also c

aff7ts the mineralogy of carbonate fluorapatite greatly. Chien

and Black (1976) showed that the free energy of formation

( !J.G' f, CA) and the related solubility product constant (KCA)

increase systematically ~1ith the number of moles of carbonate

per mole of francolite. This result clearly establishes the

metastability of the members of the francolite series with

respect to fluorapatite -the stable end member (McClellan,

1980). Thus geological processes like weathering, metamorphism

and diagenesis work to change the francolite towards its

stable form ~.e. fluorapatite. This concept of systematic

alteration complements the 'Original constant composition'

hypothesis presented by McArthur (1970) and explains much of

the variability observed in francolite compositions. Smith and

Lehr ( 1 SIG 0) found that when phosphorite is heated between

500-800°C, the absorbed moisture is expelled, organic matter

is destroyed and francolite alters progressively to

fluorapat1te. Nathan {1984) observed that heating causes loss

of structural co2

in two phases; viz. (1) between 500-700°C

{20-80%) and (2) between 700-1000°C,all the remaining co2 is

9!.1

expelled simultaneously with a proportional (equivalent) amount of fluorine. These studies further strengthened that

the C02 oc:curs in two different configurations in francolite

as co3 - 2 and as co3

.F-J

The weathering of carbonate apatite is responsible for

development of many nonapatitic phosphates in sedimentary

formations like Crandallite, Millisite, Vivianite. These all

secondary phosphates are J:elatively low in abundance.

MINEROL~GICAL CHARACTERS OF LOHARA PHOSPHORITES

The characterization of mineral phases bears great

significance in understanding the genesis. Further these data

may also be utilized for selecting suitable beneficiation

processes of these phosphorites, if need arises.

The characterization of mineral phases is based on the

X-ray ctj f fraction and infra-red studies of bulk samples of

phosphor i tcs and phosphatic siliceous clays, treated samples

of phosphorites, and clay fractions of phosphatic siliceous

clays. x-ray diffraction techniques were also employed for

determination of co2 contents, following methods of

Gulbrandsen (1970) and crystallite size measurements after Rau

{1962).

on the basis of morphology, field associations and

pclroq 1~,1 p11Ic cii<~r~,,cL<'rs, Khan and Mukherjee (1988) have

'100

recognized two types of phosphorites from this area, viz; (1)

Phosphorites and (2) Ph h · · osp at~c s~liceous clays. Hence the

mineralogical characters of both the types have been studied

in detail and described separately in the following paragraphs.

Phosphorite

The characteristic X-ray diffractograms (Figs. 13 and

14) of bulk samples of phosphorites reveal presence of

carbonate fluorapatite as the only apatite mineral phase

alongwith gangues comprising of quartz and rarely calcite in

decreasing order of abundance. Whereas the Silverman solution

treated samples do not give much change in x-ray diffraction

pattern (Fig. 15) except elimination of calcite peaks and

increase in sharpness of the apatite peaks. X-ray diffraction

data are presented in Table VII.

These phosphorites are rich in carbonate content, and

are essentially carbonate fluorapatite. The co2 content

(measured by x-ray method) varies from 4.48 to 2.12% for bulk

samples while 3. 00 to 4. 30% in extracted samples which is

quite high compar1ng to the co2 content in other Indian

Proterozoic phosphorites (Table-VIII). The cell dimension for

the bulk phosphorite samples varies from a 0 9.3249 to 9.3208 ~,

c 0 b. 8 58 9 to 6. 917 5 $., whereas cell dimensions in extracted

phosphorites show occasional increase(Table-VII).

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112 2.7640

40 2.7720

2.7715 8

2.7781 51

2.7798 46

A

300 2. 6862

38 2.6889

58 2

. 6971 10

2.6999 62

2.7023 56

A

202 2.6143

16 2.6099

26 2.6231

5 2.6276

28 2.6299

28

A

301 -

--

-2.5558

1 2.5149

7

c 110

--

--

--

--

2.4609 7

Q

110 2.4462

10 2.442

9 2.4546

6 2.4596

10 2.4320

5

A

212 2.2783

10 2.276

10 2.2796

8 2.2868

13 2.2879

11

A

310 -

-2.243

23 2.2365

5 2.2469

24 2.2469

23

Q

111 2.2394

18

A

221 -

--

-2.1266

5 2.2136

6 2.2115

5

A

311 -

-2.123

10 -

-2.1298

10 2.1332

9

A

113 2.0578

5 2.053

5 2.0617

1 2.0645

7 2.0654

6 1-4 0 0".:

-con

td ..

1.

z:. 3

. 4

. 5

. 6

.

A

CCIQ'

2.0

23

8

3 .

. A

2

Ql

1.9

72

6

5 1

. 994 5

Q

Z!ll

. -

-.

A

2n 1

.93

02

15

1.9

30

23

A

31

2

. .

1.8

76

0

12

A

32

0

. -

-.

A

21

3

1. 8

34

0

17 1

. 8370 2

9

0 112

1. 81

64

14

. .

A

321 1

.79

02

9

1. 7951

14

A

41

0

1.7

62

0

8 1

.76

40

13

A

40

2

1 • 742 2 9

1.7

45

2

12

A

004 1

.72

27

7

1.7

17

7

14

Q

202 .

-1

.67

09

8

---

20 d

ifference

in

(00

4-4

10

) 1

.30

1

.45

C0

2 W

t.S

4.4

8

2.2

7

a 0A

cell dim

ension 9

.32

49

9

.32

52

c oA

cell

dimension

6.8

76

0

6.8

63

4

Cry

stallite

stze ao

. 510

Cry

stallite

siz

e

C0

-651

7.

8.

9.

2 .03

2S

1

2.0

00

5

1. 97 i9

2

1. 9831

1. 9

74

8

3

1.9

35

3

3 1

.93

59

1.8

80

8

3 -

--

1.3

58

5

1. 8

37

7

5 1

. 8402

1.8180 12

1.8

21

3

1.7

93

4

2 1

.79

45

1.7

66

8

2 1

.76

89

1. 7463

2 1

.74

82

1.7

23

4

3 T

. 7270

--

1.6

73

9

1.4

05

1

.35

2.9

3

3.74

9.3

38

2

9.3

57

0

6.8

87

0

6.9

06

0

454 477

567 588

10

. 1

1.

5 2

.02

51

6 2

.00

09

25 1

. 9372

-1

.88

42

14 1

.85

83

29 1

.84

03

i1

15 1

.79

55

13 1

.76

73

13 T

. 7476

15 1

.72

65

7

1.3

2

4.1

8

9.3

59

3

6.8

94

2

451

588

12

.

3 5

22

13 5

28

12

12

11

12

cont~

0 '.I

T•b

le-Yll

: X

-ray diffra

ction

dat~

of p

hosp

horite

(Bu

lk

and C

onceD

trate) and

ph

osph

atic silic

eo

us

clay

s.

Htn

eral HW

A' 15 (B

ulk

) NW

A15

(Bu

lk)

SWA' 15

(Bu

lk)

swa9

(Bu

lk)

SEB15

(Bu

lk)

Phase

hk 1 d •A

Ill

d •A

Ill

d •A

1

/l d

•A

Ill d

•A

Ill m

ax Olax

max

max

max

1.

2. 3

. 4

. 5

. 6

. 7

. 8

. 9.

10. 11.

12.

Q

100 ND

NO -

4. 2 704

45 4.2663

67

A

zoo 4.0675

5 4.0823

5 4

. 0638 4

4.025 3

A

"ill -

--

3.8783 4

c 102

--

--

--

--

3.833 3

A

002 3.4583

9 3.4295

7 3.4596

8 3.4543

8 3.433

6

Q

1 0 1 3

.34

02

100

3.3313 100

3.3

54

6

100 3. 3545

100 3

.34

4

100

A

102 3.1798

5 -

3. 1~32

5 -

-3.161

4

A

210 3.0737

5 3.0532

.\ ?.0

70

6

4 3.0665

4

c 104

--

3. 0313

5 -

--

3.0228 19

A

211 2.8060

13 2.7899

8 z.sa

s3

14 2.8065

11 2. 7892

9

A

112 2.7832

5 2.7656

5 2

. 7840 7

2.7

89

9

6 2.7642

7

A

300 2.7062

7 2.6936

5 2.7073

8 2.7086

6 2.6880

6

A

202 2.6329

5 -

-2.6322

4 2.6297

4 2.6148

3

c 110

--

--

--

--

2.482 4

Q

110 2 .4464

17 2.4499

17 2

.46

29

16

2.4642 21

2.4486 4

Q

102 2.2756

16 2.2763

9 2.2874

14 2.2858

15 2.2758

15

A

310 2.2541

4

Q

111 2.2437

9 2.2336

9 2.2437

9 2.2432

10 2.2315

8

Q

200 2. 1802

11 2. 1236

13 2.1336

10 2.1332

15 2.1231

9

A

400 -

--

--

--

-2.0867

3 ..... 0 co -

con

td ..

I. z

3. 4.

5. 6

. 7.

8.

<

Q

201 1. 9885

6 1.9766

8 1. 9889

7

A

222 1.9372

3 1.9372

3 1. 9398

3

A

312 1

. 8846 2

-1. 8861

2

c 116

--

-A

213

1.8431 4

1.8396 4

Q

112 1.8198

24 1. 8005

3

A

321 1.7985

2 -

-A

410

1.7712 2

1. 71·1:..

2 1.7712

2

A

402 1

. 7498 2

-1

. 7507 2

A

004 1. 7 262

2 \

., J..GL

z 1. 72 76

3

Q

202 1.6698

8 1.6751

7

z• d

ifference

1. 14S 1. 48

1.4

0

In \004-410)

C02 Wt.S

2.2

7

1.82 3

.00

a QA ce1l

dimension

9.3792 9. 325 5

9.3778

c 0A

cell

dim

ension

6.9103

6.8633 6.9134

Cry

stallite

size

a'

412 -

42S

Cry

stallite

s\ze c

' 717

441

• •

2&

"differen

ce \n

(300)

-(0

02

). co

2 wt., calcu

lated a

fter G

ulbrandsen (1

97

0).

Cry

stallite

size

\n 'A

by

peak broadnlng m

ethod.

Cell

d\mens\on

in 'A

calculated

from

a' •

(41

0),

(31

0),

(21

0),

(300) c'•

(00

4),

(002)

Blanks

space

ind

icatin

g

peaks n

ot foun

d.

ND = N

ot an

alysed.

9. 1

0.

1.9848 10

1.9403 3

1.8879 2

1.8402 4

1.8110 31

1. 7988 2

!. 7721 2

1.7510 2

1.7270 2

1.6754 10

1.46

2. 12

9.3784

6.9195

I 1 . 1

2.

1.9770 6

1.9255 3

1.8700 4

1.8309 4

1.7825 12

1. 7760 8

1. 7159

2

1.6751 7

• 7.28

1.17

9. 350 7

6.8642

...... 0 ~

l .. .._

Y!Il =

ea.p

.rtso. o

f •1n

eralog

1c•1

ch

ara

cte

rs of lo

b•rA

~te w

ith o

ther P

reca•b

rian

Ph

osp

ho

rites of

Ind

ia

5. L

U.CAL .i: TT A

PATITE

MIN

ERAL

UM

ITCELL

DIM

ENSIO

N

CO W

t. C

RY

STALLITE

SIZE

H

OST

ROCK

••• PH

ASE

• /A"

C A

0 P

dCE

NT

a

A0

C

A0

1. C~MB'..!M

Carb

on

ate flu

ora

patite

9.3~53

6.8

68

8

1.5

3-2

.27

5

00

-60

0

55

0-8

10

$

1liceo

us

or A

n·!l"lra P

raa

esh

to

to

C

alcareou

s 9

.36

62

6

.88

16

'. I(

loAP~R

Carbonat~~

fluo

rap

atite

9

.33

0

6.8

80

up to

1 .2

5

00

-70

0

60

0-1

00

0

Si llceo

us

Ma::~ JC P

radesh w

ith

Cru

n.d

alfite to

to

9.3

56

6

.39

6

l. Jt!A

i3UA

C

arbo

nate

fluo

rap

atite

9

.34

12

6

.86

33

0

.2-1

.67

5

00

.59

0

65

0-8

00

Ca l

careo~s

:-lvdl'l:;a Prar.e~h

to

to

9.3

64

7

6.8

82

4

~

0.8

-1.8

2

64

0-6

50 6

70

-80

0

4.

JAH

AR

KO

nf, C

arbo

nate

fiuo

rap

atite

9

.36

0

6.8

56

C

alcareou

s

~UA;PUR to

to R

aj11sthan 9

.36

8

6.8

79

'. :1£~i'1U~H

MA

TA

Carb

on

ata flu

ora

patite

9

. 3 506 6

.89

75

2

.40

5

50

-65

0

50

0-7

00 Ca 1

car eou s

U:.'·: PU

R

to

to

Raj~strnn

9.3

61

0

6.8

80

9

5.

?iTHCr.A'.lP.~H C~rbonate

!luo

rap

atite

9

.34

87

6

.87

85

0

.94

-1.7

2

55

0-6

50

5

50

-72

0

Calcareo

us

Ut t.::r

Pr'"'·:lesh to

to

9.3

60

6

6.8

86

8

7 • LO~IU.

u':"G C

arbo

nate

fJuo

rap

atite

9

.32

49

6 .8

& 33

1.1

7-4

.48

4

12

-63

3

44

1-7

17

S

iliceo

us

r1aChy~ Prade~h

to

to

9.3

79

8

6.9

19

5

averag

e a vel" age

9-3

53

9

6.8

91

2

..... ~-~-

~

·-----~ ----

'.., ~.-: .. f'·r

1 ~c

;:, S

rlvasta

va

( l98

2) ·

c

111

As explicit in the Table-VII, the crystallite size for

a0 (for 300 hkl reflection) varies from 412 Jt to 510 ?, and for

c 0 (for 002 hkl reflection) from 414 ~ to 717 X in bulk

samples, and a0 517 }\, c 0 530 to 588 R in ex!":racted samples.

Although SEM microphotographs reveul 2-5 pm size of apatite

cryrstals, which implies ttnt good arnoun:: of recrystallization

has taken place (Plate-XXI). '

The infra-red spectra (Fig. 16) of bulk samples show

characteristic c-o band doublet ut 1425-1452 cm-l region with

-l P-0 band at 1025-1052 em , confirm the presence of carbonate

ion in apatite lattice, as the C-O band characteristic of

calcite also occurs at 867 cm-l region (Tab le-U<), but is

eliminated from the infra-red spectra of extracted samples

(Table-IX). The absence of 630 cm-l and 3560 cm-l region

bands rules out the possibility of presence of any hydroxyl

group within the apatite lattice (Fig. 17).

Phosphatic siliceous clays

X-ray diffractograrns O'ig. 18) and X-ray data

(Table-VIII) reveal that in phosphatic siliceous clays quartz

is dominating over apatite followed by clay minerals and

occasional calcite. The co2 content (1.17-3.0%) within these

samples is comparatively less than phosphorites, nevertheless

they arc also carbonate fluorapatites. The cell dimensions for

these pl>vspl>orites vary from a 0 ~ 9.3255 to 9.37~2 " and

112

B- 11

SEC_13.5

4000 3100 2200 1300 400

F -1 requency ern

Fig .16: Infrared spectra ot !'hosohorites{ bulk l.

~~---.J'-......... """"-""""'-·.

NEe_ 14 (Treated)

NEB- 14

NW B _ 14 (Treated l

4000 3100

Fig.17:1nfrared

2200

Frequency -1

em

1300

spectra of respective

bulk phosphorites treated samples.

113

40

and

T••l•-1

1

B•nos

c-:;

P-0

P-·~

~-c

:-.)

'-0

>

• -

~

:-0

' 1-;

1-C

c I r-. ~

ex

z ... il"'i!'~

~ t :; ,.,

-1

J•fr~-red a~~toa bands

(em

) o

f ph

osph

orites (au

lk aa4 to

ao

eatra

te) and

ph

osph

atic silic

eo

us cla

ys.

NEA10

NEB14

Bulk

::an. B

ulk C

on.

1450-1425 1450-1425

1447-1427 1447-1425

1087 1085

1087 1085

1033 1033

1033 1033

?5

5

955 953

955

860 855

860 860

595 590

595 593

560 567

560 565

867 -

867 -

757 775

767 772

687 687

687 ~85

0.8

13

0

.49

8

0.684 0.498

9. 3 315 9

. 34 06 9.3760

9.3

54

0

Ph

osph

orites

SEB9

Bulk

SEC l3 .S

Bu

lk

NWB6 B

ulk

84

Bu

lk

Carbonate

fluo

rap

atite

1447-1427 1450-1420

1450-1425 1447-1420

1037 1087

1090 '1080

1036 1033

1040 1033

950 955

950

860 860

855 853

597 595

595 596

567 560

560 560

Ca

lcite

867 867

867

Qu

artz

767 767

773 767

G87

687 687

685

-0.697

0.625 0.764

0.6

22

9.3570 9.3798

9. 3 59 3

9. 3 382

B 11

Bulk

1447-1420

1G80

1077

959

860

593

553

767

584

0.580

9.3249

012

Bulk

1450-1425

·1:30

1036

860

595

556

771

687

0.39c

9.3252

Ph

osph

atic siliceou

s clays

I SWA15

Bu

lk

1440-1415

1080

590

767

687

I 9. 3778

sws9 B

ulk

--1080

598

770

687

9.3784

(3

Bu

lk

1447-1413

1080

595

767

687

9.3255

f-.' ...... ...

20

10

30

20

1 0 -

30 -

20

10

30

20

10

SWA'_l5

0.

Q A A A A 0.

11 "~ . -'·-

NWA_15 Q

Q 0.

SWB_ 9 a

0 0

' NWA-15

0

0

Q

Q

Q

a u

a 0

0

0

0 0

Degrees 2 e

a

A A

Q

0

0

A A

..

::: g N

"' ..

c

Fig.1e: X_ ray diffractograms of phosphatic SiliceOUS clayS.( A:apalito,O.=quartz, C:calcit•)

0 0

" ,Q

A

0

!2 " 0.

A

0 S'

" a·

0

N 0 8 ..

25

11~

116

6. !3642 to 0

6.9195 A. As presen·ted in Table VIII, the

cystrallite size ran~es for a0 = 412-533 ~and for C 0 = 441-717 o Of A, which is comparat-ively smaller than apatite~phosphorites.

The infra-red spectra (Fig.

at 1423-1447 cm-l region with

19) reveal weak C-O band doublet

1080 cm-l P-0 band, this also

confirms the presence of carbonate in apatite lattice. 'I' his

weak C-O band doublet has been interpre·ted for less amount of

carbonate substitution within these apatites. The Si-0 bands

are prorainent ( Table:U<) and a broad hump like structure in

-1 3550-3600 em region, has been interpreted for the 0-H bands,

v1hich possibly accounts for hydroxyl ion in apatite lattice, or

in clays.

DISCUSSION

The phosphorites of Lohara area occur in siliceous host

rocks unlike most of the other Indian Precambrian Phosphorites,

Carbonate fluorapatite is the only apctl- i te mineral phase

present in these phosphorites as evidenced by X-ray and

infra-red analysis. The major diluent (gangue) observed is

quartz, while iron oxide, calcite, illite and kaolinite are

present in subordinate amount. X-ray diffraction patterns for

these phosphorites show sharp, well defined diffraction J.ines

suggesting well ordered lattice and hence high degree of

crystallinity.

X-t·dy di11racllon !Jattc,rns (Fi<J. 13 & 14) of bulf.

SWA- 15

sws_g

C_3

L-~-L~~L_~_L~~~~~_L~~_l_L~J-LJ-~L-~_Li_~_L~

4000 3600 3200 2800 2400 2000

-1 Frequency c m

1600

Fig.19;1nfrorecl spectra of ohasohotic s"1ticeous cloys.

1<.00 800 400

118

while in phosphatic siliceous clays only some of the major

diffraction lines of carbonate fluorap:atite (Table-VIII) are

observed alongwi th t->romincnt quartz peaks dominating over

apatite. This also reveals an abundance of quartz over

carbonate fluorapatite in the phosphatic siliceous clays.

Selected samples of phosphorites extracted with Silverman

solution show complete removal of calcite peaks both in IR and

X-ray analysis alongwith increased sharpness in the peaks.

The> co2 content as detected in bulk samples by

Gulbrandsen's method ( 1970) is hie:~ her in these lJhosphor i tes

than the extracted samples, which is possibly due to the

presence of c-o band at 867 cm-l region, characteristic of

carbonates of calcite.

Minor variation in unit cell dimensions (Table-VIII) is

observed in these apatites. These variations indicate that

-2 they were affected by the variable substitution of co3 for

-3 PO 4 , ~~hich resulted ~n the formation of carbonate fl.uorapa-

tite (McClellan and Lehr, 1969; Gulbrandsen, 1970; McClellan,

1980; Trautz, 1960).

'l'be crystallite size of these apatites were measured .fit

and calculated following the procedure of Rau (1962) and

LeGeros ct al. (1967) usin~ tl1e Scher~er equation. The

0 . 441-717 i\. Th~s ~mplics that mi.neralo were not subJcctcd Lo

rccryst,,lllZullon .:1nd ,;izc rc!JOrted here is <.1 prim.:Jry fc.Jtur,,

118

though some recrystallization is observed in SEM micrographs

(apatite crystals of 2-5 pro observed}.

Lehr et al. (1967) have reported a decrease in

crystallite size ,,lith increasing carbonate, sodium and magnesium

substitution. This decrease in crystallite size is possibly due to

-2 the concentration effects of Na, Mg and co

3 in apatite structures,

and causes inhibition of crystallization (Lehr et al., 1967). In

these apatites no significant correlation between a• and co2 is

observed (Fig. 20a), hm·1ever co2 index usually employed for

-2 knowing the extent of co3 substitution ( Lehr et al. , 1967;

Banerjee et al., 1980) reveals a crude negative relationship with

a• cell dimension (Fig. 20b). The a• cell dimension decreases with

-2 increase in co2 index and confirms that co3 has

some extent in apatites.

substituted to

In few samples (Fig. 14} peak reversal is also observed

that is 112 peak shows more intensity than 300 peak, this reversal

in peak intensity may be assigned to either presence of discrete

manganogoethite in the rock, or to the presence of small amount of

Sr, Ba or Mn substitution for Ca (Srivastava, 1982). However,

since ~n these phosphorites, content of these trace elements is

less, such a possibility is doubtful.

120

Bulk samples - •

Concenlrole -· 9 38 A •• • •

• .37

• • <( .36 • • Ill

X . 35 • 0

0

- .34 • • 0

~ ·.33 - • • • 0> • • c:: • <II . 32 -'

.31

9.30 2 3 4 5

%C02

9 .~9 B

• ·38 • <( • Ill

-37 X 0

0 -36 • -0 • .c. .3 5 ~

01 c <II

.3 4 I -' •

.3 3 • • •

9.32 30 35 40 l.S so 55 60 65 70 75 80 85

C02 Index

Fig.20:Length of a aXIS plotted against %C02 (A) and

C02 index ( sl.

121

Structural formulae of Lohara phosphorites are obtained

to knm1 the general chemical composition of these

pllOSJJhorites. 'I'he formulae are calculated usiny the complete

analysis of three bulk samples in which free carbonate is not

recoded either in X-ray or under microscope, or in I R spectra.

l. Ca9.2Na0.3Ng0.3/(P04)5 (C03)1/Fl.84

Charges : Cations + l9.72,Anion - 18.84

2. Ca9.2Na0.7Mg0.1/(P04)5 (C03)1/Fl.3

Chages : Cations = + 19.29, Anion = - 18.34

Chases : Cat ions = + 19.85, Anions = - 20.20

The above fomulac clearly reveal that these apatites

are carbonate substituted one, i.e. carbonate fluorapatite. In

the first tvlO samples higher positive charge is in conformity

to the well known fact about the loss of negative charges in

marine phosphorites of the world (Baturin, 1971; McClellan and

Lehr, 1969). This charge imbalance is however maintained if an

-3 atom ofF is added to form tetrahedral anion (C0

3F). In the

sarnple No.3, the excess of negative charge in addition to -3

formation of (C0 3F) anion, is possibly for the substitution of

-~ -3 (Si0

4) for (P0 4 ) .

CONCLUSION

Tile above x-ray and infra-red spectral studies have

led to conclude that

1. The f->hOSI,Jl!Or i tes of

sul.Jstituted variety

fluora}Jatite.

Lohara area

of apatite

arc

i.e.

122

carbonate

carbonate

2. The lenyth of a0 axis contracts \lith the increase of co2

content in apatite structure.

3. The apatite of phosphatic siliceous clays lS less

substituted fluorapatite •

4. The minute crystallite size of these apatites are

indicative of their primary precipitation.