SEC )N1)AItV ENRICHMENT OF TIN DEPOSJTS.S

BY S. C. UN

iNTRODUCTION

During the last decades, the processes of enrichment of sulpliide. ores

have been extensively studied in the geological laboratory. The work was

chiefly done on copper, silver, and gold deposits. The secondary enrichment

of tin sulphide has nOt been observed in the field; but it is believed by many

that the formation of wood-tin is due to the solution of tin ores, stannite and

cassiterite, especially the former. As stannîte and cassiterite are generally

considered as very insoluble even in strong acids, some experiments were

made to find out whether the minerals are really insoluble in dilute acidsolutions such as are found in nature. In this connection the formation ofwood-tin is discussed. These experiments were outlined by Dr. V. H.Einmons To him and to Dr. J. W. Gruner thanks are due for valuablesuggestions and assistance.

SUMMARY OF EVIDENCE SUGGESTING THE FORMATION OF

SECONDARY TIN MINERALS

1. So far only ono instance of secondary stannite seens to bavebeen meiitiooecl in literature. lt is a piece of stannite psoudomorphous after

arsenopyrit.e(J found in Cornwall.

2, Secondary wood-tin is quite abundant. Collins has described a

number of specimens from Cornwall Masses of very compact dark brown

wood-tin are found to fill the interstices between crystals of orthoclaso.(2 A

thin layer of dark l5rown wood-tin coating tho sides and summits of large

quartz crystals is also described. (3

A thesis submitted to the faculty ol the Graduate School of the University ofMinnesota in 1923. Accepted for puhication by the board of Editors of the Geolog-ical Society of China upon the recommadation of the President ('. Y. Wang.Wedding, H., Zeitschr. d. geol. Ges., vol. 13, p. 139, 186].Ooliin,, J. H., Øi some Coruish tin stones and tin-chapeh. Mineralogical Maga-zine, voL 4, p. 105, 1880.Collins, .7. II., op. cit.

flu.Uef in öf the eologicot Society of CliM

g That wood-tin may form relatively rapidly is shown by the

following instances. A long buried tin ingot was found to be coated with an

oxide. The microscopic examination of this crust showed the structure ofwood-tin,,U An irregular mass of wood-tin weighing about one and a half

pounds was found in the hearth of an old furnace.(2 There was a central

portiOn of metallic tin running lengthwise through the mass, M certainpoints this had entirely disappeared, but its former presence was shown by

the structure of the mass. '.Phe wood-tin in tnese two cases was probably

formed by the oxidation of the metallic tin.

In Penhalls and Blue Hills mines in Cornwall great anantitias

of wood-tin were found near the various intersections of the tin iodes by so

called "gossans," and also near faults. This occurrence suggests that this

minorai was deposited from solution. (3

That the deposition of tin oxide is possible in ñature from an

aqueous sohition even at ordinary temperature and normal atmospheric pres-

sure, seems to be proved by the partial replacement of iragments Of stag's

antlers by tin oxide in the tin placers of Cornwall, according to Oollins

fi. Titi oxide occurs in some mineral waters, notably in the warm

spring or Ajar Panas in Selanger on Malacca, where the siliceous sinterdeposited by the springs contains, according to St. Meunier, 5 per cent of tin

oxide. (5 Posepny(6 has also given five analyses of water from hot springs all

showing tin to he present

7. Minerals indicative of formation under 'atmospheric possure,

such as chalcedony and opal, are associated with tin oxide. Even a psou10-

morphic rep1acoment of marcasite by tin oxide is reported. (

Collin8, .1. H, op. cit. p. 111.Readden, W. P., Soim, products found in the hearth of au old furnace upon thedismantling of the Tretliellan Tin Works Truro, Cornwall, Am. Jour Soc., yo1..155, i. 9O .1898.

Cins, J. JI., op. cit, p. 109.collins, J. H., op. cit.. p. fl5.Weed, W. H,,'Translatièn oc Beck's Nahire of Ore Deposits, p. 428.Posepny, P., Gerisis ot ore deposits. Am. Inst. Min. Eng. Trans., vol. 23, p. 48.Wittlich, E., Zienerze in der Sierra von Guanajuato. Zöit. fur prakt. Geol., pp.121-]2a, 10)0.

I)

4)ö)

S. C, Lin:Secondary Enrichrnnt of Ti Drpoaite 27

8. Large .nasses of secondary cassiterito together with secondary

silver ores occurring in the oxidized zone of Bolivian tin deposits have been

reported by Singewald (1

1. in the Black Hills, South Dakota, a number of specimens O!

secondary tin minerals were found. Titus Ulkp(2 identified one snecimen

from the Etta Mine to be a copper-bearing decomposed cassiterite pseudomori

plions after stannite, or after the undecomposed brown-black cassiterite. B.c

called it "cuprocassiterite." W. P. Headden (3 describing similar specimens

from the same locality, believes thorn to be alteration products of stannite,.

The mineral is describèd as earthy in texture and from ditty green to browish

yellow in color. The mass is traversed by a fine network of sums anfi threads

of impure oide of tin. In the deeper portions, small fragments of original

stannite still remain.

OCCURRENCE OF Tfl'I

Tin is one of the rarer metals and its minerals are not numerous.Native Lin is occasionally found in trifling quantities as small grains in WestAustralia. (4 According to E. Tornebohm, (5 native tin is not uncommonly

found in Pitkaranta, Finland. The peculiar feature of occurrence in thetlitrict is that the native tin is enveloped in a layer of chalcopyrite whichla in turn surrounded by pyrite forming a sort of zonal arrangement. Tbè

tin ore of chief importance is the dioíkle, cassiterile, Sn02' but several

sul-phosalts are also known. They are

Starmite Ou 2FOSnS4

Teallito PbSnS2

Cylindrite Pbö FeSn 4Sb2 Si4

Franckeite Pb5FeSnBSb2SI4Tin boron minerals, nordenskioldine, hulsite, and paigeite, are very

rare, having been oniy found in a few places.

Sngewald, .J. T., Gnetie relations of tin deposits. Econ. Gaol vol. 7, p. 23, 1912.U1k, Thus, A new mineral iii the Black Hills: Am. Inst. Min. Eng. Trans., vol.21, pp. 240-241, 1893.Ileadden, W. P., Shennite and some of its alteration products from the Black Hills,South Dakota. Am. Jonr. Sei., vol. 145, pp 105-110, 1893.Sinpsen. E. S., Ann. Rept. GaoL Survey West Australia, ts99, p. 52.Tornotehm; A; E.. Pitkaruntamalintalt. Geulogieka Foreflingeni TrhndllngeBand 13, p. 329, Stockholm, 1891..

Bullrtin of tite (eoloqicu1 cietj of Cltina

Cassiterite has been noted as an original constituent of iguouirocks, but it more commonly occurs in VeinS or stringers of quartz nuder

conditions which indicate an opigenetic origin. As a rule tin-bearing veins

aro found in or near highly siliceous rocks, such as peginatites and altered

granites Sometimes cassiterite associates with quartz porphyry, as at Mount

Bischoff) Tasmania.(I At some localities of tin oro in Mexico, the associated

rock i rhyolite. (2 At Ohongkat Parit,(S in Perak, cassiterite oro is found in

limestone.

The typical mode of occurrence of cassiterite is in quartz veins cut-

ting granite, the va1ls of the latter rock having been altered to greisen. The

asociated minorais are of deep seated origin and include minerals containing

duortno, iron, and lithium. lu veins found at moderate or shallow depths)

cassiterite is rare, although not unknown.

\\To1..tjn is cassiterite that occurs in hotryoidal and reniform shapes,

with concentric and radiated fibrous structure. Its color is usually brownish

in varying sl ctdes which give it somewhat the appearance of dry wood (For

occurrence see elsewhere in this paper).

Stannite is found in many tin deposite in Bolivia, particularly atPotosi.( It was also known at Wheal Rock) Cornwall, and at Cam Breawhere it constituted a considerable vein, and was accompanied by pyrite,sphalrite, and other minerals. It has been found in considerable quantitiesin granite at SL Michael's Mount.(5 At Zinnwald in the Erzegebirge, itoccurs with sphalorite and galena. AL Lost River, Alaska, stannite isassociated in small quantity with galena and woiframite in a gangue of topazand fluorite. (6 It has been mined from the (onrad and King Conrad mines,

Ward, L. K., Tasmania tin deposits. BUll. (3eol. Survey Tasrnana, no. (t, 1509.Kernpon, C. W, The tin deposits of Durango. Am. Inst. Min. Eng., Trans., vel.25, p. 997, 1895.Rumbold, W R., The tin deposits of the Kiuta Valley Federated Malay States.Am. Inst. Min. Eng. Trans., vol. 87, p. S84, 19O(!lndt. À. F., The Potosi, Bolivia, silver district Aii. Inst.. Min. Eng. Trans., vol19, p. 90, 1591.Dana, E. System of mineralogy, p. 8..

t) Jnopf, 4.. Geology of the Seward Peninsula tin deposits, Alaks. L'. S. (ieol.Survey Bull. 858, p. 18, 1908.

S. C. Li::Seco ndary Enrichnnf f T'n Deposits

Howell, New South Wales,( and oecurs in notable quantity inth Conahand Silver Queen mines at Zeehan and at Heeinskirk. (2 At Zeehan it was

associated with galena, chalcopyrite, and pyrite, and t Silver Queen mine

the ore carried about 90 ounces o silver and 0.15 ounces of gold per ton.(

In Queensland it has been found in several mines in the Walsh and Tineroo

mineral fields, (4 and on Stewart's titi claim, near Watsonville, it is associated

with cassitofite, chalcopyrito, arsenopyrite, pyrite, wolframite, chlorite, and

hydrous iron eide.

CHEMISTRY OF TIN

Tin is one of the more stable metals towards air and water atordinary temperature. In the potential series it stands between cadmiumand Icati. It forms two series of compounds, in which it s respectively

divalent and tetravalent.

Salts of stannous series can not be readily obtained when dissolving

tin in dilute acids. The most easily formed is the chloride, SnCl2' which ís

almost the only fairly well known st.annous salt. Stannous chloride solution

is unstable, on keeping it at room temperature black stannous oxide, SnQthe anhydride of stannous hydroxide, is deposited. The siannous salts readily

pass into stannic compounds, and are therefore strong reducing agents. A

very dilute solution of stannous chloride would be completely oxidized within

30 minutes.

Stannic salts -hydrolize completely in water. In an aqueous solution

of tannic chloride, stannic hydroxide slowly separates out. The separation

is hastened buy saturating the. solution with a base. (5 A gelatinous precipi-.

tate, Sn (OFT) 4' is formed. The hydroxide undergoes decomposition, being

converted slowly into another less soluble form.

J vd.rews, E. (J., The geology of the New England Plateau, with special reference tothe granite of northern New England: New South Walea (4cc). Purvey Rae., vol.8, 1905, pt. S, p. 146.JiarweU, Uondcr, The occorrence of t:miiite in Anetrahia catrdiai, Min. Stanl-ard, vol. 40 p. .577, 1908.Petfered, H'. E., Catalogue of the minerals of T,nnania. Tasmanian Dept. Mines, .

67, Hobert, 1910.IJunstan, B., Queensland Mineral Index and (3uide. tueens1an4 (4eoI. SflrVe Feb.241, p. 911, Brisbane, 1913.

b) Piely, A., Translation of Ostwald' Prineiples of Inorgenic Chcm!try, p..786.

2GO BiUein of fh4 Gtcioqía1 Society of Chnct

"Neither chloride nor sulphate of staunous or stannic tin is stablen

óxygenated solutions such as he mineral waters of upper zones in suiphide

ores.'(lTin oxide, cassiterito Sn02, is found in nature and is so resistant to

solution that il is rarely much corroded. "In some of the Oonish tincopper

veins the copper and iron suiphides are leached out from the upper parts of

the veins and only tin remains, showing that in sulphate solutions tin, oxide

is niore stable than copper sulphîdes."(2

Neither simple stannous nor stannic suiphide occurs in nature.Staunous suiphido is obtained as a dark.brown precipitate when hydrogen

suiphide is led into an acid solution of stannous salt. Stannic suiphide is

formed likewise by precipitation, and is yellow in coLor. Stannic sulphjde

loses sulphur when strongly heated, and leaves stannous suiphide. It is notmuch affected by dilute acids, but interacts with alkali sulpWdes. such as

ammonium snlphide, giving a soluble complex sulphide. namely ammonium

suiphostannate:SflS2+(NH4)2S(NH4I 2SnS

Stnnons 'sulphide is not affected by semblé suiphides, but polysuiphides,

such as yellow ammonium suiphide, give with it the above mentioned sulpho

stannate.SnS+ (NH 4)252 (NH4 ) 2SflS3

The suiphostannates are unstable in acid solutions. "H ferrous iron replaces

the alkali and a molecule of chalcorit.e is added, the formula becomes that of

sfaimite, Ou 2FeSnS4. The cheruical similarity of stannite to snipharsenates

and suiphantimonates, which are commonly regarded as secondary, suggests

that the prócipitation of secondary staunite is possible, but few examples of

secondary stannite are recoMed (3

SOLUBILITY OF CASSITERITE AND STANNITE

C. Doelter(4 bas shown experimentally that cassiterite is prceptib.

ly soluble in water. He states that at 800 ci. in 22 days, was lost 0023G gram

from 0.7084 gram of tin oxide. But his reEults are questioned by J. l .Goldsberry,

I) Emmons, W. H., The enrichment of oro dopóith: LI. S. Genl. Survey Bull. 625, p399, 1917.

2) Ern,nornq, W. Ji., op. cit., ¡. .;1) Emmons, W. H., op. cit., p. 400.4) Emnon, W. ¡L, op. cit., p. 309.

S. C. Lin:&condary Enrichmnt of Tin Deùosits 261

who claimed that no trace of tin oxide was dissolved in distilled water after

several hours boiling. (1 J H. Collins, (2 in testing the solubility of cassiterite,

dissolved half a gram each of five different samples of cassiterite and wood-

tin in dilute 2804 (one part acid to five parts water) with two grams of

pure zinc. His analyses show that considerable tin was dissolved. The

solution of tin in this experiment was caused by the nasceat hydrogen evolved

in the dissolving of the zinc. According to Goldsberry, (3 eassiterite is only

very slightly soluble even in concentrated 112 SO4, and a faint trace of tin

was detected by him in N/lO 112804 solution with stannite.

In order to determine the solubility of cassiterite and stannito inwaters such as are présent in the oxidizing zone of ore deposits, the following

experiments were made. Samples of stannite and cassiterite were crushed in

an agate mortar and impurities wore carefully picked out. The samples

were then pulverized to pass. through the 100-mesh screen. The stannite was

from East Pool Mine, Cornwall, England, one specimen (No. 1) containing

pyrite, chalcopyrite, and woiframite, the other (No. 2) containing pyrite,chalcopyrite, and arsenopyrite as impurities. Tbe cassiterite was fromEhrenfriedersdorf, Saxony, having apatito and fluorite for the gangue. Five-

gram portions of the samples were put in pyrex flasks (volume 250 cc.) in

contact with the following solutions:

5 grams of cassitorite in

50cc. N/20 H2SO425cc. N/20 112504 + 26cc. N/20 Fe2 (SO4) 3*25 cc. N/20 H 2504 + 25 cc. N/20 FoSO 4 **

50 cc. N/20 HOI

<5) 25cc. N/20 1101 + 25cc. N/20 H2SO425cc. N/201101 + 25cc.N/2OFe2 (SO4)a'i'25 cc. N/20 HOi + 25 cc. N/20 FöSO4

50 cc. N/20 Na 2003

Ern»ion, W. IL, op. cit., p. 399.col1in.. J. H., On solubility of cessiterite: Inst. Min. and Met. Trans, vol. 13, p. 486.Einrnon, W. H., op. cit.

C Fe2 (804)3 oIution was made by dissolving 2.5 granis of Fe, (SO4)3 in 1 liter ofN/20 H,SO4FeSO4 sólution Was made by dissolving 3.8 granis of Fe804 in I liter of N/20H5s04

Bu11(in of the Goiogicai Socicty (f China

5 grains of 4anuitet in

(9) 50cc. N/20 H2SO4(l0) 25 cc. N/20 H 2SO4 + 25 cc. N/20 (SO4) 3 *

(II) 25 cc. N/20 H2SO4 + 25 cc. N'2() FOSO '°(t2) 0 cc. N/20 1101

25 cc. N/20 11J1 +25 cc. N/20 112304

25cc. N/20 HOi + 25cc. N/20 Fe2 (SO4) t°25 cc. N/20 1101 + 25cc. N/20 FeSO4**50 cc. N/20 Na 2003

Since tin deposits are almost always accompanied by fluorine

minerals, a pinch of powdered fluorite was added te each flask. It was

thought that any fluorite ions that might possibly form temporarily would

influence the solubility of the tin minerals.

To the following solutions no fluorite was added:

5 grams of cassiterite in

50cc. N/20 11250450 cc. N/20 1101

5 grams of stannito (Spec. No. t) in

50 cc. N/20 H2SO450 cc. N/20 HOI

The flasks were corked and shaken once a day. After a period of

two months, 20 cc. of solution was taken from each flask and tested for tin.

Standard qualitative methods were followed for separating tin from the other

metals. Metallic zinc was then added to the hot hydrochloric acid solution

to reduce any tin present. Ammonium mol'bdate was added immediately

after all zinc had been dissolved. A stannous chloride solution is colored blue

when ammonium mnolybdate is addd. This test will detect one part of tin

in a million and a half of solution.( The amount of tin in the solution was

estimated by colorimetric comparison with a standard tin solution. On

account of the very minute amount of tin in solution, it is highly probable

Spciiiicn No. 1 wa ned iii Nos. 9, 10, 11, 12; pecirneri No. 2 wa need in No..13, 14, 15, 10.

1) 1.ongsOcj Ciiei. Newn, vol. 80, p. 282, 1889. Rogers, Jour. Chem. Soc., vol. 22p. 220, 1900.

S. C. Lin:Secondary Enrichrneng of Tin Deposits 263

that any otiier quantitative determination of the metal would not have given

more accurate results. Tin u/as found to be present in all the solutions with

stannite, except 2003. No tin was detected in the solutions containing

cassitorite. The accompanying table shows the results.

TABLE

Showing solubility of stannite in dilute acid and carbonate solutions

after two months.

The experiments show that the solution of stannite and transporta.

tion of tin in ground water is not only possible but probable. The liydro-

chloric acid solution seems to be most effective in attacking the stanuito. As

suiphates of tin hydrolize more easily than chlorides, a sulphuric acid solution

would be expected to carry less tin. The difference in the soluhility between

stannous chloride and stannous sulphate may have also caused the difference

in the results. (At 19° C. 1 liter of water dissolved 188 grams stannoussulphate; at 15° C. 1 liter of water dissolved 2,698 grams stannouschloride.) (1

It is also seen that there is more tin in solution No. 12 than inNo. 20. The difference may be due to the influence of the fluorite. Between

the ferne and ferrous sulphates, the ferric seems to have dissolved more tin,

because it is a stronger solvant than the ferrous. Sodium carbonate has no

effect on the stannite.

1) EnnwÑ, II., Op. (it., p. *IS.

No. Solution Miera1s used

9 50 cc. N,'20 H8OI Staniiite (spec. 1) + fluorite 5

lo

il

12

25ce.N/20HSO4+25.N,20Fe2(SO4) ,, ,, ,, ,, 8

25cc.N/2OI-TSO4 +25cc..N/2OFeSO4 ,, H 2

2050cc. N, 20 HOI ,, ,, ,,

13 25 cc. N/20 HO! + 25 cc. N720 HSO4 ,, (spec. 2) ,, 15

ii 25cc. N/20 1101+25 cc. N/20 Fe, SOy ,, ,, ,, ,, 2

15 25 cc. N/20 HO! + 25cc. N, 20 Fe504 ,, ,, ,, F:int tr,tcc

10 50 cc. N/20 Na2CO, H

19 50cc. H2SO4 (spec. I) ,, $

20 50 cc. HOI ,, » , it)

2E4 Rulltir. of (he ologca Society of Chi.

THE FORMATION OF WOOD-TIN

As mentioned above, secondary tin suiphides are practically absent,

though their formation is possible. Duc to the readiness with which stannic

tin hydrolizos in dilute solutiOns, tin can not be carried very far. The result

of the hydrolysis of sannic tin is the formation of stannic hydroxide which is

held in colloidal suspension. Precipitation of the colloid leads to the forma-

tion of woop-tin.

The occurrence of wood-tin indicates that the dioxide has beendeposited from solution upon substances whose shape it often assumed. Thus

we find it as incrustation in plate-like masses which formed upon quartz or

feldspar; or in reniform or botryoldal aggregates; frequently in perfectstalactities which are often hollow. Radiating structures are very common

Granular and compact masses have also been observed. J. H. CollinsU ju

examining the wood-tin microscopically, said, "It seems to be formed by the

rèsolution and crystallization of the original impure tin deposit, and ofmaterials dissolved from the surrounding rock and vein substance;" and"it seems likely enough that the original deposit was a gelatinous mass of

mingled oxides of tin and iron." It seems reasonable that the wood-tin was

originally deposited as a gol, lwcauso the alkalinity iii the lower zoné of ore

doposits will throw the stannic hydroxide down as a gelatinous precipitate. (2

This hydroxide would gradually ho turned into dioxide which is moro stable.

The enormous masses of wood-tiu(3 occurring with secondary silver

ores at Potosi, Bolivia, also suggests the secondary enrichment of tin,

deposited as oxide instead of sulphkle. The source of the tin is probably the

stannite which is fóund in great quantity in that district. Another great

wood-tin district is the Cornwall where stannito deposits constituted consider-

able veins. At Lost River, Alaska, stannite is also reported to be present in

the lotie deposits, and the wood-tin occurring in the alluvium there is prob-

ably genetically related to the stannite. From l5C lbs. of graval4 3+ lbs. of

Gollins, J. H., Additional notes on wood-tin: Mineralogical Magazine, vol. 16,p. 30, 1911.Findly's. Alex., op. cit.&ngewald, J. T., op. cit.Mackneni, .1. M., The occurrence of gold in Great En him and Ireland: Trans. luit.Min. Eng., vol. 25, p. 49t, 1903, London.

I) Iiga1is, JI. IL, The Tin de1wiit of Durniigo, Mexico: Trae. Am. Insi Min. Eng.vol. 25, p. 146163, 1895.

'2) Playfm'd, E. D, Tin ii At tr&in: Eng. Min. Journ,, vol. 87, p. 272, 1909.

S. C. Lin :Secondary Enrichment of Tin Deposits 2'5

wood-tiii were gathered in Goidmine River, Wicklow, Ireland. In thiscounty stannite is also known to occur. The placer wood-tin deposits in

Durango, Mexico, are undoubtedly supplied from the veins. (1 The source of

the tin is probably from the country rock, the analyses of which show it to be

stanniferrous. Wood-tin is also found in the alluvial deposit near PineCreek, Northern Territory, Australia.(2 Stannite may be expected to befound in the district as it is so often found with other tin deposits iiiAustralia.

CONCLUSION

Field evidence seems to show that no secondary tin sulphides due to

doscendmg waters occur. On the other hand, wood-tin often suggests theorigin of descending water depositioii, and most probably the tin in it isderived from staunite. The experiments performed show that the stannite is

disgolved in dilute acid solutions. Tin carried in such solutions is probably

in the stannic form, partly as colloidal hydroxide. The colloid is thrown out

of solution in favorable ilee, tuni later changes In t3 anhydrous form,

SiiO'