RECENT studies undertaken in this Institute on the crystal structure

Transcription

1 522 Structural similarities f sme secndary lead minerals By L. FANFANI and P. F. ZANAZZI Istitut di Mineralgia dell 'Universita di Perugia, Perugia (Italy) [Taken as read 2 Nvember 1967] Summary. Sme structural analgies fund in tsumebite, vauquelinite, frnacite, and brackebuschite make it pssible t place these secndary lead minerals in the same series with general frmula Pb2MeU(Z)(XO.)( TO.), where MeIl is a bivalent catin as CuH r }InH, Z is the hydrxyl grup r the water mlecule, X is S, Cr, r V, and Y is P, As, r V. Sme erystallchemical cnsideratins are develped in rder t explain hw the replacement f these elements affects the crystal structure. in these substances. RECENT studies undertaken in this Institute n the crystal structure f frnacite and vauquelinite shw that clse similarities exist between these and tw ther lead minerals: tsumebite and brackebuschite. The results allw us t place these minerals in the same series, accrding t their structural relatinships, and, thugh their structures are quite cmplicated, t develp sme crystallchemical cnsideratins cncerning the effects that replacements f ins f differing sizes may cause in these structural framewrks. The structure f brackebuschite, Pb2Mn(V04)2H20, was determined in 1955 (Dnaldsn and Barnes). The unit-cell parameters are a = 7,68 A, b = 6.15 A, c = 8,88 A, f3 = III 50' and the space grup. 2. rs P2r/m = C2h. In 1966 Nichls determined the structure f tsumebite, Pb2Cu [P04S040H], which has the space grup P2r/m = C~h and parameters a = 8'70A, b = 5.80 A, c = 7.85 A, f3 = '. Bideaux, Nichls, and Williams (1966) described a new mineral, analgus t tsumebite, with frmula Pb2Cu[As04S040H], the same space grup as tsumebite and parameters a = 8.85 A, b = 5.92 A, c = 7.84 A, f3 = '. Ccc, Fanfani, and Zanazzi (1967) determined the crystal structure f frnacite, Pb2Cu[ As04Cr040H]. Its parameters are a = 8.10 A, b = 5,90 A, c = A, f3 = 1l0 00' with space grup P2r/c = C~h' Finally in' 1967 Fanfani and Zanazzi carried ut the structure f vauquelinite, Pb2Cu[P04Cr040H] ;)ts space grup is P2r/n = C~h and

2 STRUCTURES OF LEAD MINERALS 523 its lattice cnstants are a = A, b = 5.81 A, c = 9,56 A, f3 = 94 33'. Ntwithstanding the differences in space grups and the apparent diversity in the parameters, the cells f these minerals shw remarkable analgies bth in symmetry and in dimensins, if suitable rientatins are chsen. In rder t display the similarities f cell parameters, the crystallgraphic cnstants f the five minerals are set ut tgether with their space grups in table 1. TABLE I. Chemical frmulae, parameters and space grups f the five minerals* Tsumebite 'As-tsumebite' Vauquelinite Frnacite Brackebuschite * Pb,Cu[PO.SO,OHJ Pb,Cu[AsO,SO,OHJ Pb,Cu[PO,CrO, HJ Pb,Cu[AsO,CrO,OHJ Pb,Mn(VO,).H,O a 7.85 A 7,84 2 x , b 5.80A , ,70 A x 8,68 2 x ,88 f ' ' ' ' ' P21!m P21!m P21!n P21!c P21!m With the name 'As-tsumebite' we indicate the analgue f tsumebite where arsenic takes the place f phsphrus. In tsumebite and 'As-tsumebite' the parameters a and care inverted in respect f thse assumed by Nichls. The parameters f vauquelinite are thse when the directins [101 J and [loij are chsen as a and c axes respectively. A shrt utline f the crystal structure f tsumebite, which will be chsen as reference member in this series, will better explain the structural relatinships ccurring between the five minerals. The crystal structure f tsumebite is shwn in fig. 1. All the heavy atms ccupy special psitins in the space grup C~h:Cu2+ins are at inversin centres; the tw lead ins, phsphrus, and sulphur f the asymmetric unit lie n planes at y =! and!. Phsphrus and sulphur shw the usual tetrahedral crdinatin, with tw xygen atms necessarily n the mirrr plane and tw xygen atms ne abve and ne belw the same plane. Each Cu2+ in jins tw hydrxyls at distances f 1.88 A, tw xygen atms f the phsphric anin at 2.03 A and tw xygen atms f an 804 tetrahedrn at a larger distance f 2.37 A. The tw independent lead ins crdinate eight and ten xygen atms respectively, with bnd lengths in a range frm 2.43 A t 3.11 A, frming very irregular plyhedra. In the ther minerals the atmic array is very similar t that ne f tsumebite. In table II the atmic crdinates f vauquelinite, frnacite, and brackebuschite are cmpared with thse fund in tsumebite, after a cnvenient rientatin f the cell and a shift f the rigin. Table II cnfirms the structural analgies and therefre it is justifiable t

3 524 L..FANFANI AND P. ~F. ZANAZZI ON cllect these lead minerals int a series with the general frmula Pb2M en (Z)(X04)( Y04), where Men is a divalent catin, such as Cu2+ r Mn2+, Z is the hydrxyl grup r the water mlecule, X is S, Cr, r V, and Y is P, As, r V. a s 0 po cuo PbO FIG. 1. The crystal structure f tsumebite prjected alng the b axis. It is nw interesting t see hw the substitutin f P by As r V and f S by Cr r V affects the crystal structure. The replacement f phsphrus in tsumebite by arsenic des nt change the symmetry f the crystal structure but it causes an increase in band c, withut any change in a. Analgus behaviur ccurs n passing frm vauquelinite t frnacite. A structural interpretatin f these experimental data can be fund as fllws: each P04 tetrahedrn is cnnected t the chains f Cu crdinatin ctahedra alng the b axis thrugh tw xygen atms, frming the edge f the tetrahedrn parallel t b, s that ne f these xygen atms is tied by a Cu2+ in and the

4 ther ne by anther adjacent Cu2+ in, as sketched in fig. 2. Therefre it is plain that any increment f the length f this edge is reflected in the length f b. Nevertheless the actual increase f the b parameter is less than wuld be expected if the psitins f the xygen atms arund Cu remained unaltered, because it wuld imply t large a distrtin in the TABLE II. Crdin3tes f heavy atms in tsumebite, vauquelinite, frnacite, and brackebuschite. The unit cells f vauquelinite, frnacite, and braekebusehite are suitably riented (see table 1) and shifted fr better cmparisn with tsumebite. Crdinates cited as 0,!, t, r! are in special psitins. Atm Tsurnebite Vauqllelinite Frnacite Brackebllschite x 0, ,740 Pb, y 4; I ; I Z I 0' " x ' Pb2 y ; I " z 0,393 0, x }!e~I y 0 0 0,008 0 z x 0 0-0'014 0 JJ1e~I y I I I z " 0 0" -0,003 " 0 x X y 3 0, " ' " x ' r y 3 0'72H 0,740 3 z " '338 0,337 " 0.329,}! e" is :\fnh fr brackebusehite and CuH in the ther minerals. X is S in tsumebite, Cr in vauquelinite and frnacite, and V in brackebusehite. r is P in ts:.enebite and vauquelinite, As in frnacite, and V in brackebuschite. Cu crdinatin pseud-ctahedrn. In the change frm the phsphrus t the arsenical member there must be a slight change in angles and distances in the planar crdinatin arund Cu and the resulting increase in b is nly 0.1 A. The different behaviur f the a and c parameters in the passage frm the phsphrus t the arsenical member can be explained thus: the larger size f As04 cmplexes des nt affect in the same way the bnds between the Pb and Cu ins and the xygen atms f these cmplexes, the effect depending n the reciprcal rientatin f As-O and Cu-O r Pb-O bnds. When, fr certain bnds, the effect is large a prprtinate

5 526 L. FANFANI AND P. F. ZANAZZI ON defrmatin f the crdinatin plyhedra arund Pb and Cu in angles r in bnd values cannt take place; nly an increase f the cell parameters related t the rientatin f these bnds can yield a defrmatin small enugh t maintain the general features f the structure. This happens when the lnger As-O bnds shrten Cu-O bnds 1 2' 1 4' ' 1 4 ~C FIG. 2. A schematic view f the chain frmed by Cu crdinatin plyhedra and PO. tetrahedra in the directin f the b axis. Fr key t the several atms see fig. 1. and cause an excessive distrtin in the planar crdinatin arund Cu. Because f the directin f these Cu-O bnds parallel t the c axis, as shwn in fig. 2, a significant increase f this parameter ccurs. The lead crdinatin plyhedra are generally mre defrmable than the Cu plyhedra, the range f Pb-O distances and O-Pb-O angles being very large; it fllws that an increase in the parameters can happen nly if very shrt Pb-O bnds, unable t be further shrtened, ccur in a directin apprximately parallel t these parameters. In tsumebite and in vauquelinite the nly bnd near t a critical value is Pb(1)-O(3), with a distance f 2.43 A and 2.40 A respectively in the tw minerals and a

6 STRUCTURES OF LEAD MINERALS 527 directin apprximately parallel t the c axis; this again is in accrd with the increase f c. In the a directin, n the cntrary, the bserved Pb-Op distances are quite regular and s the a parameter des nt change. The replacement sulphur-chrmium requires mre ntable structural mdificatins than the replacement phsphrus-arsenic. In fact, as is shwn in table I, in the passage frm tsumebite t vauquelinite besides the increase f the a parameter there is a change in the space grup. Since the difference in size f Cr04 and 804 tetrahedra is much greater than that between AS04 and P04 tetrahedra, strict maintenance f the structure is impssible. A general rearrangement f the atmic array takes place: Pb ins mve slightly alng the b axis frm y = t and! tgether with chrmium and phsphrus atms, whse crdinatin tetrahedra rtate thrugh a small angle alng an axis almst parallel t c, while Cu ins remain in special psitins at inversin centres but with a very distrted crdinatin arund them. In vauquelinite, referring t the symmetry elements f the space grup P21!m t which tsumebite belngs, the mirrr planes and alternate inversin centres and screw axes disappear as shwn in fig. 3. The resulting space grup is P21!n thugh the atmic arrangement shws the presence f a pseudcell with dimensins and symmetry crrespnding t thse f the unit cell f tsumebite. The mdificatin f the parameters in cnsequence f the larger size f Cr04 tetrahedra can again be related t the lengths and directins f the bnds that Cu and Pb frm with the xygen atms f the tetrahedrn arund the chrmium. The directin f the Cu-OCr bnds parallel t a requires the lengthening f this parameter. A similar effect arises frm the rtatin f the chrmium tetrahedrn, which wuld cause an excessive shrtening f the bnds Pb-OCr apprximately arranged in the a directin. By cntrast, the c parameter des nt change because f the greater lengths and cnsequently greater pssibility f variatin f Pb-OCr bnds in its directin. The almst unchanged value f the b parameter is explained when we nte that the remval f lead, chrmium, and phsphrus ins frm y = t and!, cnsequent n the lack f mirrr planes, allws a mre cmpact arrangement alng this directin in vauquelinite. The atmic array in frnacite, which represents in this series the member cntaining chrmium and arsenic, may nce mre be interpreted by referring t tsumebite, taking int accunt hwever that the replacement As-P affects a struc ural pattern already distrted by the

7 52R L. FANFANI AND P. F. ZANAZZI ON replacement Cr-S. Since the larger size f the As04 tetrahedrn mstly affects the crdinatin arund the cpper in, as happens frm tsumebite t its arsenic analgue, we are led t think that the planar crdinatin arund Cu, very irregular in vauquelinite, needs a rearrangement fr the new replacement. It happens therefre that Cu ins leave inversin centres and rearrange themselves arund screw axes. There "" "" "" '" a b c :FIG.3. The array f the symmetry elements (a) in tsumebite, (b) in vauquelinite, and (c) in frnacite. fllws a slight structural mdificatin alng As-Cu-As directins, which explains hw in frnacite and in vauquelinite, thugh the space grup in bth is C~h' the symmetry elements are differently arrayed in these directins, as is shwn in fig. 3. Thugh at first sight imprbable, it can easily be shwn that the transpsitin f centres and screw axes des nt require any remarkable structural changes, if we cnsider that in bth minerals the same pseud cells, crrespnding t the cell f tsumebite, are present, and that therefre in bth structures all the symmetry elements f the space grup P21lm = C~h must cexist partially as true and partially as pseud-elements. The remarkable result in the rearrangement is the regular planar crdinatin arund Cu, very similar in bnd lengths and angles t that ccurring in tsumebite; hwever the displacement f cpper ins frm the special psitins at inversin centres causes the pseudcell crrespnding t tsumebite t be less well defined in frnacite than in vauquelinite. Brackeb]Jschite can be cnsidered as a particular member in this series; it differs cnsiderably in its cell parameters, a being much shrter and b much lnger than in the ther minerals, and in its chemical cm-

8 STRUCTURES OF LEAD MINERALS 529 psitin, as manganese takes the place f cpper and vanadium f sulphur and phsphrus in tsumebite; the hydrxyl grup is evidently replaced by a water mlecule. The structure f brackebuschite is clearly related t that f the minerals already described but it is nt well enugh knwn t allw any particular cnsideratin; there is dubt, t, abut its space-grup, which may be P21!m r the nn-centrsymmetrical P2I = C~. In this cnnexin it seems unlikely that this mineral belngs t the space-grup P21!m because the larger size f V04 tetrahedra leads us t expect a cnsiderable distrtin f the atmic array f tsumebite and cnsequently a lwer symmetry. Acknwledgement. The authrs wish t express their thanks t Dr. M. C. Nichls fr his private cmmunicatin n crystal structure f tsumebite befre publicatin. References BIDEAUX (R. A.), NICHOLS (M. C.), and WILLIAMS (8. A.), Amer. Min., vl. 51, p Ccc (G.), F ANFANI (L.), and ZANAZZI (P. F.), Zeits. Krist., vl. 123, p DONALDSON (D. M.) and BARNES (W. H.), Amer. Min., vl. 40, p FANFANI (L.) and ZANAZZI (P. F.), Zeits. Krist., in press. NICHOLS (M. C.), Amer. Min., vl. 51, p [Manuscript received 3 March 1967] Mm