3.5.1
Occurrence and geochemistry
Lithiophilite-triphylite occurrences are minor in the four areas studied, in comparison with the four major lithium minerals, and only partial analyses of the series have been analysed in order to relate the Mn/ (Mn+Fe) ratio of lithiophilite generally to other Mn-minerals in each specific area. The series and their alteration products from the Karibib area have been extensively studied by and Keller and von Knorring (1989) and Keller (1991). The Li-Mn-Fe phosphates described in this study, have been discussed before lithian mica, and after amblygonite in the lithium sequence, owing to their similarlity with amblygonite, both in composition, and in the crystallization sequence with regard to late fluids.
t
Members of the lithiophilite-triphylite series Li(MnFe)PC>4, are commonly observed in lithium pegmatites generally as minor concentrations. They are usually confined to the quartz core forming coarse-grained nodules (rounded and irregular aggregates) up to 1 metre in size where they occur in the Okatjimukuju pegmatites, Karibib. More details of the Okatjimukuju occurrence are given in Keller and von Knorring (1989) and Keller (1991).
Lithiophilite nodules occur in quartz with alteration phosphates at the White City pegmatite, Tantalite Valley, Namibia. In Tantalite Valley, lithiophilite and Mn-Fe phosphates are only present to any degree in the White City pegmatite, with minor amounts occurring in the Homestead pegmatite. At the White City pegmatite lithiophilite contains 8.24% Li2O with 0.40% Na2O and MnO 38% with 6.76% FeO, the same mineral from Homestead has 8.03% Li2O and 0.71% Na2O. The Homestead lithiophilite contains a high percentage of MnO and with only 0.36% FeO and 0.23% Fe2O3(Table 3.19,anal. 158) it represents the lithiophilite end-member.
Lithiophilite at the Homestead pegmatite occurs in a matrix of dark purple lithium mica with cleavelandite, the lithiophilite end-member is very rare and with an Mn/Mn+Fe ratio approaching 1, it represents an Fe-poor, Mn-rich, i.e. an extremelly fractionated environment (von Knorring, 1976). Other phosphate minerals analysed from Tantalite Valley include triplite [(Mn,Fe,Mg,Ca)2PO4(F,OH)] (Table 3.19 anal. 159) from the White City which is manganese-rich with 47.82 % MnO and 11.79% FeO. The alteration products of lithiophilite have, however, not been examined in detail, from this pegmatite area.
Lithiophilite is subject to late metasomatic alteration to a large number of secondary rare and complex phases. Lithiophilite from Noumas 1, Namaqualand (Table 3.19. anal. 160) occurs with triploidite, (Mn,Fe2+)2(PO4)(OH), strengite, phosphosiderite [Fe3+PO4.2H2O], and stewartite [MnFe23+(OH.PC>4)2.H2O as secondary replacement or alteration products, and with apatite, all of which occur in the quartz core (Baldwin, 1979).
TABLE 3.19
Partial analyses of lithiophilite and triplite from pegmatite
156. 157. 158. 159. 160. P2O5 44.82 43.02 43.91 31.65 Fe2C>3 0.23 FeO 13.44 6.76 .36 11.79 MnO 31.55 38.00 44.60 47.82 31.84 CaO 0.02 0.03 - 1.73 MgO 0.03 - 0.56 Li20 8.24 8.03 Na20 0.40 0.71 k2o 0.16 0.06 0.00 F 0.04 0.06 8.00 H2O+ 1.12 H2O- 0.16 Total 96.68 99.24 101.55 -O=F 0.02 0.03 3.36 96.66 99.21 98.19 Mn/(Mn+Fe) 0.70 0.85 0.80
156. Lithiophilite, Rubicon, Karibib, Namibia. (156,157,159, analysts: R.Davies and J.RJBaldwin) 157. Lithiophilite, White City, Tantalite Valley, Namibia
158. Lithiophilite, Homestead, Tantalite Valley, Namibia (von Knorring, 1976) 159. Triplite (WC4), White City, Tantalite Valley, Namibia
From the Kenhardt-Marydale district, lithiophilite and alteration products metastrengite (Fe3+PO4.2H20) and heterosite (Fe3+PO4) have been identified from the Straussheim 1 pegmatite (von Knorring, 1985).
In the Karibib area, lithiophilite containing 31.55% MnO and 13.44% FeO occurs at the Rubicon pegmatite. Lithiophilite from Noumas, Namaqualand with 31.84% MnO is very similar in Mn content to the Rubicon lithiophilite and consequently the Mn/Mn+Fe ratio in lithiophilite is lower in both the former pegmatites with 0.70 at Rubicon in comparison with the Tantalite Valley pegmatites where the Mn/Mn+Fe ratio in lithiophilite is 0.85 at White City, and practically 1 at Homestead, Tantalite Valley (Table 3.19). It will be shown that the Mn/(Mn+Fe) ratios for lithiophilite are compatible with the same ratios for tantalite (for each field area).
Lithiophilite may also occur in cleavelandite assemblages and London and Burt (1982c) suggest that the mineral displays two different replacement sequences depending on whether the lithiophilite is embedded in quartz or albite. The least alteration occurs in quartz. At the Rubicon mine, Okongava Ost 72, Karibib, von Knorring (1985) has observed the following secondary minerals: heterosite, sicklerite, hureaulite Mns(PO4)2 (PO3(OH)]2.4H20), tavorite, barbosalite, frondelite, strengite, phosphosiderite, bermanite and stewartite. For further details , the reader is referred to Keller and von Knorring (1989). In the pegmatites on Okatjimukuju 55, Karibib, numerous phosphates mostly of secondary origin have also been reported by von Knorring (1985a). The secondary minerals so far identified in the Rubicon pegmatite do not contain Ca or Na , but Ca is represented in mitridatite [Ca3Fe43+(PO4)4(OH)6.3H2O], collinsite [Ca2(Mg,Fe)(PO4)2-2H2O] and apatite, and Na in eosphorite [NaLiAl2 (Al2Si2)Oio(OH)2l in the pegmatites on Okatjimukuju 55.
In the Karibib area replacement of lithiophilite to form hureaulite and triploidite involved hydration and removal of lithium. Replacement phosphates containing Ca is supportive evidence for postmagmatic fluids rich in Ca involving Ca-for-Mn cation exchange comparable with montebrasite alteration to crandallite, and similarly eosphorite containing Na is supportive evidence for Na metasomatism, which has already been reported from the Karibib area. Ca metasomatism is evident in Tantalite Valley in the formation of triplite. In Namaqualand, at Noumas, replacement of lithiophilite by triploidite, and stewartite involved hydration and removal of lithium, replacement by strengite and phosphosiderite involved hydration and the removal of Li and Mn; replacement by apatite, if it is secondary, involved Ca metasomatism (see Fig. 3.8). However, London and Burt (1982c)report that stewartite, and strengite may be the result of weathering rather than metasomatism in the White Picacho District pegmatites, Arizona, as these secondary phases are not present on any freshly mined samples.