Contrib Mineral Petrol (2015) 169:19 DOI 10.1007/s00410-015-1107-x ORIGINAL PAPER Topotaxial reactions during the genesis of oriented rutile/hematite intergrowths from Mwinilunga (Zambia) Aleksander Recˇnik · Nadežda Stankovic´ · Nina Daneu Received: 25 July 2014 / Accepted: 7 January 2015 / Published online: 6 February 2015 © The Author(s) 2015. This article is published with open access at Springerlink.com Abstract Oriented rutile/hematite intergrowths from samples. Using a HRTEM and high-angle annular dark-field Mwinilunga in Zambia were investigated by electron micros- scanning TEM methods combined with energy-dispersive copy methods in order to resolve the complex sequence of X-ray spectroscopy, we identified remnants of ilmenite lamel- topotaxial reactions. The specimens are composed of up lae in the vicinity of rutile exsolutions, which were an impor- to several-centimeter-large euhedral hematite crystals cov- tant indication of the high-T formation of the primary fer- ered by epitaxially grown reticulated rutile networks. Fol- rian–ilmenite crystals. Another type of exsolution process was lowing a top-down analytical approach, the samples were observed in rutile crystals, where hematite precipitates topo- studied from their macroscopic crystallographic features taxially exsolved from Fe-rich parts of rutile through inter- down to subnanometer-scale analysis of phase composi- mediate Guinier–Preston zones, characterized by tripling the tions and occurring interfaces. Already, a simple morpho- {101} rutile reflections. Unlike rutile exsolutions in hematite, logical analysis indicates that rutile and hematite are met near hematite exsolutions in rutile form {301}R|{030}H equilibrium the �010�R{101}R||�001�H {110}H orientation relationship. interfaces. The overall composition of our samples indicates However, a more detailed structural analysis of rutile/hema- that the ratio between ilmenite and hematite in parent ferrian– tite interfaces using electron diffraction and high-resolution ilmenite crystals was close to Ilm67Hem33, typical for Fe–Ti- transmission electron microscopy (HRTEM) has shown that rich differentiates of mafic magma. The presence of ilmenite the actual relationship between the rutile and hosting hema- lamellae indicates that the primary solid solution passed the tite is in fact �010�R{401}R||�001�H {170}H. The intergrowth miscibility gap at ~900 °C. Subsequent exsolution processes is dictated by the formation of {170}H |{401}R equilibrium were triggered by surface oxidation of ferrous iron and remo- interfaces leading to 12 possible directions of rutile exsolu- bilization of cations within the common oxygen sublattice. tion within a hematite matrix and 144 different incidences Based on nanostructural analysis of the samples, we identi- between the intergrown rutile crystals. Analyzing the poten- fied three successive exsolution processes: (1) exsolution of tial rutile–rutile interfaces, these could be classified into four ilmenite lamellae from the primary ferrian–ilmenite crystals, classes: (1) non-crystallographic contacts at 60° and 120°, (2) (2) exsolution of rutile lamellae from ilmenite and (3) exsolu- {101} twins with incidence angles of 114.44° and their com- tion of hematite precipitates from Fe-rich rutile lamellae. All plementaries at 65.56°, (3) {301} twins at 54.44° with com- observed topotaxial reactions appear to be a combined func- plementaries at 125.56° and (4) low-angle tilt boundaries at tion of temperature and oxygen fugacity, fO2. 174.44° and 5.56°. Except for non-crystallographic contacts, all other rutile–rutile interfaces were confirmed in Mwinilunga Keywords Ilmenite · Hematite · Rutile · Topotaxy · Exsolution · Intergrowth · Geothermometer Communicated by Chris Ballhaus. Introduction A. Recˇnik (*) · N. Stankovic´ · N. Daneu Department for Nanostructured Materials, Jožef Stefan Institute, Jamova Cesta 39, 1000 Ljubljana, Slovenia Various intergrowths of rutile with structurally related min- e-mail: [email protected] erals are known in nature. They are found in igneous and 1 3 19 Page 2 of 22 Contrib Mineral Petrol (2015) 169:19 metamorphic rocks (Force et al. 1996) and form either as a crystallographically different intergrowths. Studying mor- result of topotaxial replacement reactions through decom- phological characteristics of rutile intergrowths with dif- position of Ti-rich minerals or by epitaxial growth of rutile ferent structurally related oxides, Armbruster (1981) also on structurally related mineral precursors. Out of these, the described two possible orientation relationships (ORs) of most spectacular are rutile/hematite overgrowths, forming rutile with the corundum-type structure. Figure 1 shows splendid, up to several-centimeter-large crystals clusters, the two ORs, where three equivalent 210H oxygen arrays mutually linked through well-defined orientation rela- are aligned either parallel to 001R (Fig. 1a) or one of the tionships (Armbruster 1981). The final product of these 101R (Fig. 1b) axes of rutile (laws IV and V; Armbruster replacement processes is rutile forming reticulated sagen- 1981): ite networks where individual crystals are interconnected OR-1: �001� {010} ||�210� {001} through complex crystallographic laws. R R H H (1) The most common precursors for the rutile/hematite OR-2: �101� {010} ||�210� {001} intergrowths are members of the ilmenite–hematite tie-line R R H H (2) in FeO–TiO2–Fe2O3 ternary system. The phase composi- OR-1 is the most simple of the two laws. Rutile exsolu- tion in this system depends on the Fe–Ti ratio, temperature tions according to this law intersect simply at 60° or 120°. and oxygen fugacity, fO2, while the effect of pressure is less These angles, however, produce non-crystallographic junc- important (Buddington and Lindsley 1964; Lindh 1972). At tions between the rutile crystals. Crystallographic relations the elevated temperatures, the compounds on the rhombo- between the rutile crystals exsoluted following OR-2, on hedral (ilm–hem) tie-line form a complete solid solution, the other hand, are more complex, but form well-defined while at lower temperatures they become immiscible, and crystallographic junctions corresponding to known spe- 3 depending on the amount of Fe +, they separate into exso- cial boundaries in rutile; e.g., 54.4° for {301} twin and lutions of hematite in ilmenite host or ilmenite in hematite 114.4° for {101} twin. Armbruster (1981) described sev- host (Lindsley 1973; Ghiorso 1990; Lindsley 1991; McEn- eral examples of intergrowths as observed on natural as roe et al. 2005). In nature, there are many examples of well as synthetic rutile crystals. Depending on the forma- lamellar intergrowths of hematite and ilmenite from micro- tion conditions (p T, geochemical environment), they − scopic grains in igneous and metamorphic rocks to large follow either of the two laws. Force et al. (1996) reported macroscopic crystals in pegmatite differentiates of mafic to OR-1 when describing the exsolutions of rutile in ilmenite, ultramafic magmas (Ramdohr 1969; Haggerty 1971). On while crystallographic junctions corresponding to OR-2 cooling, the activity of oxygen is increased and ilmenite are formed exclusively by growth twinning. They also may oxidize to hematite and rutile (Carmichael and Nich- suggested that the angles between the rutile domains can ols 1967; Zhao et al. 1999). Understanding the transient be used as the criterion to distinguish between topotaxial stages of phase transformations in this system is important transformations and growth twinning in rutile. Recent in geothermometry research (Burton 1985; Harrison et al. investigations of {101} and {301} rutile twins from Dia- 2000; Meinhold 2010). mantina in Brazil revealed that they also form by topotax- The formation of oriented intergrowths is commonly ial replacement of Al-rich Fe–Ti oxyhydroxide precursors explained by topotaxial transformation of the precursor with the tivanite-type structure (Daneu et al. 2007, 2014). phase into reaction products along specific crystallographic It has been demonstrated that topotaxial reactions result in orientations (Dent Glasser et al. 1962). Such reactions can the formation of well-defined crystallographic junctions only occur among structurally related minerals, such as between the rutile domains according to OR-2, forming hematite, ilmenite and rutile. These minerals are related complex sagenite intergrowths. The twin boundaries were through a common hexagonal close-packed (hcp) oxygen shown to accommodate remnants of dehydrated precur- sublattice that extends along the basal planes of the corun- sors, i.e., precipitates of corundum on (101) twin bounda- dum-type structure (e.g., hematite, ilmenite) and a or b ries and few nanometer-thick lamellae of Al-rich ilmenite planes of rutile. In rhombohedral ilmenite (R3¯) and hema- on (301) twin boundaries. tite (R3¯c), the arrays of oxygen atoms in basal planes inter- In the majority of previous works on mineralogical sam- sect at 60°, whereas in rutile structure they are regularly ples, determination of OR between the rutile and the pre- spaced along the c direction and slightly compressed along cursor minerals relies upon morphology and macroscopic a and b directions. This gives rise to tetragonal distortion of features, such as the angles between the adjacent rutile the rutile’s oxygen sublattice yielding the angles of 57.22° domains (Ramdohr 1969; Armbruster 1981; Force et al. between 101 and 001, and 65.56° between 101 and 1996). If the angles