Abstract
A crystal from the type locality Ajo, AZ, yielded just enough intensity from streaked diffractions using synchrotron x-rays at the Advanced Photon Source to solve the crystal structure with composition (K + Na)3Cu20Al3Si29O76(OH)16⋅∼8H2O; triclinic, P1̄, a = 13.634(5) Å, b = 13.687(7), c = 14.522(7), α = 110.83(1)°, β = 107.21(1), γ = 105.68(1); refined to a final R = 12.5%. Electron microprobe analysis yielded a similar chemical composition that is slightly different from the combined chemical and electron microprobe analyses in the literature. The ajoite structure can be described as a zeolitic octahedral-tetrahedral framework that combines the alternate stacking of edge-sharing octahedral CuO6 layers and curved aluminosilicate layers and strings. Channels bounded by elliptical 12-rings and circular 8-rings of tetrahedra contain (K and Na) ions and water. The Al atoms occupy some of the Si tetrahedral sites. Each Cu atom has near-planar bonds to four oxygen atoms plus two longer distances that generate a distorted octahedron. Valence bond estimates indicate that 8 oxygen atoms of 46 are hydroxyl. Only one alkali atom was located in distorted octahedral coordination, and electron microprobe analyses indicate K and Na as major substituents. The water from chemical analysis presumably occurs as disordered molecules of zeolitic type not giving electron density from diffraction. The high R factor results from structural disorder and many weak intensities close to detection level. The crystal chemistry is compared with shattuckite, Cu5(SiO3)4(OH)2, and planchéite, Cu8Si8O22(OH)4⋅H2O, both found in oxidized copper deposits of Arizona but only the former directly with ajoite.
Keywords: microcrystal, microporous, copper silicates
Hydrothermal ore deposits supply most of the copper plus significant molybdenum, gold, and other metals important for human welfare. Great advances in understanding the different types of hydrothermal ore deposits have occurred over the past three decades (1). In general, various granitic bodies intrude the crust in continental margins and interiors that are composed of volcanic rocks and sediments. Hot brines of many chemical types (2) permeate the existing rocks to generate diverse mineral assemblages (3). This series of papers concentrates on the porphyry copper deposits of Arizona. Paper I described the crystal structures of chenevixite and luethite, two copper Fe/Al arsenate hydroxide minerals (4). This paper covers ajoite, a rare, beautiful, blue-green mineral (5) that has challenged crystallographers for four decades. Structural relations in copper oxysalt minerals are reviewed in ref. 6, and static and Jahn–Teller chemical-bonding effects in Cu(II) oxysalts are reviewed in ref. 7.
Ajoite occurs rarely in the oxidized zone of porphyry copper deposits in Arizona (3). Type crystals are blue-green laths or plates originally listed as monoclinic but now are known to be triclinic from x-ray diffraction study (8). The first sample for chemical analysis (5) was believed to be contaminated with quartz, and a new combined chemical and electron microprobe analysis (8) is given in reference books.
Many specimens in several museums were examined, and finally the present specimen no. 159940 from the National Museum of Natural History of the Smithsonian Institution was chosen on the basis of uniform slightly greener color and “fresher appearance” than other slightly bluer specimens. After examining many crystals, one was found that gave streaked diffractions with intensity just good enough to solve the main features of the structure but with a poor agreement factor (∼12.5%). The structure has Si/Al and Na/K disorder as well as poorly defined water molecules, and the agreement factor reflects the poor crystal quality and is similar to those for large-pore zeolites. It is likely that the other bluer ajoite crystals have undergone partial dehydration and rehydration that has damaged the regularity of the crystal structure.
Materials and Methods
A crystal of ajoite, 60 × 15 × 5 μm3, was mounted on the tip of a glass fiber tapered to 1 μm. Data sets were collected at the GeoSoilEnviro and ChemMat-Consortium for Advanced Radiation Sources (CARS) sectors 13 and 15 at the Advanced Photon Source (Argonne, IL). The data set used in the refinement was collected using radiation from a diamond (111) crystal at a wavelength of 0.56954 Å and focused using horizontal and vertical Rh-coated float glass Kirkpatrick–Baez mirrors to produce a 100 × 100-μm2 beam. Data were collected using a Bruker 6000 SMART charge-coupled device detector at a fixed angle of 28° 2Θ and frame widths in ϕ of 0.3° with a 2-sec counting time per frame. The charge-coupled device detector was mounted on a Huber 4-circle diffractometer with the ω axis of the diffractometer in the plane of the ring. Two full rotations of the φ axis yielded 1,200 frames with χ = 0° and 1,200 with χ = 270°. The symmetry is triclinic: space group P1̄. Unit-cell dimensions, a = 13.634(5) Å, b = 13.687(7) Å, c = 14.522(7) Å, α = 110.833(11)°, β = 107.208(13)°, γ = 105.680(10)°, were refined by least squares using 863 reflections. Data were integrated and corrected for Lorentz, polarization, and background effects using Bruker software (SAINTPLUS). Systematic errors such as beam decay and absorption were corrected with the program SADABS on the basis of the intensities of equivalent reflections. A total of 44,861 reflections was obtained from 3 to 58° 2θ; of the 18,567 unique reflections (RINT = 3.0%), 12,110 were classed as observed (|Fo| > 4σF). The crystal structure was refined using SHELXTL 6.12, F2, and isotropic temperature factors to a final R of 12.5%. When scaled to Cu, the largest peak on the final difference Fourier was ∼3 electrons per Å3. Final atomic coordinates are shown in Table 1, and selected bond distances are shown in Table 2.
Table 1.
Atomic coordinates (×104) and isotropic displacement parameters, U, (Å2 × 103) for ajoite
| x | y | z | U | |
|---|---|---|---|---|
| Cu(1) | 905 (1) | 2149 (1) | 4795 (1) | 6 (1) |
| Cu(2) | 2917 (1) | 1663 (1) | 4821 (1) | 6 (1) |
| Cu(3) | −3123 (1) | 3137 (1) | 4739 (1) | 7 (1) |
| Cu(4) | −1117 (1) | 2633 (1) | 4739 (1) | 7 (1) |
| Cu(5) | 6937 (1) | 674 (1) | 4870 (1) | 7 (1) |
| Cu(6) | −5122 (1) | 3618 (1) | 4779 (1) | 8 (1) |
| Cu(7) | 4904 (1) | 1143 (1) | 4814 (1) | 5 (1) |
| Cu(8) | 8985 (1) | 236 (1) | 4971 (1) | 6 (1) |
| Cu(9) | 948 (1) | 4663 (1) | 4917 (1) | 8 (1) |
| Cu(10) | 2895 (1) | 4106 (1) | 4811 (1) | 8 (1) |
| Si(1) | 6643 (1) | 3950 (1) | 7006 (1) | 5 (1) |
| Si(2) | 701 (1) | 3064 (1) | 7035 (1) | 5 (1) |
| Si(3) | 4619 (1) | 4426 (1) | 7031 (1) | 7 (1) |
| Si(4) | 2748 (1) | 2604 (1) | 7058 (1) | 5 (1) |
| Si(5) | −713 (1) | 4667 (1) | 2873 (1) | 6 (1) |
| Si(6) | 1296 (1) | 4192 (1) | 2741 (1) | 8 (1) |
| Si(7) | 6719 (1) | 1773 (1) | 7118 (1) | 5 (1) |
| Si(8) | 7100 (1) | −524 (1) | 2641 (1) | 7 (1) |
| Si(9) | 5006 (1) | −103 (1) | 2554 (1) | 6 (1) |
| Si(10) | 8838 (1) | 1424 (1) | 7223 (1) | 6 (1) |
| Si(11) | −2735 (1) | 2659 (1) | 2552 (1) | 7 (1) |
| Si(12) | −2213 (1) | 602 (1) | 1340 (1) | 10 (1) |
| Si(13) | 8941 (1) | 292 (1) | 8708 (1) | 9 (1) |
| Si(14) | 3420 (1) | 763 (1) | 1332 (1) | 11 (1) |
| Si(15) | 95 (1) | 1594 (1) | 1263 (1) | 10 (1) |
| Si(16) | −4746 (1) | 3171 (1) | 2625 (1) | 7 (1) |
| O(1) | 64 (3) | 2350 (3) | 5707 | 6 (1) |
| O(2) | 2093 (3) | 1867 (3) | 5737 (3) | 6 (1) |
| O(3) | 6127 (3) | 964 (3) | 5795 (3) | 8 (1) |
| O(4) | 7754 (3) | 377 (3) | 3951 (3) | 9 (1) |
| O(5) | 3784 (4) | 1511 (4) | 3979 (3) | 12 (1) |
| O(6) | 5675 (3) | 819 (3) | 3852 (3) | 9 (1) |
| O(7) | 4062 (4) | 1326 (3) | 5717 (3) | 10 (1) |
| O(8) | 1812 (4) | 2064 (3) | 3988 (3) | 11 (1) |
| O(9) | −2284 (4) | 2968 (4) | 3830 (4) | 14 (1) |
| O(10) | 9850 (4) | 41 (4) | 4113 (4) | 14 (1) |
| O(11) | −152 (4) | 2624 (3) | 4004 (3) | 11 (1) |
| O(12) | 8203 (3) | 566 (3) | 5921 (3) | 8 (1) |
| O(13) | −4252 (4) | 3529 (3) | 3914 (3) | 11 (1) |
| O(14) | 2084 (4) | 4287 (3) | 5734 (3) | 11 (1) |
| O(15) | −92 (3) | 5202 (3) | 4187 (3) | 8 (1) |
| O(16) | 6069 (3) | 3324 (3) | 5678 (3) | 8 (1) |
| O(17) | 4066 (3) | 3796 (3) | 5711 (3) | 7 (1) |
| O(18) | 7972 (4) | 2725 (3) | 5554 (3) | 11 (1) |
| O(19) | 1816 (4) | 4601 (3) | 4041 (3) | 11 (1) |
| O(20) | 5889 (4) | 4500 (3) | 7500 (3) | 13 (1) |
| O(21) | 2109 (4) | 5035 (4) | 2467 (4) | 16 (1) |
| O(22) | 9944 (4) | 2541 (4) | 7561 (4) | 15 (1) |
| O(23) | −1907 (4) | 3607 (4) | 2371 (4) | 16 (1) |
| O(24) | 7999 (4) | 1910 (4) | 7636 (3) | 15 (1) |
| O(25) | 6778 (4) | 3047 (4) | 7452 (4) | 15 (1) |
| O(26) | 2969 (4) | 272 (4) | −6 (4) | 20 (1) |
| O(27) | −935 (4) | 5595 (4) | 2513 (4) | 16 (1) |
| O(28) | 9270 (4) | 820 (4) | 7927 (4) | 18 (1) |
| O(29) | 6896 (4) | −1836 (4) | 2402 (3) | 15 (1) |
| O(30) | −2887 (4) | 1410 (4) | 1764 (4) | 17 (1) |
| O(31) | −4690 (4) | 4287 (4) | 2449 (4) | 16 (1) |
| O(32) | −3963 (3) | 2689 (3) | 2113 (3) | 12 (1) |
| O(33) | 73 (4) | 4247 (4) | 2335 (4) | 16 (1) |
| O(34) | 9320 (5) | −756 (5) | 8510 (5) | 30 (1) |
| O(35) | 3984 (4) | −1238 (3) | 2325 (3) | 14 (1) |
| O(36) | 1120 (4) | 2912 (4) | 2001 (4) | 19 (1) |
| O(37) | 4489 (3) | 417 (3) | 1793 (3) | 12 (1) |
| O(38) | −901 (4) | 1485 (4) | 1699 (4) | 19 (1) |
| O(39) | 3887 (4) | 3713 (3) | 7466 (3) | 14 (1) |
| O(40) | 7808 (4) | −187 (4) | 2002 (4) | 16 (1) |
| O(41) | 5847 (3) | −557 (3) | 2126 (3) | 12 (1) |
| O(42) | 9606 (4) | 1315 (4) | 9971 (4) | 24 (1) |
| O(43) | 1918 (4) | 3038 (3) | 7518 (3) | 14 (1) |
| O(44) | −6046 (4) | 2217 (4) | 1934 (4) | 17 (1) |
| O(45) | 7562 (4) | −253 (4) | 8291 (4) | 19 (1) |
| O(46) | −6211 (4) | 4054 (4) | 3988 (4) | 14 (1) |
| K | 4955 (4) | −269 (3) | 1 (4) | 38 (1) |
| W(1) | 2462 (16) | 2448 (15) | −39 (15) | 73 (7) |
| W(2) | 1059 (12) | 3156 (13) | −31 (12) | 61 (5) |
Table 2.
Bond lengths (Å) for ajoite
| Cu octahedra | |||
| Cu(1)–O(8) | 1.938 (5) | Cu(6)–O(46) | 1.959 (4) |
| Cu(1)–O(11) | 1.969 (4) | Cu(6)–O(13) | 1.964 (4) |
| Cu(1)–O(1) | 1.988 (3) | Cu(6)–O(17) | 1.979 (4) |
| Cu(1)–O(2) | 2.012 (4) | Cu(6)–O(16) | 1.994 (4) |
| Cu(1)–O(10) | 2.501 (5) | Cu(6)–O(5) | 2.482 (4) |
| Cu(1)–O(14) | 2.516 (4) | Cu(6)–O(46) | 2.747 (5) |
| Cu(2)–O(5) | 1.935 (5) | Cu(7)–O(5) | 1.952 (4) |
| Cu(2)–O(8) | 1.968 (4) | Cu(7)–O(7) | 1.977 (4) |
| Cu(2)–O(2) | 1.975 (4) | Cu(7)–O(3) | 1.984 (4) |
| Cu(2)–O(7) | 1.989 (4) | Cu(7)–O(6) | 1.984 (4) |
| Cu(2)–O(17) | 2.509 (4) | Cu(7)–O(3) | 2.517 (4) |
| Cu(2)–O(12) | 2.640 (4) | Cu(7)–O(16) | 2.569 (4) |
| Cu(3)–O(18) | 1.959 (4) | Cu(8)–O(10) | 1.954 (5) |
| Cu(3)–O(13) | 1.975 (4) | Cu(8)–O(10) | 1.967 (5) |
| Cu(3)–O(9) | 1.981 (5) | Cu(8)–O(12) | 1.983 (4) |
| Cu(3)–O(16) | 1.983 (4) | Cu(8)–O(4) | 1.989 (4) |
| Cu(3)–O(19) | 2.673 (4) | Cu(8)–O(2) | 2.493 (4) |
| Cu(3)–O(6) | 2.728 (4) | Cu(8)–O(1) | 2.500 (4) |
| Cu(4)–O(11) | 1.924 (4) | Cu(9)–O(14) | 1.962 (4) |
| Cu(4)–O(18) | 1.952 (4) | Cu(9)–O(19) | 1.973 (4) |
| Cu(4)–O(9) | 2.015 (4) | Cu(9)–O(15) | 1.978 (4) |
| Cu(4)–O(1) | 2.026 (3) | Cu(9)–O(15) | 1.983 (4) |
| Cu(4)–O(15) | 2.546 (4) | Cu(9)–O(11) | 2.392 (4) |
| Cu(4)–O(4) | 2.663 (4) | Cu(9)–O(9) | 2.792 (5) |
| Cu(5)–O(3) | 1.979 (4) | Cu(10)–O(46) | 1.942 (5) |
| Cu(5)–O(4) | 1.981 (4) | Cu(10)–O(14) | 1.966 (4) |
| Cu(5)–O(12) | 2.017 (4) | Cu(10)–O(17) | 1.991 (4) |
| Cu(5)–O(6) | 2.019 (4) | Cu(10)–O(19) | 1.994 (4) |
| Cu(5)–O(7) | 2.390 (4) | Cu(10)–O(8) | 2.402 (4) |
| Cu(5)–O(18) | 2.430 (4) | Cu(10)–O(13) | 2.793 (4) |
| Si tetrahedra | |||
| Si(1)–O(25) | 1.614 (4) | Si(5)–O(33) | 1.612 (5) |
| Si(1)–O(21) | 1.614 (5) | Si(5)–O(15) | 1.616 (4) |
| Si(1)–O(20) | 1.615 (4) | Si(6)–O(36) | 1.600 (5) |
| Si(1)–O(16) | 1.622 (4) | Si(6)–O(21) | 1.617 (5) |
| Si(2)–O(27) | 1.614 (5) | Si(6)–O(19) | 1.619 (4) |
| Si(2)–O(43) | 1.614 (5) | Si(6)–O(33) | 1.629 (5) |
| Si(2)–O(22) | 1.619 (5) | Si(7)–O(25) | 1.606 (5) |
| Si(2)–O(1) | 1.624 (2) | Si(7)–O(24) | 1.611 (5) |
| Si(3)–O(31) | 1.613 (5) | Si(7)–O(35) | 1.615 (4) |
| Si(3)–O(17) | 1.613 (4) | Si(7)–O(3) | 1.630 (4) |
| Si(3)–O(20) | 1.622 (5) | Si(8)–O(40) | 1.612 (5) |
| Si(3)–O(39) | 1.624 (4) | Si(8)–O(29) | 1.628 (4) |
| Si(4)–O(29) | 1.610 (4) | Si(8)–O(41) | 1.631 (4) |
| Si(4)–O(39) | 1.615 (4) | Si(8)–O(4) | 1.634 (4) |
| Si(4)–O(43) | 1.616 (5) | Si(9)–O(37) | 1.611 (4) |
| Si(4)–O(2) | 1.618 (4) | Si(9)–O(41) | 1.619 (4) |
| Si(5)–O(23) | 1.598 (5) | Si(9)–O(6) | 1.627 (4) |
| Si(5)–O(27) | 1.601 (4) | Si(9)–O(35) | 1.628 (4) |
| Si(10)–O(28) | 1.607 (5) | Si(13)–O(45) | 1.645 (5) |
| Si(10)–O(12) | 1.615 (4) | Si(13)–O(28) | 1.650 (5) |
| Si(10)–O(22) | 1.623 (5) | Si(14)–O(26) | 1.657 (5) |
| Si(10)–O(24) | 1.626 (5) | Si(14)–O(45) | 1.667 (5) |
| Si(11)–O(30) | 1.597 (5) | Si(14)–O(37) | 1.679 (4) |
| Si(11)–O(9) | 1.622 (5) | Si(14)–O(44) | 1.690 (5) |
| Si(11)–O(32) | 1.623 (4) | Si(15)–O(34) | 1.634 (6) |
| Si(11)–O(23) | 1.629 (5) | Si(15)–O(42) | 1.650 (5) |
| Si(12)–O(26) | 1.658 (5) | Si(15)–O(38) | 1.652 (5) |
| Si(12)–O(38) | 1.663 (5) | Si(15)–O(36) | 1.653 (5) |
| Si(12)–O(40) | 1.681 (5) | Si(16)–O(44) | 1.619 (5) |
| Si(12)–O(30) | 1.687 (5) | Si(16)–O(13) | 1.619 (4) |
| Si(13)–O(34) | 1.613 (6) | Si(16)–O(31) | 1.622 (5) |
| Si(13)–O(42) | 1.628 (5) | Si(16)–O(32) | 1.622 (4) |
| K octahedra | |||
| K–O(30) | 2.731 (7) | K–O(30)#6 | 2.863 (6) |
| K–O(37) | 2.761 (6) | K–O(37)#8 | 2.878 (6) |
| K–O(26) | 2.829 (7) | K–O(26) | 2.993 (6) |
Electron microprobe analysis was done on four crystals with 5–8 analyses completed on a polished surface of crystalline fragments mounted in epoxy using standard wavelength-dispersive techniques and standards: K, asbestos microcline; Na, amelia albite; Si and Al, anorthite; and Cu, metal. The Si, Al, and Cu analyses were equal within experimental error, but the average Na and K analyses for each crystal varied. Atoms per 20 Cu atoms: K 0.20, 1.29, 0.23, and 2.15; Na 0.15, 0.65, 0.10, and 1.57; (Na + K) add up to 0.35, 1.94, 0.33, and 3.72 atoms per unit. Because of ajoite instability in the electron beam (8), we decided to prefer the conventional bulk analyses for alkalis and not to make electron probe analyses of the crystal used for x-ray diffraction.
Bearing in mind the evidence for half the bulk water being zeolitic, the formula in ref. 8 was amended to give (K + Na)∼3Cu20Al3Si29O76(OH)16⋅∼8H2O for bulk analysis. This assumes that one cation, K, fills the atomic site obtained from x-ray diffraction, and the remaining K and Na are located in the cavity associated with water molecules. No doubt the alkali and water contents of ajoite vary from spot to spot depending on the salt content of hydrothermal brines during original growth and the humidity after collection. Possibly ion exchange has occurred between K+, Na+, and H+.
Results and Discussion
The ajoite structure consists of alternating CuO6 octahedral sheets and SiO4 tetrahedral layers composed of chains and strings. Note that sites labeled Si may contain some Al. Fig. 1 is a projection down the a axis showing oxygen and hydroxyl atoms at the vertices of octahedra and tetrahedra. A sheet of edge-shared octahedrally coordinated Cu atoms lies at c ∼ 0.5 (Fig. 1). Above and below the Cu layer are bands of Si tetrahedra (centered at c ∼ 0.0 and c ∼ 1.0). Each band contains four subunits. Each subunit sharing vertices with the Cu sheet consists of nonlinear chains of edge-sharing 6-rings. These chains are traced in Fig. 2, a projection down c, with double arrows. The second subunit is a string of four Si tetrahedra denoted as Si12, Si15, Si13, and Si14 (Fig. 2, single arrows). When the two subunits are connected, the tetrahedral layer is based topologically on a nonplanar twisted two-dimensional net with Si atoms in rings of 5, 6, and 7 (Fig. 2) with the 5-, 7-, and elliptical 6-rings resulting from the linkage between chains and strings. A center of symmetry generates two identical subunits and a double string of four Si tetrahedra. Fig. 3, a projection down b, shows in more detail the stacking of the four layers. Each gap between strings is occupied by K. The three-dimensional octahedral-tetrahedral net contains a two-dimensional channel system defined by elliptical 12-rings along a (Fig. 1) and circular 8-rings along b (Fig. 3). As is typical of Cu(II)–O linkages, each Cu atom has four short distances to adjacent oxygen atoms in a square and two longer distances to generate opposing vertices of a distorted octahedron. The short distances generate strings of edge-shared squares.
Fig 1.
Ajoite structure projected down a showing the CuO6 layer, SiO4 bands, and 12-ring channels. K atoms are shown as black circles.
Fig 2.
SiO4 tetrahedral layer looking down c. The block double arrows trace nonlinear chains of 6-rings, block single arrows denote four-SiO4 strings, and 5, 6, and 7 give the number of tetrahedron in rings.
Fig 3.
Ajoite structure projected down b showing the positions of CuO6 layers, SiO4 6-ring chains, four-SiO4 strings, K sites, and 8-ring channels.
The chemical analyses indicate that 3 of the 32 tetrahedral sites are occupied by Al. From the distances in Table 2, the larger Al atoms should be occupying ∼1/2 of sites Si(12) 1.67 Å; Si(14) 1.67 Å and 1/4 of Si(13) 1.63 Å, Si(15) 1.65 Å. This accounts for the three Al atoms per unit cell obtained from the microprobe analysis. All these tetrahedra reside in the strings of four tetrahedra, near the K site and the sites containing 1/2 Al share oxygen atoms with K. All other tetrahedra have mean T–O distances near 1.61–1.62 Å, indicating occupancy with only Si. It should be noted that bonding to other species may perturb mean T–O distances in frameworks up to 0.02 Å, and a linear plot from Si–O ∼ 1.61 to Al–O ∼ 1.74 Å cannot be strictly used to predict Al substitution in Si positions. Whether ordered domains or a superstructure occurs rather than random disorder is not clear from the present x-ray data. A thorough transmission electron diffraction study of ajoite is desirable.
Ajoite contains three types of oxygen atoms: those bonded to Si plus 3 Cu, those bonded to 2 Si/Al, and those bonded to 3 Cu. The valence sum for tetrahedral Si plus 3 octahedral divalent Cu, and for 2 T, is ∼2, whereas for 3 Cu is ∼1. This means that to achieve charge balance, the O atoms bonded to 3 Cu must really be OH groups. There are 16 of these oxygen atoms, so the chemical formula is written with (OH)16.
A position on a center of symmetry in ajoite was split into two positions with half occupancy to explain the elongated electron distribution. The distances to adjacent oxygen atoms are large enough for most of the site to be occupied by K rather than Na. No clear positions for Na and water were obtained from the diffractions, indicating structural disorder. Fig. 3 shows a K position in distorted octahedral coordination, which accounts for only one of the cations. The distances in Table 2 range from 2.73 to 2.99 Å. Additional (K and Na) and water from the chemical analysis presumably reside in the channels, as disordered molecules do in zeolites, and are not detected. Note that atoms listed as W in Table 1 are possible sites for water molecules or Na+. For zeolitic materials, the details of ionic bonding are complex.
Of the other minerals in the Arizona porphyry deposits, shattuckite and planchéite (9) come closest in chemical bonding. The crystal structure of shattuckite has octahedral Cu–O layers, but the cross-linkage is by pyroxene-type silicate chains. There are no zeolitic water molecules and no alkali or aluminum cations. In planchéite, the octahedral sheets are cross-linked by amphibole-type silicate chains, and a small amount of water is indicated. The linkage of short Cu–O distances in both structures also generates an edge-shared string.
Conclusions
The crystal structure of ajoite opens up a further window into zeolitic (microporous) materials. Ion-exchange experiments are needed to determine the selectivities and possible industrial use. Removal of the zeolitic water molecules may permit molecular sorption or catalysis. A theoretical study of polygonal linkages may allow invention of new octahedral-tetrahedral frameworks, and a family of copper silicate zeolitic materials may ensue. Finally, we emphasize the importance of synchrotron x-ray facilities for the x-ray structure determination and the need for complex spectroscopic techniques to help resolve the many details left undetermined. Ajoite has not been reported yet in corroded copper art objects but might be found along with planchéite.
Acknowledgments
We thank Peter Burns for parallel study and manuscript review, the curators of the Smithsonian National Museum of Natural History facility for specimens, and Ian M. Steele for the electron probe analysis. J.J.P. thanks the Materials Research Science and Engineering Center at the University of Chicago for support. This work was performed at the GeoSoilEnviroCARS (GSE-CARS) Sector 13, and ChemMat-CARS Sector 15 of the Advanced Photon Source at Argonne National Laboratory. GSE-CARS and ChemMat-CARS are supported by the National Science Foundation, Department of Energy, and W. M. Keck Foundation. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Energy Research, under Contract W-31-109-Eng-38.
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