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Armalcolite
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Armalcolite
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Armalcolite
Armalcolite from Myanmar (grain size 5 mm)
General
CategoryTitanium mineral
Formula(Mg,Fe2+)Ti2O5
IMA symbolArm[1]
Strunz classification4.CB.15
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupBbmm
Unit cella = 9.743(30)
b = 10.023(20)
c = 3.738(30) [Å], Z = 5
Identification
ColorGray to tan in reflection, opaque
Mohs scale hardness<5
LusterMetallic
Specific gravity4.64 g/cm3 (measured)
Optical propertiesBiaxial
References[2][3][4]

Armalcolite (/ˌɑːrˈmɑːlkəlaɪt/) is a titanium-rich mineral with the chemical formula (Mg,Fe2+)Ti2O5. It was first found at Tranquility Base on the Moon in 1969 during the Apollo 11 mission, and is named for Armstrong, Aldrin and Collins, the three Apollo 11 astronauts. Together with tranquillityite and pyroxferroite, it is one of three new minerals that were discovered on the Moon.[5] Armalcolite was later identified at various locations on Earth and has been synthesized in the laboratory. (Tranquillityite and pyroxferroite were also later found at various locations on Earth).[6] The synthesis requires low pressures, high temperatures and rapid quenching from about 1,000 °C to the ambient temperature. Armalcolite breaks down to a mixture of magnesium-rich ilmenite and rutile at temperatures below 1,000 °C, but the conversion slows down with cooling. Because of this quenching requirement, armalcolite is relatively rare and is usually found in association with ilmenite and rutile, among other minerals.

Occurrence

[edit]
The Apollo 11 crew, after whom Armalcolite is named. Left to right are Neil Armstrong, Michael Collins, and Buzz Aldrin.

Armalcolite was originally found on the Moon, in the Sea of Tranquility at Tranquility Base, and also in the Taurus–Littrow valley and the Descartes Highlands. The largest amounts were provided by the Apollo 11 and 17 missions. It was later identified on Earth from samples of lamproite dikes and plugs taken in Smoky Butte, Garfield County, Montana, US.[7] On the Earth, it also occurs in Germany (Nördlinger Ries impact crater in Bavaria), Greenland (Disko Island), Mexico (El Toro cinder cone, San Luis Potosí), South Africa (Jagersfontein, Bultfontein and Dutoitspan kimberlite mines), Spain (Albacete Province and Jumilla, Murcia), Ukraine (Pripyat Swell), United States (Knippa quarry, Uvalde County, Texas and Smoky Butte, Jordan, Montana) and Zimbabwe (Mwenezi District).[2][8] Armalcolite was also detected in lunar meteorites, such as Dhofar 925 and 960 found in Oman.[9]

Armalcolite is a minor mineral found in titanium-rich basalt rocks, volcanic lava and sometimes granite pegmatite, ultramafic rocks, lamproites and kimberlites. It is associated with various mixed iron-titanium oxides, graphite, analcime, diopside, ilmenite, phlogopite and rutile. It forms elongated crystals up to about 0.1–0.3 mm in length embedded in a basalt matrix.[10] Petrographic analysis suggests that armalcolite is typically formed at low pressures and high temperatures.[2]

Synthesis

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Armalcolite crystals up to several millimeters in length can be grown by mixing powders of iron, titanium and magnesium oxides in the correct ratio, melting them in a furnace at about 1,400 °C, letting the melt crystallize for a few days at about 1,200 °C, and then quenching the crystals to the ambient temperature.[11][12] The quenching step is required both for laboratory and natural synthesis in order to avoid conversion of armalcolite to a mixture of magnesium-rich ilmenite (Mg-FeTiO
3) and rutile (TiO2) at temperatures below 1,000 °C.[13] This conversion threshold temperature increases with pressure and eventually crosses the melting point, meaning that the mineral cannot be formed at sufficiently high pressures. Because of this conversion to ilmenite, armalcolite has a relatively low abundance and is associated with ilmenite and rutile.[14] Consequently, the relative amount of ilmenite and armalcolite can be used as an indicator of the cooling rate of a mineral during its formation.[15]

Properties

[edit]
Crystal structure. Colors: green – Mg, blue – Ti, red – oxygen.

Armalcolite has a general chemical formula (Mg,Fe2+)Ti2O5. It forms opaque masses which appear gray (ortho-armalcolite) to tan (para-armalcolite) in reflection, with gray varieties being most common, especially in synthetic samples. The crystal structure is the same for the ortho- and para-armalcolite. Their chemical composition does not differ significantly, but there is a difference in the MgO and Cr2O3 content which was attributed to dissimilar coloration.[13][16] Armalcolite is a part of the pseudobrookite group which consists of minerals of the general formula X2YO5. X and Y are usually Fe (2+ and 3+), Mg, Al, and Ti. End members are armalcolite ((Mg,Fe)Ti2O5), pseudobrookite (Fe2TiO5), ferropseudobrookite (FeTi2O5) and "karrooite" (MgTi2O5). They are isostructural and all have orthorhombic crystal structure and occur in lunar and terrestrial rocks.[8][10][17]

Chemical composition of most armalcolite samples can be decomposed into a sum of metal oxides as follows: TiO2 (concentration 71–76%), FeO (10–17%), MgO (5.5–9.4%), Al2O3 (1.48–2%), Cr2O3 (0.3-2%) and MnO (0–0.83%). Whereas the titanium content is relatively constant, the ratio of magnesium to iron varies and is usually lower than 1.[2][10] A so-called Cr-Zr-Ca variety of armalcolite is distinguished which has an elevated content of Cr2O3 (4.3–11.5%), ZrO2 (3.8–6.2%) and CaO (3–3.5%). These varieties are not distinct and intermediate compositions are also found.[13] The iron-poor (magnesium-rich) modification of armalcolite has the same crystal structure and occurs in the Earth's crust as the mineral unofficially named "karrooite".[15][18]

Most titanium is present in armalcolite in the 4+ state, owing to the reducing synthesis environment, but there is a significant fraction of Ti3+ in lunar samples. The Ti3+/Ti4+ ratio in armalcolite can serve as an indicator of fugacity (effective partial pressure) of oxygen during the mineral's formation. It also allows one to distinguish lunar and terrestrial armalcolite, as Ti3+/Ti4+ = 0 for the latter.[13]

Since armalcolite's formula is (Mg,Fe2+)Ti2O5, it follows the general formula of XY2O5 where the X=(Mg and Fe2+), Y=Ti, and O is oxygen. Both X and Y sites are octahedrally coordinated and the radius ratio between the cations and the anions in armalcolite are at three to five ratio equaling 0.6 making the structure octahedral. Armalcolite is a titanium-rich mineral that falls under the magnesianferropseudobrookite mineral group with Fe2+Ti2O5 and MgTi2O5 as end members.[8] Due to having octahedral symmetry, armalcolite has solid solution (cation substitution) between multiple elements Fe2+, Fe3+, Mg, Al, and Ti; this is because of their similarities in atomic radii and charge. The crystallographic structure exhibited by armalcolite is an orthombic-dipyramid, thus falls in the orthorhombic category and has a 2/m 2/m 2/m point group and space group of Bbmm. Inside the M1 sites for armalcolite it is ideal for iron to reside there due to the larger size of iron and for M2, magnesium and titanium have a distribution between the two sites. In the metal sites, titanium has an eightfold; magnesium and iron with a four coordination.[13][16] The magnesium and iron ratio in armalcolite decreases with decreasing temperature from 0.81 at 1,200 °C to 0.59 at 1,150 °C. Once the armalcolite reaches 1,125 °C it is replaced with ilmenite, FeTiO3, which lacks both magnesium and iron.[7]

The crystal structure of armalcolite is close to that of distorted brookite. It is based on deformed octahedra, with a titanium atom in the center and six oxygen atoms at the corners. Magnesium or iron ions are located in the interstitial sites; they do not contribute significantly to the lattice framework, which is held by Ti-O bonds via the corners of the octahedra. However, these ions affect optical properties, rendering the mineral opaque in contrast to the transparent titanium dioxide TiO2.[13]

See also

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References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Armalcolite

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from Grokipedia
Armalcolite is a rare orthorhombic in the pseudobrookite group, with the idealized (Mg,Fe²⁺)Ti₂O₅, characterized by its opaque, bluish-gray appearance and metallic luster. It was first discovered in 1969 within basaltic lunar soil samples collected during the mission from the Sea of Tranquillity, marking it as one of the initial minerals identified exclusively from extraterrestrial material at the time. The mineral's name derives from the surnames of the mission's astronauts—Neil Armstrong, Edwin "Buzz" Aldrin, and Michael Collins—reflecting its historical tie to humanity's first . Composed primarily of titanium dioxide (71–76 wt% TiO₂), with significant magnesium oxide (5–11 wt% MgO) and iron oxide (12–18 wt% FeO), armalcolite forms small, prismatic to tabular crystals typically 100–300 μm in size, often associated with ilmenite and exhibiting a pseudo-brookite-type crystal structure with lattice parameters a ≈ 9.74 Å, b ≈ 10.02 Å, and c ≈ 3.74 Å. Its physical properties include a Mohs hardness of about 5–6, a specific gravity of approximately 4.3–4.9, and reflectivity values around 14–15% in reflected light, making it distinguishable under microscopic examination. On Earth, armalcolite was first identified in 1983 in kimberlite rocks from the Jagersfontein locality in South Africa, where it occurs as a Cr-Ca-(Nb,Zr)-bearing variety, and has since been reported in rare igneous settings such as basalts, lamproites, granites, and kimberlites in locations including Greenland, Germany, the United States, and Western Australia. These terrestrial occurrences, often in high-temperature, low-pressure environments that cool rapidly, highlight armalcolite's formation under conditions akin to those on the early Moon, and potential resources for future space exploration, such as titanium extraction for metals and oxygen.

Discovery and Etymology

Apollo 11 Samples

Armalcolite was first identified in 1969 within basaltic rocks returned from () by the mission, marking it as one of the initial new minerals discovered in lunar samples. The mineral was independently recognized by six research groups examining the samples shortly after their return on July 24, 1969, during initial post-mission analyses at facilities like the Lunar Receiving Laboratory. These basalts, representing the fine-grained igneous rocks of the lunar maria, provided the primary context for the discovery, with armalcolite appearing as accessory phases in the rock fabric. Specific occurrences were documented in several Apollo 11 samples, including crystalline basalts such as 10022-37 and 10071-28, as well as microbreccias like 10059-27, 10067-8, 10068-25, and 10084-64. Armalcolite grains, typically isolated and ranging from 100 to 300 micrometers in size, were embedded in the fine-grained matrix alongside dominant minerals pyroxenes, , and interstitial glass, often forming part of the groundmass in these vuggy or vesicular basalts. In most cases, the grains were mantled or rimmed by , suggesting a paragenetic relationship during , though exceptions occurred in feldspar-rich fragments such as 10084-12. Initial analyses employed electron microprobe techniques to characterize the mineral, revealing its high titanium content— with TiO₂ comprising 71.1 to 75.6 weight percent—along with significant FeO (11.90 to 18.01%) and MgO (5.52 to 11.06%), and trace amounts of Cr, Al, Mn, Ca, V, and Zr. This composition highlighted armalcolite's enrichment in titanium compared to associated phases like ilmenite, and its relation to the pseudobrookite series was noted early in the investigations. The discovery was first publicly announced on January 30, 1970, in Science, with the formal description provided by Anderson et al. later that year in the proceedings of the Apollo 11 Lunar Science Conference.

Naming and Historical Context

The name armalcolite is a portmanteau derived from the surnames of the three astronauts—Neil A. Armstrong, Edwin E. "Buzz" Aldrin, and Michael Collins—specifically combining the initials "Arm," "Al," and "Col" to honor their achievement in the first human landing on the . This naming was proposed by the team analyzing the lunar samples returned from the mission, reflecting the mineral's initial discovery in collected at on July 20, 1969. The tribute underscores the historic significance of as the culmination of the U.S. space program's early efforts to explore the lunar surface, with armalcolite emerging as one of the first new minerals identified exclusively from extraterrestrial material at the time. The mineral's formal description and naming were published in the proceedings of the Apollo 11 Lunar Science Conference in 1970, marking a key moment in planetary mineralogy. This practice of naming lunar minerals after mission-related elements became a in the field, exemplified by tranquillityite, which was also discovered in samples and named for , the landing site. The International Mineralogical Association (IMA) officially approved armalcolite as a valid mineral species in 1970, solidifying its place in geological nomenclature and highlighting the interdisciplinary impact of on Earth-based sciences.

Chemical Composition

Molecular Formula

Armalcolite is defined by its ideal chemical formula , representing a between magnesium and iron in a 1:1 ratio at the divalent cation site. This composition was first established through electron microprobe analyses of samples from the mission, confirming armalcolite as a titanium-rich . The formula reflects a stoichiometric arrangement where titanium predominantly occupies octahedral sites in the +4 , while the Mg and Fe^{2+} ions share a single site per . The stoichiometry of armalcolite breaks down to one atom of Mg or Fe^{2+}, two atoms of Ti^{4+}, and five atoms of oxygen per , yielding a molecular weight of 207.95 g/mol. This end-member composition positions armalcolite as the magnesium-bearing analogue within the pseudobrookite group, closely related to the iron-dominant end-member of the pseudobrookite group, pseudobrookite (Fe₂TiO₅). Natural specimens of armalcolite often incorporate minor impurities such as Cr and Al, though these do not alter the core formula.

Elemental Variations and Impurities

Armalcolite exhibits compositional variations primarily through between the end-members MgTi₂O₅ and FeTi₂O₅, where Fe²⁺ substitutes for Mg²⁺, resulting in ferrian armalcolite in iron-rich samples and magnesian varieties in magnesium-enriched ones. These substitutions are common in natural samples and are detected via electron microprobe analysis, with the Fe/Mg ratio influencing the mineral's stability under varying oxygen fugacity conditions. Trace elements in armalcolite include Cr (up to 2-3 wt% as Cr₂O₃), Al (up to 2.5 wt% as Al₂O₃), Mn (<1 wt% as MnO), V (<1 wt% as V₂O₃), Ca (<1 wt% as CaO), and Zr (<2 wt% as ZrO₂ in most cases, though up to 6 wt% in Zr-armalcolite varieties). These impurities, often substituting at octahedral sites, are present in concentrations typically below 1 wt% each except for Cr and Al, and their incorporation is analyzed using microprobe techniques to reveal zoning patterns, such as slight increases in Fe and decreases in Ti from core to rim in some crystals. Lunar armalcolite from Apollo samples generally shows higher TiO₂ content (70-78 wt%) compared to terrestrial occurrences, which have lower TiO₂ (50-68 wt%) and often elevated Fe³⁺ due to more oxidizing conditions. For example, analyses of samples (e.g., 10022) yield compositions with 71-76 wt% TiO₂, 12-18 wt% FeO, and 5-11 wt% MgO, while high-Ti basalts (e.g., 74241) reach 70-78 wt% TiO₂, 6-10 wt% FeO, and 6-11 wt% MgO. In contrast, terrestrial armalcolite from paragneiss xenoliths displays 50-68 wt% TiO₂, 7-45 wt% total FeO/Fe₂O₃, and 0.2-5 wt% MgO, with trace Cr₂O₃ (0.02-0.19 wt%) and ZrO₂ (0.07-1.84 wt%).
Sample TypeTiO₂ (wt%)MgO (wt%)FeO (wt%)Key Traces (wt%)Source
Apollo 11 (e.g., 10022)71-765-1112-18Cr₂O₃ 1-2, Al₂O₃ 1-2
Apollo 17 (e.g., 74241)70-786-116-10ZrO₂ up to 6 (Zr-armalcolite)
Terrestrial (Mexico xenoliths)50-680.2-57-45 (incl. Fe₂O₃)V₂O₃ 0.4-3, ZrO₂ 0.07-1.8

Crystal Structure and Physical Properties

Crystal System and Morphology

Armalcolite belongs to the and adopts the pseudobrookite-type structure with Bbmm. This structure features a framework of edge-sharing octahedra occupied by and magnesium/iron cations, consistent with its composition in the pseudobrookite group (X₂YO₅, where X = Mg, Fe²⁺ and Y = Ti). The unit cell parameters for ortho-armalcolite from lunar samples are a = 9.743(5) , b = 10.001(5) , and c = 3.728(2) , yielding a of approximately 363.7 ³ with Z = 4. Para-armalcolite exhibits slightly smaller dimensions: a = 9.712(20) , b = 9.997(20) , and c = 3.735(8) . These values reflect variations in cation ordering and composition observed in Apollo mission samples, with the structure refined from single-crystal diffraction data. In natural occurrences, armalcolite typically forms anhedral grains ranging from 100 to 300 μm in size, often intergrown or mantled by . Euhedral prisms are rare, but lamellar and acicular habits have been noted, particularly in coarser-grained lunar basalts. Two morphological varieties are distinguished in samples: ortho-armalcolite, which appears as equant grains with gray reflectance, and para-armalcolite, characterized by elongated, acicular forms showing tan coloration in reflected light. These differences arise from crystallization conditions rather than distinct polymorphs, as both share the same and structural topology related to pseudobrookite. X-ray powder diffraction patterns of armalcolite confirm its orthorhombic , with characteristic d-spacings including 3.468 (100% intensity, 111 reflection), 2.763 (25%, 211), 2.454 (25%, 220), and 1.958 (80%, 040). Additional peaks at approximately 2.52 and 1.48 correspond to higher-order reflections like (312) and (444), respectively, aiding identification in lunar analyses.

Optical and Mechanical Properties

Armalcolite appears opaque with a metallic luster and exhibits a bluish-gray color in reflected light. It displays distinct , varying from pale gray to dark blue-gray, and shows strong under reflected light . The mineral's reflectivity in air is measured at R1 = 14.1% and R2 = 15.2% at 450 nm, decreasing to R1 = 13.0% and R2 = 14.1% at 640 nm, resulting in positive bireflectance. Mechanically, armalcolite has a hardness of approximately 5 on the , softer than . Its measured specific gravity is 4.94 g/cm³. The mineral exhibits a subconchoidal , consistent with its brittle in the pseudobrookite group. Variations in color and may arise from impurities, as noted in compositional analyses.

Natural Occurrence

Lunar Sites

Armalcolite was first identified in samples collected from during the mission in 1969, where it occurs as an accessory in high-titanium mare basalts from . These basalts, characterized by elevated TiO₂ contents, contain armalcolite as isolated, rectangular grains typically 100-300 µm in size, often mantled by . Subsequent Apollo missions expanded the known distribution of armalcolite across lunar mare regions. Compositionally similar armalcolite grains were reported in low- and very low-titanium s from the site in , samples from Fra Mauro (though primarily breccias with basalt fragments), and mare s from the site in Hadley Rille and site in . The mission at Taurus-Littrow valley yielded the most diverse armalcolite occurrences, including both "ortho-armalcolite" (prismatic, early-crystallizing) and "para-armalcolite" (anhedral, late-stage), in high-titanium s. In lunar rocks and , armalcolite typically constitutes less than 1 vol% as an accessory phase, though it can reach up to ~1 vol% in Ti-rich oxide assemblages, and is commonly associated with and in the groundmass or as inclusions in pyroxenes. Its distribution is predominantly in basalts, especially high-Ti varieties, with Zr-free compositions typical of these settings; however, Zr-rich armalcolite variants occur in some non-mare, high-alumina basalts from highland terrains, indicating a broader but less common presence beyond mare regions. Additional lunar sample-return missions since Apollo 17 include China's (2020) from and (2024) from the lunar far side (Apollo basin). Zr-rich armalcolite occurs in samples. Ongoing laboratory analyses of archived Apollo samples and recent Chang'e returns continue to refine understanding of armalcolite's compositional variations and paragenesis through advanced techniques like electron microprobe and backscatter diffraction.

Terrestrial Localities

Armalcolite occurs rarely on Earth, primarily in high-titanium and ultramafic rocks such as lamproites, kimberlites, and basalts, often in trace amounts compared to its more abundant presence in lunar samples. The first terrestrial discoveries followed the lunar findings in , with initial reports in the early from volcanic and kimberlitic settings, highlighting its association with reduced, high-temperature environments. These occurrences are typically subhedral to anhedral grains up to 300 µm in size, intergrown with minerals like , , , and . One of the earliest and most studied terrestrial localities is Smoky Butte in , , where armalcolite appears in dikes and plugs dated to approximately 27 Ma. Here, it forms abundant, Ti-rich crystals in olivine-armalcolite-phlogopite hyalolamproites, associated with Ti-phlogopite, , and , reflecting rapid quenching in a low-pressure, mantle-derived . Another notable U.S. site is the Knippa quarry near , where armalcolite occurs in Tertiary volcanic rocks as metallic, grey grains. In , armalcolite has been documented in pipes, including Jagersfontein, Bultfontein, and Dutoitspan mines near Kimberley, where it appears in hypabyssal-facies rocks with and . These finds, from the , indicate formation under conditions with low oxygen fugacity, often as part of Fe-Mg-Ti oxide assemblages in differentiated . Additional rare occurrences include the El Toro cinder cone near , , in sillimanite-bearing paragneiss xenoliths from volcanics, where armalcolite formed via reactions involving and during decompression at 900–1200 °C. In , it is reported from the impact crater in , , within impactite glasses, and from lamproites in Cancarix and Jumilla, , associated with Cr-Zr-rich phases in high-MgO, high-SiO₂ magmas. Greenland's hosts armalcolite in basalts with and Fe-Ti oxides. Globally, these sites are scattered across volcanic provinces like the Basin and Range (), Kaapvaal Craton (), and Central , with no significant economic deposits due to its trace-level abundance.

Formation and Synthesis

Geological Formation Processes

Armalcolite primarily forms through crystallization within high-titanium basaltic magmas on the , where it emerges as a late-stage accessory mineral under conditions of low oxygen . These reducing environments, with oxygen (fO₂) typically below 10⁻¹³ atm, favor the stability of armalcolite by maintaining in a reduced state, often incorporating Ti³⁺, which distinguishes it from more oxidized terrestrial counterparts. occurs near the liquidus in magmas containing greater than 10 wt% TiO₂, associating armalcolite with phases like , , and , before it reacts with the evolving melt as progresses. The process unfolds at temperatures between 1000°C and 1200°C, with peak stability around 1200°C in lunar high-Ti basalts. Pressure plays a minimal role in lunar settings due to the Moon's low gravity and shallow magmatic depths, typically less than 400 km, allowing armalcolite to persist without significant breakdown. However, its stability is limited; at higher temperatures above 900°C or under prolonged subsolidus conditions, armalcolite decomposes into assemblages including , , Ca-rich , , and . Preservation occurs primarily in rapidly quenched rocks, such as those from volcanic eruptions, which halt these reactions and lock in the mineral's structure. On Earth, armalcolite forms rarely through enrichment during fractional of magmas, particularly in Ti-rich variants, where it appears after significant differentiation, around 50% in some modeled systems. This process concentrates in the residual melt, promoting armalcolite at high temperatures of 900–1200°C and low pressures below 10 kbar, often in volcanic or xenolithic contexts. As a metastable phase, it develops in cooling lavas or during rapid decompression of mantle-derived materials, such as in xenoliths transported quickly to the surface, via reactions like + → armalcolite under less reducing conditions than on the . Slower cooling on Earth typically leads to its decomposition into more stable oxides, limiting its persistence compared to lunar occurrences.

Laboratory Synthesis Methods

Armalcolite was first synthesized in the laboratory shortly after its discovery in lunar samples, using solid-state reactions to replicate its composition. In 1970, researchers mixed chemically pure oxides of iron, magnesium, and (MgO, FeO or Fe₂O₃, and TiO₂) and heated the mixtures in sealed, evacuated silica glass tubes at 1300°C for 2 hours, followed by rapid to . This method produced orthorhombic crystals with controlled Fe/Mg ratios, such as Fe₀.₄Mg₀.₆Ti₂O₅ and end-member compositions like MgTi₂O₅ and FeTi₂O₅, verified by diffraction (XRD) patterns matching those of natural armalcolite. To mimic lunar high-pressure conditions, subsequent experiments in the employed piston-cylinder apparatus for synthesizing armalcolite under elevated pressures. Synthetic armalcolite with compositions near (Mg,Fe)Ti₂O₅ was produced at pressures up to 1.4 GPa and temperatures of 1100–1200°C, using silver-palladium containers to control oxygen . These conditions stabilized the orthorhombic phase, but armalcolite decomposed to plus above approximately 1.4 GPa at 1200°C, highlighting challenges in maintaining phase stability at higher pressures. XRD analysis confirmed the synthetic products' structural similarity to natural samples, with lattice parameters adjustable via Fe/Mg ratios. Melt techniques have also been used to grow armalcolite crystals from titaniferous basaltic compositions. In these approaches, mixtures enriched in TiO₂ are melted at temperatures around 1300–1400°C under low pressure (0.1 MPa) and variable oxygen , then rapidly quenched to form pseudobrookite-structured armalcolite. This method allows for larger crystal sizes but requires precise control of cooling rates to avoid decomposition into secondary phases like . Modern synthesis focuses on producing nanocrystalline armalcolite for applications in sensors and composites, often via solid-state step- of precursors. One approach involves wet-milling a of CaO, MgCO₃, Fe₂O₃, and TiO₂ in , drying the , compacting into pellets, and in air at stepwise temperatures up to 1050°C (e.g., 350–1050°C ramps with soaking times of 1–3.5 hours). This yields armalcolite nanocrystals (Fe₂MgTi₃O₁₀ phase) with sizes around 50–100 nm, confirmed by XRD matching orthorhombic patterns of natural armalcolite and field-emission scanning electron microscopy for morphology. Challenges persist in achieving pure orthorhombic stability without impurities, particularly at lower temperatures, and in tailoring Fe/Mg ratios for specific properties, often requiring multiple cycles.

References

  1. https://eml.geoscience.wisc.edu/wp-content/uploads/sites/1364/2023/01/Anderson-et-al-1970-Armalcolite.pdf
  2. https://digitalcommons.usf.edu/cgi/viewcontent.cgi?article=1007&context=geologia
  3. https://www.csiro.au/en/news/all/articles/2024/june/armalcolite-mineral
  4. https://adsabs.harvard.edu/full/1970GeCAS...1...55A
  5. https://www.lpi.usra.edu/publications/books/lunar_sourcebook/pdf/Chapter05.pdf
  6. https://www.webmineral.com/data/Armalcolite.shtml
  7. https://www.mindat.org/min-341.html
  8. https://webmineral.com/data/Armalcolite.shtml
  9. https://rruff.info/uploads/PA11LSC1a_55.pdf
  10. http://www.minsocam.org/msa/ammin/toc/Articles_Free/1995/Hayob_p810-822_95.pdf
  11. https://doi.org/10.1016/0012-821X(74)90104-6
  12. https://www.handbookofmineralogy.org/pdfs/armalcolite.pdf
  13. https://www.handbookofmineralogy.org/pdfs/Pseudobrookite.pdf
  14. https://adsabs.harvard.edu/full/1973LPSC....4..777H
  15. https://www.sciencedirect.com/science/article/pii/0012821X74901046
  16. https://www.researchgate.net/publication/396713518_Impactor_relics_of_CI-like_chondrites_in_Chang%27e-6_lunar_samples
  17. https://minds.wisconsin.edu/bitstream/1793/48099/1/Armalcolite.pdf
  18. https://academic.oup.com/petrology/article-abstract/28/4/645/1578949
  19. https://pubs.geoscienceworld.org/msa/ammin/article/60/7-8/566/543121/Armalcolite-Ti-phlosopite-diopside-analcite
  20. https://www.sciencedirect.com/science/article/abs/pii/0016703783902016
  21. https://ntrs.nasa.gov/api/citations/19760017596/downloads/19760017596.pdf
  22. https://rruff.geo.arizona.edu/doclib/am/vol63/AM63_1209.pdf
  23. https://link.springer.com/article/10.1007/s00410-020-01769-y
  24. https://rruff.info/doclib/am/vol80/AM80_810.pdf
  25. https://www.sciencedirect.com/science/article/pii/001670377790268X
  26. https://www.cambridge.org/core/journals/mineralogical-magazine/article/synthetic-armalcolite-and-pseudobrookite/D1BA3B7D4FF84FAF4FA9A7247618EDB6
  27. https://pmc.ncbi.nlm.nih.gov/articles/PMC4813867/
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