Vibrational Investigation of Pressure-Induced Phase Transitions of Hydroxycarbonate Malachite Cu2(CO3)(OH)2

At a Glance

Metadata	Details
Publication Date	2020-03-19
Journal	Minerals
Authors	Jing Gao, Xueyin Yuan
Institutions	Chinese Academy of Sciences, Chinese Academy of Geological Sciences
Citations	12
Analysis	Full AI Review Included

Technical Documentation & Analysis: High-Pressure Vibrational Studies Using MPCVD Diamond Anvils

Document Prepared for: High-Pressure Geophysics and Materials Science Divisions Subject: Analysis of “Vibrational Investigation of Pressure-Induced Phase Transitions of Hydroxycarbonate Malachite $\text{Cu}_2(\text{CO}_3)(\text{OH})_2$“

Executive Summary

This research successfully leverages the mechanical and optical superiority of high-quality diamond anvils to probe the stability of the hydroxycarbonate mineral malachite ($\text{Cu}_2(\text{CO}_3)(\text{OH})_2$) under deep Earth mantle conditions. The findings critically depend on the structural integrity and spectroscopic transparency of the Diamond Anvil Cell (DAC) used.

Application: In-situ Raman and Infrared (IR) spectroscopy of malachite up to ~29 GPa at $25\text{ °C}$ (Room Temperature, RT).
Core Diamond Requirement: The experiment necessitated the use of high-purity, optically superior Type-II diamond anvils (300 µm culet) to ensure minimal absorption and maximize light throughput for vibrational analysis.
Key Achievement: Identification of three distinct pressure-induced phase transitions at ~7 GPa, ~15 GPa, and ~23 GPa, marked by discontinuities in vibrational mode shifting.
Geophysical Significance: The stability of hydrogen bonding in malachite is confirmed up to ~23 GPa, supporting the hypothesis that hydroxycarbonates can transport water and carbon deep into the Earth’s transition zone.
Structural Mechanism: Phase transitions are driven by increasing deformation of the $[\text{CuO}_6]$ octahedron and subsequent reordering/reorientation of hydrogen bonds and $[\text{CO}_3]^{2-}$ units.
6CCVD Value Proposition: The extreme GPa requirements validate the demand for custom, high-optical-grade Single Crystal Diamond (SCD) material, a core specialty of $\text{6CCVD}$, for replicating and extending this critical geoscience research.

Technical Specifications

The following parameters define the operational envelope and specific findings of the high-pressure spectroscopic investigation:

Parameter	Value	Unit	Context
Maximum Applied Pressure	~29.2	GPa	Limit of compression tested in Raman/IR
Temperature	25	°C	Room temperature (RT) conditions
Diamond Anvil Type Used	Type-II	N/A	Specified for high optical transparency
Diamond Culet Diameter	300	µm	Defined working area of the DAC
Sample Chamber Diameter	150	µm	Drilled into Rhenium gasket
Pressure Transmitting Medium (Raman)	Silicone oil	N/A	Used for hydrostatic pressure
Pressure Transmitting Medium (IR)	Dried KBr layers	N/A	Used for hydrostatic pressure and as IR window
First Phase Transition	~7	GPa	$[\text{CuO}_6]$ octahedron deformation and rotation
Second Phase Transition	~15	GPa	Hydrogen bonding reordering; new mode development
Third Phase Transition	~23	GPa	$\text{O-C-O}$ softening and $\text{O-H}$ stretch blueshift
Raman Excitation Source	488	nm	Solid-state continuous-wave laser
Raman Spectral Resolution	$1.105$	$\text{cm}^{-1}$	High resolution required for mode splitting analysis
FTIR Spectral Range Monitored	$650-5000$	$\text{cm}^{-1}$	Encompasses $\text{O-H}$ stretches and $[\text{CO}_3]^{2-}$ modes
FTIR Resolution	4	$\text{cm}^{-1}$	Signal acquisition quality

Key Methodologies

The experiment relied on precise sample preparation and measurement calibration to achieve accurate in-situ spectroscopic data under ultra-high pressure:

DAC Assembly: A symmetrical-type DAC was assembled using Type-II diamond anvils (300 µm culet).
Gasket and Chamber Preparation: A Rhenium gasket (250 µm initial thickness) was pre-indented to 50 µm. A 150 µm hole was drilled via electro-spark erosion to serve as the sample chamber.
Sample Loading (Raman): A malachite chip was loaded into the chamber with silicone oil serving as the pressure-transmitting medium (PTM).
Sample Loading (Infrared): A thin 10 µm malachite chip was sandwiched between dried KBr layers, which acted as both the PTM and the infrared window.
Pressure Calibration: Pressure was monitored by measuring the shifts of ruby fluorescence lines.
Raman Data Acquisition: Spectra were collected using a $488\text{ nm}$ laser ($\text{20 mW}$ power) in backscattering geometry. A resolution of $1.105\text{ cm}^{-1}$ was achieved using a $300\text{ g/mm}$ grating.
FTIR Data Acquisition: Measurements were performed using a Bruker VERTEX 70V instrument and Hyperion 2000 microscope in a $20 \times 20\text{ µm}^2$ aperture. Each spectrum accumulated 640 scans to ensure a high signal-to-noise ratio at $4\text{ cm}^{-1}$ resolution.
Data Processing: Spectra were fitted using a Voight function via Jandel Peakfit software, requiring squared correlations ($R^2$) $\ge 0.995$.

6CCVD Solutions & Capabilities

This research underscores the critical dependence of high-pressure geoscience and physics experiments on the highest quality diamond materials. The use of Type-II diamond anvils (known for purity and optical transmission) up to nearly 30 GPa directly aligns with 6CCVD’s core strengths in custom, high-purity Single Crystal Diamond (SCD) manufacturing.

Application Requirement (From Paper)	6CCVD Solution & Capability	Engineering Advantage for Researcher
Ultra-High Pressure Capacity (> 29 GPa)	SCD Substrates & Anvil Blanks (Up to 10 mm): Our MPCVD SCD offers exceptional hardness and high yield strength, crucial for maintaining structural integrity under extreme stress.	Maximize the experimental pressure ceiling and increase the lifespan of DAC components in demanding deep Earth simulations.
Optical Clarity (Type-II requirement)	Optical Grade SCD: $\text{6CCVD}$ specializes in growth techniques yielding SCD wafers with ultra-low nitrogen incorporation (equivalent to or better than Type-II), ensuring superior transparency across the visible, IR, and UV spectra.	Guaranteed minimal parasitic absorption or photoluminescence, critical for sensitive Raman and FTIR measurements of low-energy mode shifts (e.g., $\text{Cu-O}$ vibrations).
Precision Shaping (300 µm Culet)	Custom Dimensions & Laser Shaping: $\text{6CCVD}$ provides custom polishing, cleaving, and precision laser cutting services. We can produce custom culet sizes, bevels, and specialized dimensions required for varied DAC designs.	Achieve the precise geometry necessary for uniform pressure distribution and accurate alignment within the DAC apparatus.
Sensitive Vibrational Analysis	Surface Polishing ($\text{Ra} < 1\text{ nm}$ SCD): Ultra-smooth, low-defect surfaces minimize light scattering and crystal strain effects, which can interfere with the analysis of small frequency shifts ($d\nu/dP$).	Obtain sharper spectroscopic peaks and cleaner background signals, improving the confidence ($R^2$) in identifying phase transition boundaries.
Replicating Mineral Chemistry	Boron-Doped Diamond (BDD) Capabilities: While malachite is a hydroxycarbonate, $\text{6CCVD}$’s ability to produce highly conductive, heavily BDD thin films and wafers is essential for high-pressure electrochemistry and sensing applications that often accompany geoscience studies.	Extend research beyond optics into electrical characterization of deep Earth materials, complementing spectroscopic analysis.

Engineering Support

$\text{6CCVD}$’s in-house $\text{PhD}$ materials science team possesses extensive expertise in diamond properties under extreme conditions. We are ready to assist researchers with material selection, dimension optimization, and crystal orientation choices required for similar high-pressure geophysical projects focusing on hydrogen and carbon stability in the mantle transition zone.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available).

View Original Abstract

Malachite Cu2(CO3)(OH)2 is a common hydroxycarbonate that contains about 15.3 wt % H2O. Its structural chemistry sheds light on other hydroxyl minerals that play a role in the water recycling of our planet. Here using Raman and infrared spectroscopy measurements, we studied the vibrational characteristics and structural evolution of malachite in a diamond anvil cell at room temperature (25 °C) up to ~29 GPa. Three types of vibrations were analyzed including Cu-O vibrations (300-600 cm−1), [CO3]2− vibrations (700-1600 cm−1), and O-H stretches (3200-3500 cm−1). We present novel observations of mode discontinuities at pressures of ~7, ~15, and ~23 GPa, suggesting three phase transitions, respectively. First, pressure has a great effect on the degree of deformation of the [CuO6] octahedron, as is manifested by the various shifting slopes of the Cu-O modes. [CuO6] deformation results in a rotation of the structural unit and accordingly a phase transition at ~7 GPa. Upon compression to ~15 GPa, the O-H bands redshift progressively with significant broadness, indicative of an enhancement of the hydrogen bonding, a shortening of the O···O distance, and possibly somewhat of a desymmetrization of the O-H···O bond. O-H mode hardening is identified above ~15 GPa coupled with a growth in the amplitude of the lower-energy bands. These observations can be interpreted as some reorientation or reordering of the hydrogen bonding. A further increment of pressure leads to a change in the overall compression mechanism of the structure at ~23 GPa, which is characterized by the blueshift of the O-H stretches and the softening of the O-C-O in-plane bending bands. The hydrogen bonding weakens due to a substantial enhancement of the Cu-H repulsion effect, and the O···O bond length shows no further shortening. In addition, the change in the local geometry of hydrogen is also induced by the softening of the [CO3]2− units. In this regard we may expect malachite and other analogous hydroxyl minerals as capable of transporting water downward towards the Earth’s transition zone (~23 GPa). Our results furnish our knowledge on the chemistry of hydrogen bonding at mantle conditions and open a new window in understanding the synergistic relations of water and carbon recycling in the deep Earth.

Tech Support

Original Source

DOI: https://doi.org/10.3390/min10030277

References

2012 - H2O storage capacity of olivine at 5-8 GPa and consequences for dehydration partial melting of the upper mantle [Crossref]
1994 - Seismic evidence for silicate melt atop the 410-km mantle discontinuity [Crossref]
2013 - Seismic evidence for subduction-transported water in the lower mantle. Earth’s Deep Water Cycle
1999 - The stability of dense hydrous magnesium silicates in Earth’s transition zone and lower mantle. Mantle Petrology: Field observations and high pressure experimentation: A tribute to francis R.(Joe) Boyd
2014 - Stability of hydrous silicate at high pressures and water transport to the deep lower mantle [Crossref]
2004 - Water transport into the deep mantle and formation of a hydrous transition zone [Crossref]
1967 - High-pressure reconnaissance investigations in the system Mg2SiO4-MgO-H2O [Crossref]
2014 - Melting of phase D in the lower mantle and implications for recycling and storage of H2O in the deep mantle [Crossref]
2001 - Stability field of new hydrous phase, δ-AlOOH, with implications for water transport into the deep mantle [Crossref]
2014 - Stability of a hydrous δ-phase, AlOOH-MgSiO2(OH)2, and a mechanism for water transport into the base of lower mantle [Crossref]