spore: profile
id: magnesium-fluoride
spore_version: 0.2
family: optical-crystals
name: Magnesium fluoride (single-crystal MgF2)
abbreviation: MgF2
role: VUV and UV optical crystal, and protective overcoat for aluminium mirrors
category: glasses-optics
type: optical-crystals
summary: |
  Magnesium fluoride (MgF2) is a tetragonal optical crystal with a rutile structure. Crystran transmits it from 0.12 to 7 µm and says it passes the hydrogen Lyman-alpha line at 121 nm. Its density is 3.1766 g/cc at 25°C and its melting point is 1255°C on the Crystran data sheet, while the Crystran safety sheet prints 1248°C and PubChem prints 1263°C. NASA Goddard describes aluminium mirrors protected by an MgF2 layer, and the Hubble coating it cites uses a 25 nm MgF2 overcoat. The sources read give no space radiation or outgassing test for bulk MgF2.
best_for: Vacuum-ultraviolet and excimer-laser optics, and mirror overcoats that must pass the hydrogen Lyman-alpha line at 121 nm (Crystran; NASA Goddard)
weakest_at: 'Mechanical strength and cleavage: Crystran gives an apparent elastic limit of 49.6 MPa and says the crystal can cleave on the c-axis'
identity:
  formula: MgF2
  cas: 7783-40-6
  crystal_system: tetragonal
  space_group: P42/mnm (#136), rutile structure
  source: crystran-mgf2-datasheet
  where: Class/Structure row (Tetragonal P42/mnm (#136) Rutile Structure); CAS 7783-40-6 from crystran-mgf2-msds section 3.1 and PubChem CID 24546
tags:
  environments: []
  stresses: []
  applications:
    - optics-and-cameras
properties:
  - benchmark: transmission_range
    label: Transmission range
    value: 0.12
    value_high: 7
    bound: range
    unit: µm
    scope: material
    conditions:
      thickness: not stated on the page
    method: Range printed in the Crystran data table; not a canonical property name
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Transmission Range row (0.12 to 7µm(1))
  - benchmark: refractive_index
    label: Refractive index, ordinary ray at 0.22 µm
    value: 1.413
    unit: '-'
    scope: material
    conditions:
      wavelength: 0.22 µm
      ray: ordinary (No)
    method: Value printed in the Crystran data table, which cites Laporte et al. (reference 4)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Refractive Index row (No 1.413 at 0.22µm)
  - benchmark: refractive_index
    label: Refractive index, extraordinary ray at 0.220 µm
    value: 1.426
    unit: '-'
    scope: material
    conditions:
      wavelength: 0.220 µm
      ray: extraordinary (Ne)
    method: Value printed in the Crystran refractive index table
    status: ground
    source: crystran-mgf2-datasheet
    where: Refractive index table, row 0.220, Ne column
  - benchmark: refractive_index
    label: Refractive index, ordinary ray at 5.000 µm
    value: 1.334
    unit: '-'
    scope: material
    conditions:
      wavelength: 5.000 µm
      ray: ordinary (No)
    method: Value printed in the Crystran refractive index table
    status: ground
    source: crystran-mgf2-datasheet
    where: Refractive index table, row 5.000, No column
  - benchmark: reststrahlen_peak
    label: Reststrahlen peak
    value: 20
    unit: µm
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed in the Crystran data table; not a canonical property name
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Reststrahlen Peak row (20µm (1))
  - benchmark: density
    label: Density
    value: 3.1766
    unit: g/cm3
    scope: material
    conditions:
      temperature: 25 °C
      unit_in_source: g/cc
    method: Value printed on the Crystran data sheet
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Density row (3.1766g/cc at 25°C)
  - benchmark: density
    label: Density, safety sheet (specific gravity)
    value: 3.18
    unit: g/mL
    scope: material
    conditions:
      temperature: not stated on the page
    method: Printed as SPECIFIC GRAVITY in the Crystran safety sheet; a rounded value, and it differs from the data sheet
    status: ground
    source: crystran-mgf2-msds
    where: Section 9.1, SPECIFIC GRAVITY row
  - benchmark: melting_point
    label: Melting point, data sheet
    value: 1255
    unit: °C
    scope: material
    conditions:
      form: single crystal
    method: Value printed on the Crystran data sheet; the safety sheet prints a different value (see the next melting point row)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Melting Point row (1255°C)
  - benchmark: melting_point
    label: Melting point, safety sheet
    value: 1248
    unit: °C
    scope: material
    conditions:
      form: magnesium fluoride optical crystal, no temperature stated
    method: Value printed in the Crystran safety sheet, section 9.1
    status: ground
    source: crystran-mgf2-msds
    where: Section 9.1, MELTING POINT row (1248C)
  - benchmark: melting_point
    label: Melting point, experimental entry
    value: 1263
    unit: °C
    scope: material
    conditions:
      form: magnesium fluoride, no grade given
    method: Experimental property entry in the PubChem record for CID 24546
    status: ground
    source: pubchem-24546
    where: Chemical and Physical Properties > Experimental Properties > Melting Point (1263 °C)
  - benchmark: boiling_point
    label: Boiling point, safety sheet
    value: 2260
    unit: °C
    scope: material
    conditions:
      pressure: 760 mmHg
    method: Value printed in the Crystran safety sheet, section 9.1
    status: ground
    source: crystran-mgf2-msds
    where: Section 9.1, BOILING POINT (760mm Hg) row (2260C)
  - benchmark: boiling_point
    label: Boiling point, experimental entry
    value: 2227
    unit: °C
    scope: material
    conditions:
      pressure: 760 mmHg
    method: Experimental property entry in the PubChem record for CID 24546
    status: ground
    source: pubchem-24546
    where: Chemical and Physical Properties > Experimental Properties > Boiling Point (2227 °C @760 [mm Hg])
  - benchmark: thermal_conductivity
    label: Thermal conductivity, parallel to the optic axis
    value: 21
    unit: W/(m·K)
    scope: material
    conditions:
      orientation: parallel (para)
      temperature: 300 K
      unit_in_source: W m-1 K-1 at 300K
    method: Value printed on the Crystran data sheet, which cites Kashnow and McCarthy (reference 3)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Thermal Conductivity row (21 (para))
  - benchmark: thermal_conductivity
    label: Thermal conductivity, perpendicular to the optic axis
    value: 33.6
    unit: W/(m·K)
    scope: material
    conditions:
      orientation: perpendicular (perp)
      temperature: 300 K
      unit_in_source: W m-1 K-1 at 300K
    method: Value printed on the Crystran data sheet, which cites Kashnow and McCarthy (reference 3)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Thermal Conductivity row (33.6 (perp))
  - benchmark: specific_heat
    label: Specific heat capacity
    value: 1003
    unit: J/(kg·K)
    scope: material
    conditions:
      temperature: not stated on the page
      unit_in_source: J Kg m-1 K-1 as printed
    method: Value printed on the Crystran data sheet; the printed unit has an extra m and is read here as the usual J/(kg·K)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Specific Heat Capacity row
  - benchmark: dielectric_constant
    label: Dielectric constant, parallel to the optic axis
    value: 4.87
    unit: '-'
    scope: material
    conditions:
      orientation: parallel (para)
      frequency: 1 MHz
    method: Value printed on the Crystran data sheet, which cites reference 1 (Duncanson et al. 1958)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Dielectric Constant row (4.87 (para) 5.45 (perp) at 1MHz)
  - benchmark: dielectric_constant
    label: Dielectric constant, perpendicular to the optic axis
    value: 5.45
    unit: '-'
    scope: material
    conditions:
      orientation: perpendicular (perp)
      frequency: 1 MHz
    method: Value printed on the Crystran data sheet, which cites reference 1 (Duncanson et al. 1958)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Dielectric Constant row (5.45 (perp))
  - benchmark: elastic_modulus
    label: Young's modulus
    value: 138
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet, which cites Kandil et al. 1981 (reference 2)
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Youngs Modulus (E) row (138 GPa (2))
  - benchmark: shear_modulus
    label: Shear modulus
    value: 54.66
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet, which cites reference 2
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Shear Modulus (G) row (54.66GPa (2))
  - benchmark: bulk_modulus
    label: Bulk modulus
    value: 101.32
    unit: GPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet, which cites reference 2
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Bulk Modulus (K) row (101.32 GPa (2))
  - benchmark: poissons_ratio
    label: Poisson ratio
    value: 0.276
    unit: '-'
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet, which cites reference 2
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Poisson Ratio row (0.276 (2))
  - benchmark: apparent_elastic_limit
    label: Apparent elastic limit
    value: 49.6
    unit: MPa
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet (7200 psi also printed); not a canonical property name
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Apparent Elastic Limit row (49.6 MPa (7200 psi))
  - benchmark: water_solubility
    label: Solubility in water
    value: 0.0002
    unit: g/100 g water
    scope: material
    conditions:
      temperature: not stated on the page
    method: Value printed on the Crystran data sheet; not a canonical property name
    status: ground
    source: crystran-mgf2-datasheet
    where: Data table, Solubility row (0.0002g/100g water)
  - benchmark: uv_cutoff_wavelength
    label: Short-wavelength cutoff, typical deposited MgF2 thin film (not the Crystran crystal)
    value: 115
    unit: nm
    scope: material
    conditions:
      form: deposited thin film, as a typical value in the JPL table
      band_energy_in_source: 10 – 11 eV
    method: Value printed in a JPL table of typical deposited thin films for UV optical applications; it is not a measurement of the Crystran crystal and is recorded only as a thin-film reference
    status: ground
    source: jpl-balasubramanian-2016
    where: Protective layers slide, table Properties of Typical Deposited Thin Films for UV Optical Applications, magnesium fluoride row (~λ Cut Off (nm) 115)
corrosion:
  - environment: Strong mineral acids
    headline: Magnesium fluoride reacts with strong mineral acids, and the decomposition product is hydrogen fluoride gas.
    effect: The Crystran safety sheet lists strong mineral acids as reacting with the material and as incompatible, and it names hydrogen fluoride gas as the decomposition product. No rate or concentration is printed, so the rating is qualitative.
    rating: moderate
    conditions:
      basis: qualitative reactivity statement; no rate or concentration printed
      agent: strong mineral acids
    source: crystran-mgf2-msds
    where: Sections 10.1, 10.5 and 10.6
  - environment: Water, room temperature
    headline: Magnesium fluoride is practically insoluble in water.
    effect: The data sheet gives a solubility of 0.0002 g per 100 g of water, and the safety sheet calls the material practically insoluble (section 9.1). No dissolution or corrosion test is printed.
    rating: low
    conditions:
      note: solubility figure, not a corrosion rate; temperature not stated
    source: crystran-mgf2-datasheet
    where: Data table, Solubility row; crystran-mgf2-msds section 9.1
  - environment: Storage and handling of aluminium mirrors overcoated with MgF2
    headline: An aluminium mirror with an MgF2 overcoat, made in 2011, fell to about 65% average reflectance in the 110 to 170 nm range after poor handling.
    effect: NASA Goddard says plasma treatment restored that sample to a pristine value of 85% in the 110 to 170 nm range, and its summary gives 84% in the 115 to 170 nm range for a six-year-old sample. The paper does not separate MgF2 ageing from handling damage.
    rating: moderate
    conditions:
      basis: reflectance of an aluminium mirror with an MgF2 overcoat, not of bulk MgF2
      period: six years
    source: nasa-quijada-2018
    where: Section 4.1 (Al+MgF2 paragraph) and the closing summary paragraph
risks:
  hazards:
    - text: The Crystran safety sheet (revision 2026, issued 6 July 2026) gives H315 (causes skin irritation), H319 (causes serious eye irritation) and H332 (harmful if inhaled), with the signal word Warning.
      ghs:
        - H315
        - H319
        - H332
      source: crystran-mgf2-msds
      where: Sections 2.1 and 2.2
    - text: PubChem's aggregated GHS summary for magnesium fluoride lists H315, H319 and H335 (May cause respiratory irritation) from ECHA notifications, and notes that the chemical does not meet GHS hazard criteria in 168 of 498 company reports.
      ghs:
        - H315
        - H319
        - H335
      source: pubchem-24546
      where: Safety and Hazards > Hazards Identification > GHS Classification
    - text: The same safety sheet says the material is toxic by ingestion and by inhalation of dust, with a cumulative effect, that it affects the nervous system, and that its oral LD50 in rats is above 2330 mg/kg.
      ghs: []
      source: crystran-mgf2-msds
      where: Section 11.1
    - text: The occupational exposure limit is 2.5 mg/m3 long term, expressed as fluoride, in both the Crystran safety sheet and PubChem.
      ghs: []
      source: crystran-mgf2-msds
      where: Section 8.1, OEL row; PubChem gives the same 2.5 mg/m3 as F in its Permissible Exposure Limit row
    - text: Strong mineral acids release hydrogen fluoride gas from this material, according to the safety sheet decomposition statement.
      ghs: []
      source: crystran-mgf2-msds
      where: Section 10.6
  failure_risks:
    - mode: Optic-axis orientation (slight birefringence)
      trigger: Window cut with the optic axis at an angle to the faces
      consequence: The data sheet says the material is slightly birefringent and is usually supplied with the optic axis perpendicular to the window faces, so a window cut another way changes the polarisation of light.
      severity: low
      source: crystran-mgf2-datasheet
      where: Introduction text, first paragraph
    - mode: Cleavage on the c-axis
      trigger: Mechanical stress or impact on a crystal or polished piece
      consequence: The data sheet says the crystal can cleave on the c-axis, but not easily, and gives an apparent elastic limit of 49.6 MPa.
      severity: low
      source: crystran-mgf2-datasheet
      where: Class/Structure row; Apparent Elastic Limit row
    - mode: Reflectance loss of an overcoated aluminium mirror
      trigger: Long storage or poor handling of an Al+MgF2 mirror
      consequence: NASA Goddard reports an aluminium mirror with an MgF2 overcoat that fell to about 65% average reflectance over time, and plasma treatment restored it.
      severity: moderate
      source: nasa-quijada-2018
      where: Section 4.1
  side_effects: []
  handling:
    - text: Wear PVA gloves, a laboratory coat and safety goggles or glasses where chipping or dust may occur; provide general mechanical and local exhaust ventilation; wash hands after handling.
      source: crystran-mgf2-msds
      where: Section 8.1, Exposure controls
    - text: Avoid generating dust, and take up broken pieces into containers for disposal.
      source: crystran-mgf2-msds
      where: Sections 6.1, 6.3 and 7.1
    - text: Keep away from foodstuffs and strong acids.
      source: crystran-mgf2-msds
      where: Section 7.2
  mitigations:
    - text: Specify the optic-axis orientation when ordering windows; the usual cut has the optic axis perpendicular to the faces.
      source: crystran-mgf2-datasheet
      where: Introduction text, first paragraph
    - text: Crystran normally cuts and orients windows with the c-axis normal to the surface, which is said to give minimum birefringence.
      source: crystran-mgf2-web
      where: Product page, text under the Applications paragraph
uses:
  - application: optics-and-cameras
    device: UV optics and excimer-laser windows
    example: Crystran says magnesium fluoride is used mostly for UV optics and is excellent for excimer laser applications.
    source: crystran-mgf2-datasheet
    where: Applications text, first paragraph
  - application: optics-and-cameras
    device: Aluminium mirror overcoat for space ultraviolet instruments
    example: NASA Goddard describes aluminium mirrors protected by an MgF2 overcoat, and gives the pre-launch Hubble (HST) coating as an aluminium layer with an MgF2 overcoat of 25 nm.
    source: nasa-quijada-2018
    where: Section 2.1, paragraph after the Figure 1 reference
  - application: optics-and-cameras
    device: Typical deposited metal-fluoride UV coating
    example: A JPL table of typical deposited thin films for UV optical applications lists magnesium fluoride with a cutoff near 115 nm.
    source: jpl-balasubramanian-2016
    where: Protective layers slide, Properties of Typical Deposited Thin Films table
supply:
  - subject: Fluorine feedstock, fluorspar (the mineral form of calcium fluoride)
    abundance: 'USGS: world mine production of fluorspar was 10,000 thousand tons in 2025, and world reserves are 330,000 thousand tons, of which China holds 110,000 thousand tons. The USGS chapter says aqueous HF is the primary feedstock for virtually all fluorine-bearing chemicals, but it does not name magnesium fluoride.'
    producers:
      - country: China
        share_percent: 60
        year: 2025
      - country: Mongolia
        share_percent: 15
        year: 2025
      - country: Mexico
        share_percent: 15
        year: 2025
      - country: South Africa
        share_percent: 4.1
        year: 2025
    critical_lists:
      - list: USGS 2025 final critical minerals list, as summarised by CRS report IF13145 (fluorspar listed)
        listed: true
    recycling: 'USGS: synthetic fluorspar may be produced from waste neutralisation in uranium enrichment, petroleum alkylation and stainless-steel pickling, but undesirable impurities constrain its use.'
    substitutes:
      - 'USGS: fluosilicic acid has been used as an alternative to fluorspar in the production of AlF3 and HF.'
    source: usgs-mcs2026-fluorspar
    where: World Mine Production and Reserves table, 2025 column (China 6,000; Mongolia 1,500; Mexico 1,500; South Africa 410; world total 10,000 thousand tons). Shares are calculated as country value divided by the printed world total. Recycling and Substitutes text.
  - subject: Magnesium feedstock (magnesium compounds)
    abundance: 'USGS: most U.S. magnesium compounds were produced from seawater and natural brines in 2025. World magnesite mine production was 21,000 thousand tons in 2025, with China at 12,700 thousand tons. USGS prints no production figure for magnesium fluoride.'
    producers: []
    critical_lists: []
    substitutes: []
    source: usgs-mcs2026-magnesium-compounds
    where: Domestic Production and Use paragraph; World Magnesite Mine Production table, 2025 column
documents:
  - title: 'Crystran: Magnesium Fluoride (MgF2) materials data sheet (undated PDF, data table and refractive index table)'
    type: datasheet
    url: https://www.crystran.com/wp-content/uploads/2025/09/magnesium-fluoride-mgf2-data-sheet.pdf
    year: 2026
  - title: 'Crystran: Safety Data Sheet, Magnesium Fluoride Optical Crystal (revision 2026, issued 6 July 2026)'
    type: sds
    url: https://www.crystran.com/wp-content/uploads/2025/09/magnesium-fluoride-mgf2-msds.pdf
    year: 2026
  - title: 'PubChem: Magnesium fluoride, CID 24546, compound record (pug_view JSON)'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/24546/JSON
    year: 2026
  - title: 'Quijada et al.: E-beam generated plasma etching for developing high-reflectance mirrors for far-ultraviolet astronomical instrument applications (NASA NTRS 20180005725)'
    type: paper
    url: https://ntrs.nasa.gov/api/citations/20180005725/downloads/20180005725.pdf
    year: 2018
  - title: 'Balasubramanian (JPL): Mirror Coatings for large aperture UV to IR Telescopes, HabEx Technology Meeting, 3 August 2016'
    type: report
    url: https://www.jpl.nasa.gov/habex/documents/Aug2016/Balasubramanian-Mirror-Coatings-Technology.pdf
    year: 2016
  - title: 'USGS Mineral Commodity Summaries 2026: Fluorspar'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-fluorspar.pdf
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Magnesium Compounds'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-magnesium-compounds.pdf
    year: 2026
  - title: 'Congressional Research Service: U.S. Geological Survey''s Critical Minerals List (IF13145)'
    type: report
    url: https://www.everycrsreport.com/reports/IF13145.html
    year: 2026
gaps:
  - Thermal expansion is printed as 13.7 (para) and 8.9 (perp) times 10 to the minus 6 per K. Because the printed form uses an exponent, these values are left out of the record.
  - The absorption coefficient is printed with a 10 to the minus 3 exponent, so it is left out.
  - Refractive index change with temperature (dn/dT) is printed as 2.3 (para) and 1.7 (perp) at 0.4 µm without a unit, so it is not entered.
  - Knoop hardness is printed as 415 without a unit, so it is not entered.
  - The molecular weight is printed as 62.32 without a unit on the data sheet, and PubChem gives 62.302 without a unit, so neither is entered.
  - PubChem's experimental density of 3.148 at 25 °C has no unit printed, so it is not entered.
  - 'The three melting points differ: 1255 °C on the data sheet, 1248 °C on the safety sheet and 1263 °C in PubChem. All three are recorded, and the data sheet value is used for the headline.'
  - No outgassing (TML or CVCM), ground radiation, UV ageing or atomic-oxygen result for bulk MgF2 was found in a source we could open.
  - A 2011 mirror result is the only ageing data read from a fetched source. A search summary mentions a 30-month humidity study of Al+MgF2 mirrors, but its full text was not opened, so its figures are not used.
  - The 115 nm cutoff is a typical value for a deposited thin film in the JPL table. No cutoff for the Crystran crystal itself is printed beyond the 0.12 µm transmission limit.
  - The data sheet prints the specific heat unit as J Kg m-1 K-1 and gives no temperature. The value is recorded with the unit read as J/(kg·K), and this is noted in the method.
  - The fluorspar and magnesium compound USGS chapters do not name magnesium fluoride, so no MgF2 production statistic was found. The supply shares are for fluorspar, the fluorine feedstock, and are calculated from printed totals.
  - Only Crystran's sheets were read for the grade. No other supplier's data sheet or safety sheet was opened.
  - PubChem's H335 comes from aggregated ECHA notifications. The Crystran safety sheet does not list H335, so the two hazard lists are shown side by side and not merged.
  - PubChem lists Irtran 1 as a depositor-supplied synonym. Neither the Crystran sheets nor the Crystran page print it, and the verify script cannot read it from the PubChem JSON page, so no designation is entered.
  - No media (photograph or drawing with a licence) was checked for this profile.
license: CC-BY-4.0
updated: '2026-11-01'
review:
  status: draft
  method: assistant
  checked_values: 24
  unreachable: 0
  caveats: 4
  note: 'Written by an AI assistant on 2026-11-01 from the Crystran data sheet and safety sheet (revision 2026), PubChem CID 24546, the NASA Goddard 2018 paper on FUV mirror coatings, the JPL 2016 HabEx coating slides, and the USGS 2026 fluorspar and magnesium compounds chapters, with the CRS IF13145 summary for the critical list. Caveats: the MgF2 record is for one supplier''s crystal; the melting and boiling points disagree across sources; the 115 nm cutoff is a thin-film value; thermal expansion and the absorption coefficient are left out because they are printed with exponents.'
sources:
  crystran-mgf2-datasheet:
    title: 'Crystran: Magnesium Fluoride (MgF2) materials data sheet (data table, refractive index table, transmission curves)'
    id: magnesium-fluoride-mgf2-data-sheet.pdf (undated; pages 68 and 69)
    url: https://www.crystran.com/wp-content/uploads/2025/09/magnesium-fluoride-mgf2-data-sheet.pdf
    year: 2026
    kind: datasheet
    license: supplier document, linked not reproduced
  crystran-mgf2-msds:
    title: 'Crystran: Safety Data Sheet, Magnesium Fluoride Optical Crystal (REACH format, revision 2026)'
    id: Revision 2026, issued 6 July 2026; CAS 7783-40-6; EC 231-995-1
    url: https://www.crystran.com/wp-content/uploads/2025/09/magnesium-fluoride-mgf2-msds.pdf
    year: 2026
    kind: datasheet
    license: supplier document, linked not reproduced
  crystran-mgf2-web:
    title: 'Crystran: Magnesium Fluoride (MgF2) product page'
    id: product page; the Lyman-alpha, excimer and Stockbarger text is read here
    url: https://www.crystran.com/optical-materials/magnesium-fluoride-mgf2
    year: 2026
    kind: web
    license: supplier web page, linked not reproduced
  pubchem-24546:
    title: PubChem compound record, Magnesium fluoride, CID 24546
    id: CID 24546; F2Mg; CAS 7783-40-6; EC 231-995-1; molecular weight 62.302 (computed)
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/24546/JSON
    year: 2026
    kind: database
    license: public domain (NCBI)
  nasa-quijada-2018:
    title: 'Quijada, Boris, del Hoyo, Wollack, Kozen, Walton and Dwivedi: E-beam generated plasma etching for developing high-reflectance mirrors for far-ultraviolet astronomical instrument applications (NASA GSFC and NRL)'
    id: NASA NTRS citation 20180005725
    url: https://ntrs.nasa.gov/api/citations/20180005725/downloads/20180005725.pdf
    year: 2018
    kind: paper
    license: NASA public document
  jpl-balasubramanian-2016:
    title: 'Balasubramanian (JPL / Caltech): Mirror Coatings for large aperture UV to IR Telescopes, HabEx Technology Meeting slides'
    id: JPL HabEx Technology Meeting, 3 August 2016
    url: https://www.jpl.nasa.gov/habex/documents/Aug2016/Balasubramanian-Mirror-Coatings-Technology.pdf
    year: 2016
    kind: report
    license: JPL slides, linked not reproduced
  usgs-mcs2026-fluorspar:
    title: Mineral Commodity Summaries 2026, Fluorspar (USGS, Vanessa Londono)
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-fluorspar.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-mcs2026-magnesium-compounds:
    title: Mineral Commodity Summaries 2026, Magnesium Compounds (USGS, Vanessa Londono)
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-magnesium-compounds.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  crs-if13145:
    title: 'Congressional Research Service: U.S. Geological Survey''s Critical Minerals List (IF13145)'
    id: IF13145; 2025 final list published 7 November 2025
    url: https://www.everycrsreport.com/reports/IF13145.html
    year: 2026
    kind: report
    license: CRS report, linked not reproduced
story:
  recipe:
    line: Magnesium fluoride is the compound MgF2, with one magnesium atom and two fluorine atoms per formula unit. Crystran grows it as a single crystal by the vacuum Stockbarger technique in ingots of various diameters, and the crystal has a tetragonal rutile structure.
    parts:
      - name: Magnesium
        symbol: Mg
        role: The metal. One atom per formula unit, from the formula MgF2 (PubChem lists the molecular formula as F2Mg).
        source: pubchem-24546
        where: Names and Identifiers > Molecular Formula (F2Mg)
      - name: Fluorine
        symbol: F
        role: The halogen. Two atoms per formula unit, from the same formula.
        source: pubchem-24546
        where: Names and Identifiers > Molecular Formula (F2Mg)
  risk_groups:
    - title: For people
      level: 1
      verdict: Harmful if inhaled; irritating to skin and eyes
      lead: The Crystran safety sheet (revision 2026) gives H315, H319 and H332 with the signal word Warning. PubChem's aggregated summary also lists H335.
      points:
        - label: Dust
          line: The sheet gives an occupational limit of 2.5 mg/m3 long term, expressed as fluoride, and says to avoid generating dust.
        - label: Toxicity
          line: The sheet says the material is toxic by ingestion and by inhalation of dust, with a cumulative effect, and gives an oral LD50 above 2330 mg/kg in rats.
    - title: If it fails
      lead: The failure results come from the Crystran data sheet and the NASA mirror paper. No flight failure record was read.
      points:
        - label: Cleavage
          line: The data sheet says the crystal can cleave on the c-axis, but not easily.
        - label: Mirror ageing
          line: An aluminium mirror with an MgF2 overcoat, made in 2011, fell to about 65% average reflectance in the 110 to 170 nm range after poor handling. Plasma treatment restored it to 85% (NASA Goddard).
    - title: Handling and storage
      gap: The sheets give no storage temperature or humidity limit.
      points:
        - label: Protective equipment
          line: PVA gloves, a laboratory coat, safety goggles or glasses where chipping or dust may occur, and mechanical and local exhaust ventilation.
        - label: Incompatible materials
          line: Strong mineral acids react with the material and release hydrogen fluoride gas, according to the safety sheet.
  why: Magnesium fluoride is a tetragonal optical crystal that transmits from 0.12 to 7 µm, with a refractive index of 1.413 at 0.22 µm (Crystran). It passes the hydrogen Lyman-alpha line at 121 nm, and NASA Goddard uses an MgF2 overcoat to keep aluminium far-ultraviolet mirrors reflective. The Hubble coating it cites has a 25 nm MgF2 overcoat.
  uses:
    - where: UV optics and excimer lasers
      line: Crystran says magnesium fluoride is used mostly for UV optics and is excellent for excimer laser applications.
    - where: Space telescope mirror overcoats
      line: NASA Goddard's 2018 paper gives the pre-launch Hubble (HST) coating as an aluminium layer with a 25 nm MgF2 overcoat, and lists GSFC coating work on COSTAR, GHRS, STIS and COS and optics for FUSE.
    - where: Lyman-alpha optics
      line: Crystran says the crystal transmits well into the VUV to the hydrogen Lyman-alpha line at 121 nm and beyond.
  can_do:
    - label: Range
      line: Crystran's data table gives a transmission range of 0.12 to 7 µm.
      benchmark: transmission_range
    - label: Refraction
      line: Refractive index 1.413 for the ordinary ray at 0.22 µm, and 1.334 for the ordinary ray at 5.000 µm, per the Crystran refractive index table.
      benchmark: refractive_index
    - label: Heat
      line: Thermal conductivity is 21 W/(m·K) parallel and 33.6 W/(m·K) perpendicular to the optic axis at 300 K, per the Crystran data table.
      benchmark: thermal_conductivity
    - label: Stiffness
      line: Young's modulus 138 GPa and bulk modulus 101.32 GPa, per the Crystran data table.
      benchmark: elastic_modulus
    - label: Mirror reflectance
      line: NASA Goddard reports that plasma treatment restored an aged Al+MgF2 mirror to 85% average reflectance in the 110 to 170 nm range.
  compare: The JPL 2016 HabEx slides list magnesium fluoride among typical deposited thin films for UV optical applications, with a cutoff near 115 nm, and place it between aluminium fluoride and calcium fluoride in their table. That table gives thin-film values, not bulk crystal values. For far-ultraviolet mirrors, NASA Goddard works with MgF2 and LiF overcoats and with AlF3 passivation, and its 2018 paper extends the work to AlF3. The sources read give no side-by-side ground or space test of the three fluorides, and the melting points of magnesium fluoride differ between its own sheets and PubChem.
  compare_rows:
    - label: Heat conduction
      benchmark: thermal_conductivity
    - label: Density
      benchmark: density
    - label: Stiffness
      benchmark: elastic_modulus
  handling:
    - Wear PVA gloves, a laboratory coat and safety goggles or glasses where chipping or dust may occur, and use general and local exhaust ventilation (safety sheet section 8.1).
    - Avoid generating dust, and take up broken pieces into containers for disposal (safety sheet sections 6.1 and 6.3).
    - Keep away from foodstuffs and strong acids (safety sheet section 7.2).
  good_for:
    - Vacuum-ultraviolet and excimer-laser optics, using the highest-purity grades (Crystran).
    - Protective overcoats on aluminium far-ultraviolet mirrors, where the MgF2 layer stops the aluminium oxidising (NASA Goddard).
    - Optics in water-exposed settings where low solubility matters, with a solubility of 0.0002 g per 100 g of water (Crystran).
  watch_out:
    - Optic axis. The crystal is slightly birefringent, so windows are usually cut with the optic axis perpendicular to the faces.
    - Acids. Strong mineral acids react with magnesium fluoride and release hydrogen fluoride gas.
    - Mirror ageing. An aluminium mirror with an MgF2 overcoat fell to about 65% reflectance after poor handling and was restored by plasma treatment (NASA Goddard).
    - No space test. No radiation, outgassing or atomic-oxygen result for bulk magnesium fluoride was found in the sources read.
  verdict: Magnesium fluoride is a tetragonal crystal that transmits into the vacuum ultraviolet, and Crystran supplies it for UV and excimer-laser optics. NASA Goddard's coating work shows that MgF2 overcoats protect aluminium far-ultraviolet mirrors, but the sources read give no space test of bulk MgF2 itself.