Optical calcite (Iceland spar, CaCO3)

spore: profile
id: calcite
spore_version: 0.2
family: carbonate
name: Optical calcite (Iceland spar, CaCO3)
role: birefringent polariser crystal
category: glasses-optics
type: optical-crystals
summary: |
  Optical calcite is a transparent, naturally occurring crystal form of calcium carbonate (CaCO3). It is strongly birefringent: a 2026 paper in Ukrainian Journal of Physics gives a birefringence of approximately 0.172 at 589 nm, so its ordinary and extraordinary rays separate strongly. That property has made it a classic material for polarisers and retardation plates.
best_for: Polarisers and retardation plates, where a large birefringence matters more than hardness or acid resistance
weakest_at: Low hardness (Mohs hardness of 3) and dissolving in dilute acids
identity:
  formula: CaCO3
  cas: 471-34-1
  crystal_system: trigonal
  space_group: R-3c
  source: apreotesei-2026
  where: Section 1 (Introduction), paragraph on crystal structure. The CAS number is printed for calcium carbonate in the Sigma SDS (sigma-c6033-sds), not on the calcite pages.
  designations:
    - system: Trade name
      code: Iceland spar
      source: wikipedia-calcite
      where: 'Uses paragraph: ''A transparent variety called Iceland spar is used for optical purposes'''
tags:
  environments: []
  stresses: []
  applications:
    - optics-and-cameras
properties:
  - benchmark: density
    label: Density of optical calcite
    value: 2.71
    unit: g/cm3
    scope: material
    conditions:
      basis: crystal; temperature not stated on the page
    method: Value stated in the 2026 paper, which cites earlier work for it
    status: ground
    source: apreotesei-2026
    where: Section 1 (Introduction), paragraph on physicochemical properties
  - benchmark: density
    label: Relative density of calcium carbonate powder (Sigma C6033)
    value: 2.93
    unit: g/cm3
    scope: material
    conditions:
      form: powder, not an optical-grade crystal
      temperature: 25 °C (77 °F), as printed
    method: Safety data sheet value, as printed
    status: ground
    source: sigma-c6033-sds
    where: Section 9.1 (Physical and chemical properties), item m
  - benchmark: melting_point
    label: Melting point of optical calcite
    value: 825
    unit: °C
    scope: material
    conditions:
      note: printed as 'Decomposes'; the page gives no separate decomposition temperature
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Melting point'
  - benchmark: melting_point
    label: Melting point of calcium carbonate powder (Sigma C6033)
    value: 800
    unit: °C
    scope: material
    conditions:
      form: powder, not an optical-grade crystal
      note: printed as 'Decomposes on heating'
    method: Safety data sheet value, as printed
    status: ground
    source: sigma-c6033-sds
    where: Section 9.1 (Physical and chemical properties), item e
  - benchmark: mohs_hardness
    label: Mohs hardness of calcite
    value: 3
    unit: Mohs
    scope: material
    conditions:
      basis: Mohs scale, as stated in the paper
    method: Value stated in the 2026 paper
    status: ground
    source: apreotesei-2026
    where: Section 1 (Introduction), paragraph on physicochemical properties
  - benchmark: transmission_range
    label: Transmission range of optical calcite
    value: 0.3
    value_high: 2.3
    unit: μm
    scope: material
    conditions:
      basis: as printed; no thickness or grade given on the page
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Transmission range'
  - benchmark: refractive_index
    label: Ordinary refractive index (no) at 0.51 μm
    value: 1.6653
    unit: '-'
    scope: material
    conditions:
      wavelength: 0.51 μm
      ray: ordinary (no), as printed
    method: Refractive index table on the datasheet; the page's summary line gives 1.6654 at the same wavelength
    status: ground
    source: crystran
    where: Refractive index table, row 0.51 μm
  - benchmark: refractive_index
    label: Extraordinary refractive index (ne) at 0.51 μm
    value: 1.4896
    unit: '-'
    scope: material
    conditions:
      wavelength: 0.51 μm
      ray: extraordinary (ne), as printed
    method: Refractive index table on the datasheet
    status: ground
    source: crystran
    where: Refractive index table, row 0.51 μm
  - benchmark: refractive_index
    label: Ordinary refractive index (no) at 400 nm
    value: 1.6782
    unit: '-'
    scope: material
    conditions:
      wavelength: 400 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 400
  - benchmark: refractive_index
    label: Extraordinary refractive index (ne) at 400 nm
    value: 1.4936
    unit: '-'
    scope: material
    conditions:
      wavelength: 400 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 400
  - benchmark: refractive_index
    label: Ordinary refractive index (no) at 550 nm
    value: 1.658
    unit: '-'
    scope: material
    conditions:
      wavelength: 550 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 550
  - benchmark: refractive_index
    label: Extraordinary refractive index (ne) at 550 nm
    value: 1.4862
    unit: '-'
    scope: material
    conditions:
      wavelength: 550 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 550
  - benchmark: refractive_index
    label: Ordinary refractive index (no) at 700 nm
    value: 1.6521
    unit: '-'
    scope: material
    conditions:
      wavelength: 700 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 700
  - benchmark: refractive_index
    label: Extraordinary refractive index (ne) at 700 nm
    value: 1.4833
    unit: '-'
    scope: material
    conditions:
      wavelength: 700 nm
      method_detail: Abbe refractometer, as stated in the paper
    method: Table 1 measurement, principal refractive indices
    status: ground
    source: apreotesei-2026
    where: Table 1 (Principal refractive indices and linear birefringence of calcite), row 700
  - benchmark: birefringence
    label: Linear birefringence (ne minus no) at 550 nm
    value: 0.1718
    unit: '-'
    scope: material
    conditions:
      wavelength: 550 nm
      basis: magnitude as printed in the Delta n column; the paper defines Delta n as ne minus no
    method: Computed from the Table 1 indices in the paper
    status: ground
    source: apreotesei-2026
    where: Table 1, column Delta n, row 550
  - benchmark: birefringence
    label: Linear birefringence of calcite at 589 nm
    value: 0.172
    unit: '-'
    scope: material
    conditions:
      wavelength: 589 nm
      basis: stated as approximately 0.172 at 589 nm in the Introduction
    method: Value stated in the paper's Introduction, citing refs. 1 and 4
    status: ground
    source: apreotesei-2026
    where: Section 1 (Introduction), paragraph on negative uniaxial nature
  - benchmark: thermal_conductivity
    label: Thermal conductivity, para direction
    value: 5.526
    unit: W m-1 K-1
    scope: material
    conditions:
      direction: para, as printed (the page does not define the axis)
      temperature: 273 K
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Thermal conductivity'
  - benchmark: thermal_conductivity
    label: Thermal conductivity, perp direction
    value: 4.646
    unit: W m-1 K-1
    scope: material
    conditions:
      direction: perp, as printed (the page does not define the axis)
      temperature: 273 K
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Thermal conductivity'
  - benchmark: specific_heat
    label: Specific heat capacity of calcite
    value: 852
    unit: J Kg-1 K-1
    scope: material
    conditions:
      temperature: not given on the page
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Specific heat capacity'
  - benchmark: elastic_modulus
    label: Young's modulus, para direction
    value: 88.19
    unit: GPa
    scope: material
    conditions:
      direction: para, as printed (the page does not define the axis)
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Young's modulus (E)'
  - benchmark: elastic_modulus
    label: Young's modulus, perp direction
    value: 72.35
    unit: GPa
    scope: material
    conditions:
      direction: perp, as printed (the page does not define the axis)
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Young's modulus (E)'
  - benchmark: shear_modulus
    label: Shear modulus of calcite
    value: 35
    unit: GPa
    scope: material
    conditions:
      basis: single value on the page; direction not stated
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Shear modulus (G)'
  - benchmark: bulk_modulus
    label: Bulk modulus of calcite
    value: 129.53
    unit: GPa
    scope: material
    conditions:
      basis: single value on the page; direction not stated
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Bulk modulus (K)'
  - benchmark: dielectric_constant
    label: Dielectric constant, para direction
    value: 8
    unit: '-'
    scope: material
    conditions:
      direction: para, as printed
      frequency: 10 kHz
      temperature: 293 K
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Dielectric constant'
  - benchmark: dielectric_constant
    label: Dielectric constant, perp direction
    value: 8.5
    unit: '-'
    scope: material
    conditions:
      direction: perp, as printed
      frequency: 10 kHz
      temperature: 293 K
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Dielectric constant'
  - benchmark: water_solubility
    label: Solubility of calcite in water
    value: 0.0014
    unit: g/100g water
    scope: material
    conditions:
      temperature: 25 °C, as printed
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Solubility'
  - benchmark: elastic_limit
    label: Apparent elastic limit of calcite
    value: 4.83
    unit: MPa
    scope: material
    conditions:
      basis: apparent elastic limit as printed; the test method is not described
    method: Datasheet value, as printed
    status: ground
    source: crystran
    where: Optical data table, row 'Apparent elastic limit'
corrosion:
  - environment: Dilute acids
    headline: Calcite dissolves in dilute acids.
    effect: Wikipedia states that calcite is soluble in dilute acids. The 2026 paper says its susceptibility to acidic environments requires protective measures during handling.
    rating: severe
    conditions:
      concentration: dilute acid; strength not stated in the sources
    source: wikipedia-calcite
    where: Properties section, solubility statement
  - environment: Water at 25 °C
    headline: Very low solubility in water.
    effect: Crystran lists a solubility of 0.0014g/100g water at 25°C. The 2026 paper describes calcite's solubility in water as low.
    rating: low
    conditions:
      temperature: 25 °C
    source: crystran
    where: Optical data table, row 'Solubility'
risks:
  hazards:
    - text: PubChem's GHS summary for calcium carbonate (CID 10112) lists the ECHA entries as 'Not Classified'. Most company reports say the substance does not meet GHS hazard criteria.
      ghs: []
      source: pubchem-ghs
      where: GHS Classification, ECHA entries
    - text: 'The Sigma safety data sheet for calcium carbonate powder (product C6033) says: ''Not a hazardous substance or mixture.'' Its toxicology section records mild eye irritation in rabbits (OECD Test Guideline 405).'
      ghs: []
      source: sigma-c6033-sds
      where: Section 2.1 and 2.2; Section 11.1, eye irritation
  failure_risks:
    - mode: Cleavage fracture
      trigger: Mechanical stress along the crystal's cleavage planes
      consequence: Calcite has perfect cleavage along rhombohedral planes, so a crystal can split along them.
      severity: moderate
      source: wikipedia-calcite
      where: Cleavage paragraph
    - mode: Acid attack
      trigger: Contact with dilute acid
      consequence: The crystal dissolves, and the 2026 paper says protective measures are needed during handling.
      severity: high
      source: wikipedia-calcite
      where: Properties section, solubility statement
    - mode: Heating past its decomposition point
      trigger: Heating to its melting or decomposition point
      consequence: Crystran prints the melting point as 825 °C with 'Decomposes'. Wikipedia describes calcite as infusible and says it decrepitates energetically.
      severity: low
      source: crystran
      where: Optical data table, row 'Melting point'
  side_effects: []
  handling:
    - text: Keep the powder container tightly closed in a dry and well-ventilated place. The Sigma SDS says the powder is moisture sensitive.
      source: sigma-c6033-sds
    - text: Avoid dust formation and use exhaust ventilation where dust forms, according to the Sigma SDS.
      source: sigma-c6033-sds
    - text: Keep the material away from strong oxidizing agents, acids, magnesium and aluminium, according to the Sigma SDS.
      source: sigma-c6033-sds
  mitigations:
    - text: Where nuisance dust is a concern, the Sigma SDS advises an N95 (US) or P1 (EN 143) dust mask.
      source: sigma-c6033-sds
uses:
  - application: optics-and-cameras
    device: Polarisers and retardation plates
    example: Crystran describes calcite as a strongly birefringent material used for polarisers and retardation plates.
    source: crystran
    where: Description paragraph
  - application: optics-and-cameras
    device: Optical path compensators, multi-order wave-plates and Wood-type polarisers
    example: The 2026 paper lists these uses, which exploit the contrast between the ordinary and extraordinary indices.
    source: apreotesei-2026
    where: Section 1 (Introduction)
  - application: optics-and-cameras
    device: Gun sights and bomb sights in World War II
    example: Wikipedia states that high-grade optical calcite was used in gun sights, including bomb sights and anti-aircraft weapons, during World War II.
    source: wikipedia-calcite
    where: History of use section
supply:
  - subject: Calcium carbonate feedstock (crushed stone, limestone)
    abundance: 'USGS: stone resources are plentiful throughout the world, but the supply of high-purity limestone and dolomite suitable for specialty uses is limited in many geographic areas.'
    producers: []
    critical_lists: []
    recycling: Not stated for calcite in the sources we hold.
    substitutes: []
    source: usgs-mcs2026-stone
    where: World Resources paragraph
  - subject: Optical-grade calcite (Iceland spar)
    abundance: Wikipedia says the Helgustadir mine in Iceland was once the main source of Iceland spar. The site is now a nature reserve, and calcite mining is no longer allowed there.
    producers: []
    critical_lists: []
    recycling: Not stated for calcite in the sources we hold.
    substitutes: []
    source: wikipedia-calcite
    where: Iceland spar paragraph
media: []
documents:
  - title: Sigma-Aldrich safety data sheet, Calcium carbonate, product C6033, version 5.8, revision 05/24/2016
    type: sds
    url: https://isolab.ess.washington.edu//laboratory/safety/sds/calcium_carbonate.pdf
    year: 2016
  - title: 'PubChem: calcium carbonate (CID 10112), GHS classification'
    type: database
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/10112/JSON?heading=GHS+Classification
    year: 2026
  - title: Linear birefringence dispersion of uniaxial inorganic crystal calcite estimated based on channeled spectra (Ukr. J. Phys. 71, No. 4, 308)
    type: paper
    url: https://ujp.bitp.kiev.ua/index.php/ujp/article/download/2023972/3473/31120
    year: 2026
  - title: 'USGS Mineral Commodity Summaries 2026: Stone (Crushed)'
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-stone-crushed.pdf
    year: 2026
gaps:
  - Thermal expansion and the temperature coefficient of refractive index are printed in exponent notation on the Crystran page, so they were left out.
  - Knoop hardness is printed without a unit on the Crystran page, so it was left out.
  - Growth method and manufacturing process for optical-grade calcite were not stated in the sources we hold.
  - No producer country shares for optical-grade calcite were found; the USGS stone summary gives none for this commodity.
  - No critical-minerals list status for calcium carbonate was found in the sources we hold.
  - No flight, vacuum or radiation test data for calcite were found. The planetary use mentioned in the 2026 paper is not tested in space in any source we hold.
  - Hazard data come from a calcium carbonate powder SDS and PubChem, not from an SDS for an optical-grade calcite crystal.
  - Heat and humidity ageing of the crystal were not found in the sources we hold.
  - Dispersion coefficients (Ghosh 1999, shown on refractiveindex.info) are printed as a formula with exponents, so no values were recorded from them.
  - The CAS number is printed for calcium carbonate in the Sigma SDS, not for optical calcite on the calcite pages.
sources:
  crystran:
    title: Calcite (CaCO3) optical material data, Crystran
    id: Crystran optical materials, calcite page
    url: https://www.crystran.com/optical-materials/calcite-caco3/
    year: 2026
    kind: datasheet
    license: vendor page, cited not reproduced
  wikipedia-calcite:
    title: Calcite, Wikipedia
    id: 'Wikipedia article: Calcite'
    url: https://en.wikipedia.org/wiki/Calcite
    year: 2026
    kind: web
    license: CC BY-SA 4.0
  apreotesei-2026:
    title: Linear birefringence dispersion of uniaxial inorganic crystal calcite estimated based on channeled spectra
    id: doi:10.15407/ujpe71.4.308 (Ukr. J. Phys. 71, No. 4, 308)
    url: https://ujp.bitp.kiev.ua/index.php/ujp/article/download/2023972/3473/31120
    year: 2026
    kind: paper
    license: CC BY-NC-ND (as printed on the journal's article)
  pubchem-cid10112:
    title: 'PubChem compound summary: calcium carbonate (CID 10112)'
    id: CID 10112
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug/compound/name/calcite/property/MolecularFormula,MolecularWeight,InChIKey/JSON
    year: 2026
    kind: database
    license: public domain (NIH PubChem)
  pubchem-ghs:
    title: PubChem GHS classification for CID 10112 (ECHA aggregated reports)
    id: CID 10112, GHS Classification
    url: https://pubchem.ncbi.nlm.nih.gov/rest/pug_view/data/compound/10112/JSON?heading=GHS+Classification
    year: 2026
    kind: database
    license: public domain (NIH PubChem)
  sigma-c6033-sds:
    title: Safety data sheet, Calcium carbonate, Sigma-Aldrich product C6033
    id: Sigma product C6033, CAS 471-34-1
    url: https://isolab.ess.washington.edu//laboratory/safety/sds/calcium_carbonate.pdf
    year: 2016
    kind: datasheet
    license: vendor SDS, cited not reproduced
  usgs-mcs2026-stone:
    title: 'Mineral Commodity Summaries 2026: Stone (Crushed)'
    id: USGS Mineral Commodity Summaries, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-stone-crushed.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  refractiveindex-ghosh:
    title: refractiveindex.info entry for calcite, ordinary index (Ghosh 1999 dispersion)
    id: Polyanskiy M. N., Sci. Data 11, 94 (2024); Ghosh G., Opt. Commun. 163, 95-102 (1999)
    url: https://refractiveindex.info/?shelf=main&book=CaCO3&page=Ghosh-o
    year: 2024
    kind: database
    license: site terms not checked
license: CC-BY-4.0
updated: '2026-11-02'
review:
  status: draft
  method: assistant
  checked_values: 27
  note: Drafted on 2026-11-02 by an AI assistant from pages read in this run.
story:
  recipe:
    line: Calcite is calcium carbonate, CaCO3. Each formula unit combines one calcium ion with one carbonate group, which is one carbon atom bonded to three oxygen atoms in a planar arrangement. Natural crystals differ in their refractive indices depending on how they grew (Apreotesei et al. 2026).
    parts:
      - name: Calcium
        symbol: Ca
        role: Ca2+ ions sit between the planar carbonate groups and add to the crystal's mechanical stability.
      - name: Carbon
        symbol: C
        role: The central atom of each carbonate group.
      - name: Oxygen
        symbol: O
        role: Three oxygen atoms per carbonate group, bonded to the carbon in a planar, trigonal arrangement.
  risk_groups:
    - title: For people
      verdict: Not classified hazardous in the SDS we checked
      lead: The calcium carbonate powder SDS we checked does not classify the material as hazardous. Its toxicology section records mild eye irritation.
      points:
        - label: The powder
          line: The Sigma SDS for product C6033 says 'Not a hazardous substance or mixture'. PubChem's ECHA-based entries also list the substance as Not Classified.
        - label: Dust
          line: The SDS advises avoiding dust and using an N95 (US) or P1 (EN 143) mask where nuisance dust is a concern.
        - label: Eyes
          line: The SDS reports mild eye irritation in rabbits under OECD Test Guideline 405.
    - title: For the crystal
      verdict: Dissolves in dilute acid and cleaves easily
      lead: Calcite's own properties limit where it can be used, more than any hazard to people.
      points:
        - label: Acids
          line: Calcite dissolves in dilute acids, and the 2026 paper says protective measures are needed during handling.
        - label: Cleavage
          line: Calcite has perfect cleavage along rhombohedral planes, so it can split along them under stress.
        - label: Heat
          line: Crystran prints the melting point as 825 °C with 'Decomposes'. Wikipedia describes calcite as infusible and says it decrepitates energetically.
  why: Optical calcite is a transparent crystal form of calcium carbonate with a trigonal structure. It is used where its strong birefringence is needed, so that ordinary and extraordinary light can be separated and controlled. The same crystal is soft and dissolves in acid, so it suits instruments and polarising optics more than structural or chemically exposed parts.
  uses:
    - where: Polarisers and retardation plates
      line: Crystran describes calcite as a strongly birefringent material used for polarisers and retardation plates. The 2026 paper lists optical path compensators, multi-order wave-plates and Wood-type polarisers as uses.
    - where: Historical polarisers and wartime sights
      line: Wikipedia says calcite was used as a polarizer in Nicol prisms before Polaroid plates were invented. During World War II, high-grade optical calcite was used in gun sights, including bomb sights and anti-aircraft weapons.
  can_do:
    - label: Polarisation
      line: Separates light into ordinary and extraordinary rays, which is the basis of polarisers and retardation plates (Crystran; Apreotesei et al. 2026).
    - label: Birefringence
      benchmark: birefringence
      value: approximately 0.172 at 589 nm
      line: The 2026 paper states this value, which is large enough for compact polarising optics.
    - label: Transmission
      value: 0.3 to 2.3 μm
      line: The transmission range printed by Crystran.
    - label: Reference standard
      line: The 2026 paper says calcite is used as a reference standard for characterising uniaxial media.
  compare: The density of optical calcite is recorded as 2.71 g/cm3 in the 2026 paper, and Crystran's table gives the same value. The Sigma safety data sheet prints a relative density of 2.93 g/cm3 at 25 °C for calcium carbonate powder. The two values describe different product forms, so the gap between them is not a grade comparison. Optical calcite's melting point of 825 °C (Crystran, decomposes) is also above the 800 °C (decomposes) printed for the powder, but both are reported only as decomposition points.
  compare_rows:
    - label: Birefringence at 589 nm
      benchmark: birefringence
    - label: Melting point (decomposes)
      benchmark: melting_point
    - label: Mohs hardness
      benchmark: mohs_hardness
  handling:
    - Keep the material dry and sealed, away from acids and strong oxidizers (Sigma SDS for the powder).
    - Avoid dust, and use exhaust ventilation where dust forms (Sigma SDS).
    - Protect optical calcite from acidic environments during handling, as the 2026 paper advises.
  good_for:
    - Polarisers and retardation plates, where a large birefringence is the main requirement (Crystran; Apreotesei et al. 2026).
    - Historical and instrument optics, where its polarising role is established (Wikipedia).
  watch_out:
    - Low hardness of Mohs hardness of 3, so it scratches easily and needs careful polishing.
    - Dissolves in dilute acids, so it does not suit acid environments.
    - Cleaves along rhombohedral planes under mechanical stress.
  verdict: Calcite is the classic choice for polarising optics where birefringence matters most. It is a poor choice for acid environments or for parts that need hardness.