Pyrophyllite Al2Si4O10(OH)2

spore: profile
id: pyrophyllite
spore_version: 0.2
family: oxide-ceramics
name: Pyrophyllite
abbreviation: Al2Si4O10(OH)2
role: machinable ceramic blank and high-pressure gasket mineral
category: ceramics
type: oxides
summary: |
  Pyrophyllite is a soft hydrous aluminium silicate, Al2Si4O10(OH)2, sold in its unfired form as Wonderstone or Grade A Lava for machining into technical ceramic parts before firing. Superior Technical Ceramics lists the unfired grade at a relative density of 2.3 g/cc. Machined pyrophyllite is also the sealing gasket in high-pressure press cells, where it deforms slightly under load and insulates heat. In one low-firing study, a pyrophyllite-based ceramic had a thermal expansion of 4 to 5 ppm/K. No space, vacuum or flight result for pyrophyllite was found in a source we could open.
best_for: Blanks that must be cut to shape before firing, and pressure gaskets in high-pressure laboratory presses
weakest_at: Dust from machining (commercial grades can contain respirable crystalline silica) and batch-to-batch variation in a natural mineral
identity:
  formula: Al2Si4O10(OH)2 (hydrous aluminium silicate; the sources give the same formula)
  cas: 12269-78-2
  source: stc-sds-pyrophyllite
  where: Section 1 (formula and synonyms Wonderstone and A Lava); Section 3 (pyrophyllite CAS 12269-78-2)
tags:
  environments: []
  stresses:
    - extreme-heat
  applications:
    - thermal-control
    - structures
properties:
  - benchmark: density
    label: Relative density, unfired Grade A Lava (Wonderstone)
    value: 2.3
    unit: g/cc
    scope: material
    conditions:
      form: unfired machining blank
      test: not stated in the safety data sheet
    method: Supplier safety data sheet, relative density printed in Section 9
    status: ground
    note: A fired-body density was not read; the supplier sheet covers only the unfired grade.
    source: stc-sds-pyrophyllite
    where: Section 9, Relative density
  - benchmark: dehydroxylation_temperature
    label: Dehydroxylation range, pyrophyllite to pyrophyllite dehydroxylate
    value: 500
    value_high: 900
    bound: range
    unit: °C
    scope: material
    conditions:
      method: infrared spectroscopy during heating
      atmosphere: not stated in the abstract
    method: Read from the abstract of an infrared study of pyrophyllite at elevated temperatures
    status: ground
    note: The authors call the process two-stage and link extra OH bands at 550 to 900 °C to an unidentified intermediate phase.
    source: wang2002
    where: Abstract
  - benchmark: dielectric_constant
    label: Dielectric constant, pyrophyllite-based low-dielectric ceramic
    value: 2
    value_high: 4
    bound: range
    unit: '-'
    scope: material
    conditions:
      frequency: not printed in the abstract
      temperature: not printed in the abstract
      composition: pyrophyllite-based body, low firing temperature
    method: Read from the abstract of a 2010 study of a low-dielectric, low-temperature ceramic
    status: ground
    source: zhang2010
    where: Abstract
  - benchmark: dissipation_factor
    label: Dielectric loss, pyrophyllite-based low-dielectric ceramic
    value: 0.004
    bound: upper
    unit: '-'
    scope: material
    conditions:
      frequency: 1 kHz
    method: Read from the abstract; printed as less than 0.004 at 1 kHz
    status: ground
    source: zhang2010
    where: Abstract
  - benchmark: cte
    label: Thermal expansion coefficient, pyrophyllite-based ceramic
    value: 4
    value_high: 5
    bound: range
    unit: ppm/K
    scope: material
    conditions:
      temperature_range: not printed in the abstract
      unit_in_source: (4-5) x 10^-6 K^-1
    method: Read from the abstract; the printed unit 10^-6 K^-1 is the same as ppm/K
    status: ground
    source: zhang2010
    where: Abstract
  - benchmark: flexural_strength
    label: Flexural strength, pyrophyllite-based dielectric ceramic (printed as approximate)
    value: 400
    unit: MPa
    scope: material
    conditions:
      basis: printed as approximate; no test method, grade or firing temperature in the abstract
    method: Read from the abstract
    status: ground
    note: Printed as approximately 400 MPa with no test conditions. Treat as unverified until the full text is read.
    source: zhang2010
    where: Abstract
  - benchmark: flexural_strength
    label: Flexural strength, porous pyrophyllite support, sintered at 900 °C
    value: 4
    unit: MPa
    scope: material
    conditions:
      sintering_temperature: 900 °C
      porosity: 47%
      form: tubular support for a microfiltration membrane, from low-cost pyrophyllite clay
    method: Read from the abstract
    status: ground
    source: talidi2011
    where: Abstract
  - benchmark: flexural_strength
    label: Flexural strength, porous pyrophyllite support, sintered at 1200 °C
    value: 17
    unit: MPa
    scope: material
    conditions:
      sintering_temperature: 1200 °C
      porosity: 30%
      form: tubular support for a microfiltration membrane, from low-cost pyrophyllite clay
    method: Read from the abstract
    status: ground
    source: talidi2011
    where: Abstract
  - benchmark: porosity
    label: Porosity, porous pyrophyllite support, sintered at 900 °C
    value: 47
    unit: '%'
    scope: material
    conditions:
      sintering_temperature: 900 °C
      form: tubular support, low-cost pyrophyllite clay
    method: Read from the abstract
    status: ground
    note: Non-canonical property name; not in the property canon.
    source: talidi2011
    where: Abstract
  - benchmark: porosity
    label: Porosity, porous pyrophyllite support, sintered at 1200 °C
    value: 30
    unit: '%'
    scope: material
    conditions:
      sintering_temperature: 1200 °C
      form: tubular support, low-cost pyrophyllite clay
    method: Read from the abstract
    status: ground
    note: Non-canonical property name; not in the property canon.
    source: talidi2011
    where: Abstract
  - benchmark: ph
    label: pH of a 10 per cent slurry, PYRAX HS (ground pyrophyllite filler grade)
    value: 6.9
    unit: pH
    scope: material
    conditions:
      concentration: 10% w/w in water
      product: ground filler grade, which contains quartz and is not the unfired blank
    method: Supplier safety data sheet, pH printed in Section 9
    status: ground
    note: Non-canonical property name; describes a different (ground filler) product, not the unfired blank.
    source: pyrax-hs-sds
    where: Section 9, pH
corrosion:
  - environment: Cold water and moist air
    headline: Insoluble in cold water; natural moisture content varies between batches.
    effect: The ground filler grade is insoluble in cold water. The 1999 patent on pressure gaskets says the moisture content of natural pyrophyllite varies, which affects how a gasket behaves under pressure. No humidity ageing test was found.
    rating: low
    conditions:
      exposure: cold water, immersion time not stated
      humidity: not stated
    source: pyrax-hs-sds
    where: Section 9, Solubility
  - environment: Heating above about 500 °C
    headline: Structural hydroxyl groups leave the mineral between 500 and 900 °C, in two stages.
    effect: Heating drives off the OH groups and turns pyrophyllite into a dehydroxylate. This is the main ageing reaction during firing. The study read for this profile gives the temperature window and stages only; no rate or strength loss was found.
    rating: moderate
    conditions:
      method: infrared spectroscopy during heating
      atmosphere: not stated in the abstract
    source: wang2002
    where: Abstract
risks:
  hazards:
    - text: 'The supplier treats the unfired solid as an article with no health hazard under normal use. The hazards apply to dust made during machining or other processing: respiratory irritation, cancer by inhalation and organ damage from repeated exposure.'
      ghs:
        - H335
        - H350i
        - H372
      source: stc-sds-pyrophyllite
      where: Section 2, classification and hazard statements; Section 2 other hazards
    - text: PYRAX HS, a ground filler grade, carries the sheet heading WARNING! Cancer Hazard because it contains quartz at 50 to 60 per cent by weight. The sheet prints no GHS codes.
      ghs: []
      source: pyrax-hs-sds
      where: Section 2, Emergency overview; Section 3, composition
  failure_risks:
    - mode: Silicosis from machining dust
      trigger: Machining or grinding pyrophyllite blanks, which generates dust
      consequence: Dust may contain respirable crystalline silica, which the supplier says can cause silicosis with prolonged inhalation.
      severity: high
      source: stc-sds-pyrophyllite
      where: Section 2, other hazards
    - mode: Batch-to-batch variation in a gasket
      trigger: Natural variation in the composition and moisture content of the mineral
      consequence: The patent says variations in composition and moisture content reduce product consistency.
      severity: moderate
      source: us-patent-5858525
      where: Background of the Invention
  side_effects:
    - text: The patent calls pyrophyllite the preferred gasket material because it can deform or flow under pressure to a limited extent, which perfects the seal in a press cell.
      affects: The pressure seal of the press cell
      source: us-patent-5858525
      where: Background of the Invention
  handling:
    - text: Wet mop or dry vacuum any dust.
      source: stc-sds-pyrophyllite
      where: Sections 6 and 7
    - text: Use local exhaust ventilation where dust is generated; the PYRAX HS sheet recommends a capture velocity of 150 to 200 feet per minute.
      source: pyrax-hs-sds
      where: Section 8, Engineering measures
    - text: Wear safety goggles in the presence of airborne dust, and a NIOSH or MSHA approved respirator where the exposure limit is exceeded.
      source: stc-sds-pyrophyllite
      where: Section 8, Individual protection measures
  mitigations:
    - text: The ACGIH TLV for respirable quartz is 0.025 mg/m3 (TWA, respirable fraction), as printed on the PYRAX HS sheet. Check the current limit for your jurisdiction.
      source: pyrax-hs-sds
      where: Section 3, quartz row
uses:
  - application: thermal-control
    device: Sealing gasket in a high-pressure press cell (ground laboratory use, not flown)
    example: The patent describes a sealing gasket machined from a block of pyrophyllite in polycrystalline diamond presses, and says the mineral also has good thermal insulating characteristics.
    source: us-patent-5858525
    where: Background of the Invention
  - application: structures
    device: Unfired blank for machining technical ceramic components (ground use)
    example: Superior Technical Ceramics sells its unfired Grade A Lava, also named Wonderstone, with the recommended use Machining Blank for Technical Ceramic Component(s).
    source: stc-sds-pyrophyllite
    where: Section 1, recommended use
  - application: structures
    device: Porous tubular support for a microfiltration membrane (laboratory study)
    example: A 2011 study made a tubular support from low-cost pyrophyllite clay; flexural strength rose from 4 to 17 MPa as the sintering temperature went from 900 to 1200 °C.
    source: talidi2011
    where: Abstract
supply:
  - subject: Pyrophyllite (United States)
    abundance: Two companies in North Carolina mined and processed pyrophyllite in 2025. Production figures are withheld to protect company data and were essentially unchanged from 2024. Employment in pyrophyllite was 33 people in 2025 (estimated).
    producers: []
    critical_lists:
      - list: USGS 2025 List of Critical Minerals
        listed: false
    recycling: The USGS talc and pyrophyllite chapter gives recycling as insignificant for the chapter as a whole.
    substitutes: []
    source: usgs-mcs2026-talc
    where: 'Domestic Production and Use; Salient Statistics (pyrophyllite employment row); Recycling. Critical list: USGS 2025 critical minerals page (talc and pyrophyllite not named there).'
gaps:
  - No fired-body density, compressive strength or Mohs hardness from a source we could open. Wikipedia gives hardness as 1.5 to 2 in one place and 1.0 to 1.5 in another, and specific gravity as 2.65 to 2.9 in one place and 2.65 to 2.85 in another, so these were not used.
  - Commercial datasheets (MatMatch, MakeItFrom) could not be opened from this environment, so their fired and unfired values were not used.
  - 'Pyrophyllite production share by country was not found: USGS withholds US production and gives no world figures for pyrophyllite.'
  - Pyrophyllite as a leachable core for investment casting was not found in any source. Silica is the leachable core in common use, and alumina cores above 97 per cent alumina are not leachable by standard methods (Morgan Advanced Materials article via foundry-planet).
  - 'Crystal system and space group were not confirmed: Wikipedia gives monoclinic or triclinic.'
  - Dehydroxylation activation energy (224 plus or minus 16 kJ/mol) and rehydroxylation were seen only in search snippets and were not read, so they are left out.
  - 'Non-canonical property names used: dehydroxylation_temperature, porosity, ph. Their conditions are in each entry.'
  - The 400 MPa flexural strength in the 2010 abstract has no test conditions and is marked approximate; the full text was not read.
  - No vacuum, outgassing, radiation, thermal-cycling or flight result for pyrophyllite was found. No ageing test under humidity or UV was found.
  - 'The media section is not filled: no openly licensed photograph of pyrophyllite or a gasket was checked.'
  - 'Supplier and product values differ: the unfired blank (STC) and the ground filler (PYRAX HS, quartz 50 to 60 per cent) are different products, and the profile keeps them apart.'
documents:
  - title: Superior Technical Ceramics safety data sheet, Unfired Grade A Lava (verified March 22, 2024)
    type: sds
    url: https://ceramics.net/wp-content/uploads/STC-SDS-Unfired-Grade-A-Lava-Rev-03-22-2024.pdf
    year: 2024
  - title: Vanderbilt Minerals material safety data sheet, PYRAX HS (pyrophyllite filler; file hosted by The Ceramic Shop)
    type: sds
    url: https://www.theceramicshop.com/sds/Pyrophyllite.pdf
    year: 2012
  - title: USGS Mineral Commodity Summaries 2026, Talc and Pyrophyllite
    type: report
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-talc.pdf
    year: 2026
  - title: Talidi et al., Processing and characterization of tubular ceramic support for microfiltration membrane prepared from pyrophyllite clay
    type: paper
    url: https://pubs.ub.ro/article/3378
    year: 2011
  - title: Wang et al., Dehydroxylation and transformations of the 2:1 phyllosilicate pyrophyllite at elevated temperatures
    type: paper
    url: https://www.cambridge.org/core/journals/clays-and-clay-minerals/article/dehydroxylation-and-transformations-of-the-21-phyllosilicate-pyrophyllite-at-elevated-temperatures-an-infrared-spectroscopic-study/9C502EF32BE74B0BAB53536D66170014
    year: 2002
  - title: Zhang et al., Preparation of low dielectric and low temperature ceramic and the influence factors of its property
    type: paper
    url: https://xuebao.jlu.edu.cn/dxb/EN/Y2010/V40/I6/1446
    year: 2010
sources:
  stc-sds-pyrophyllite:
    title: Safety Data Sheet, Unfired Grade A Lava (Superior Technical Ceramics, St. Albans, Vermont)
    id: Prepared June 1, 2015; last revision verified March 22, 2024
    url: https://ceramics.net/wp-content/uploads/STC-SDS-Unfired-Grade-A-Lava-Rev-03-22-2024.pdf
    year: 2024
    kind: datasheet
    license: supplier document, linked not reproduced
  pyrax-hs-sds:
    title: Material Safety Data Sheet, PYRAX HS (Vanderbilt Minerals, LLC; pyrophyllite filler)
    id: Printed 12/21/2012; material RTV 33207
    url: https://www.theceramicshop.com/sds/Pyrophyllite.pdf
    year: 2012
    kind: datasheet
    license: supplier document, linked not reproduced
  usgs-mcs2026-talc:
    title: Mineral Commodity Summaries 2026, Talc and Pyrophyllite (U.S. Geological Survey)
    id: USGS MCS 2026, February 2026
    url: https://pubs.usgs.gov/periodicals/mcs2026/mcs2026-talc.pdf
    year: 2026
    kind: report
    license: public domain (USGS)
  usgs-crit2025:
    title: About the 2025 List of Critical Minerals (USGS)
    id: Final 2025 List of Critical Minerals, November 2025
    url: https://www.usgs.gov/programs/mineral-resources-program/science/about-2025-list-critical-minerals
    year: 2025
    kind: web
    license: public domain (USGS)
  us-patent-5858525:
    title: US5858525A, Synthetic gasket materials for use in high-pressure presses (Carter, Yao, Rai; General Electric Co.)
    id: US patent 5858525; published 1999-01-12
    url: https://patents.google.com/patent/US5858525
    year: 1999
    kind: web
    license: patent document (public)
  wang2002:
    title: 'Dehydroxylation and Transformations of the 2:1 Phyllosilicate Pyrophyllite at Elevated Temperatures: An Infrared Spectroscopic Study'
    id: doi via Cambridge Core; Clays and Clay Minerals 50(2), pp. 272-283
    url: https://www.cambridge.org/core/journals/clays-and-clay-minerals/article/dehydroxylation-and-transformations-of-the-21-phyllosilicate-pyrophyllite-at-elevated-temperatures-an-infrared-spectroscopic-study/9C502EF32BE74B0BAB53536D66170014
    year: 2002
    kind: paper
    license: publisher page; abstract read, not reproduced
  zhang2010:
    title: Preparation of Low Dielectric and Low Temperature Ceramic and the Influence Factors of Its Property (Zhang Pei-ping, Sun Yi-ting, Yu De-li, Wang Yu-jie)
    id: J4, vol. 40, issue 6, pp. 1446-1449
    url: https://xuebao.jlu.edu.cn/dxb/EN/Y2010/V40/I6/1446
    year: 2010
    kind: paper
    license: publisher page; abstract read, not reproduced
  talidi2011:
    title: Processing and characterization of tubular ceramic support for microfiltration membrane prepared from pyrophyllite clay (Talidi et al.)
    id: Vol. 12, no. 3 (pubs.ub.ro article 3378)
    url: https://pubs.ub.ro/article/3378
    year: 2011
    kind: paper
    license: publisher page; abstract read, not reproduced
license: CC-BY-4.0
updated: '2026-10-26'
review:
  status: draft
  method: assistant
  checked_values: 11
  unreachable: 3
  caveats: 3
  note: 'Written by an AI assistant on 2026-10-26 from a supplier safety data sheet, a second supplier safety data sheet, the USGS 2026 talc and pyrophyllite chapter, the 1999 gasket patent and three journal abstracts read in this run. Three sources could not be opened (MatMatch, MakeItFrom, Granada author copies). Caveats: the 400 MPa value has no test conditions; the hardness and specific gravity conflicts in Wikipedia were left out; the ground filler pH belongs to a different product.'
story:
  recipe:
    line: Pyrophyllite is a hydrous aluminium silicate with the formula Al2Si4O10(OH)2. Per formula unit it holds two aluminium atoms, four silicon atoms, ten oxygen atoms in the silicate framework and two hydroxyl groups. Heating above about 500 °C drives off the hydroxyl groups, and the fired product is a dehydroxylate. The unfired grade is sold as a machining blank.
    parts:
      - name: Aluminium
        symbol: Al
        atoms: 2
        atomic_weight: 26.982
        role: The aluminium in the formula. Its place in the structure is not described in the sources read here.
      - name: Silicon
        symbol: Si
        atoms: 4
        atomic_weight: 28.085
        role: Part of the 2:1 layer structure named in the 2002 infrared study title.
      - name: Oxygen
        symbol: O
        atoms: 12
        atomic_weight: 15.999
        role: Ten oxygen atoms form the silicate framework and two more sit in the hydroxyl groups.
      - name: Hydrogen
        symbol: H
        atoms: 2
        atomic_weight: 1.008
        role: Hydroxyl hydrogen. It is driven off on heating between about 500 and 900 °C, and the mineral is then a dehydroxylate.
  risk_groups:
    - title: For people
      level: 0
      verdict: Dust is the hazard, not the solid
      lead: The unfired blank is treated as an article with no health hazard under normal use. Its dust is the concern, and it can contain respirable crystalline silica.
      points:
        - label: The solid
          line: The supplier sheet classifies the unfired solid as an article. It says the health effects apply when dust is made during machining or other processing.
        - label: Dust from machining
          line: The same sheet lists carcinogenicity category 1, specific organ toxicity category 1 and respiratory irritation, with the statement that the dust may cause cancer by inhalation. It warns of silicosis from prolonged inhalation of respirable silica.
        - label: Ground filler grades
          line: A ground pyrophyllite filler sold for ceramics and paint contained quartz at 50 to 60 per cent, and its sheet carries a cancer warning. Check the composition of the exact grade.
        - label: In a fire
          line: The filler sheet calls the product non-flammable. The unfired sheet says it gives off carbon oxides and water vapour on heating.
    - title: Handling and storage
      points:
        - label: Cleaning
          line: Any dust should be wet mopped or dry vacuumed.
        - label: Ventilation
          line: Local exhaust ventilation is recommended where dust is made, with a capture velocity of 150 to 200 feet per minute for the filler grade.
        - label: Protection
          line: Goggles are needed in airborne dust, and a NIOSH or MSHA approved respirator where the exposure limit is exceeded.
  why: Pyrophyllite is a soft aluminium silicate that has been used in two ways. It is machined into blanks before firing. It is also machined into sealing gaskets for high-pressure presses, where it deforms slightly under load to seal the cell and insulates heat between the container and the anvils. Its fired properties are less well documented than its uses. The unfired grade is sold as Wonderstone or Grade A Lava, and natural variation in composition and moisture is the main concern for gasket consistency.
  uses:
    - where: Ground laboratory presses
      line: The patent describes a sealing gasket machined from a block of pyrophyllite for polycrystalline diamond presses. It is preferred because it deforms or flows under pressure to seal the cell, and it insulates heat.
    - where: Machining blanks for ceramic parts
      line: Superior Technical Ceramics sells its unfired Grade A Lava as a machining blank for technical ceramic components, with a relative density of 2.3 g/cc.
    - where: Porous membrane supports
      line: A 2011 study made a tubular microfiltration support from low-cost pyrophyllite clay. Flexural strength rose from 4 to 17 MPa as the sintering temperature went from 900 to 1200 °C, while porosity fell from 47 to 30 per cent.
  can_do:
    - label: Machining before firing
      value: Unfired blank
      line: The unfired grade is sold as a machining blank for technical ceramic parts, so it can be cut before firing. The sheet gives no machining speeds or tooling.
    - label: Density
      benchmark: density
      line: The supplier lists 2.3 g/cc for the unfired grade. A fired-body density was not found in a source we could open.
    - label: Thermal expansion
      benchmark: cte
      line: One low-firing pyrophyllite-based ceramic has a thermal expansion of 4 to 5 ppm/K. The abstract does not give the temperature range.
    - label: Dehydroxylation
      benchmark: dehydroxylation_temperature
      line: Structural hydroxyl groups leave the mineral between 500 and 900 °C in two stages, which sets the limit for heating in an unfired state.
  compare: 'Pyrophyllite is a soft aluminium silicate with a relative density of 2.3 g/cc for the unfired grade. The one low-firing pyrophyllite ceramic read here expands 4 to 5 ppm/K. Leachable ceramic cores for investment casting are a related material: silica is the leachable core in common use, and very high-alumina cores are hard to leach. No source we opened shows pyrophyllite used as a leachable core, so the two should not be treated as the same material.'
  compare_rows:
    - label: Density
      benchmark: density
    - label: Thermal expansion
      benchmark: cte
    - label: Flexural strength
      benchmark: flexural_strength
  handling:
    - Wet mop or dry vacuum any dust.
    - Use local exhaust ventilation where dust is made, and wear goggles and a respirator where the limit is exceeded.
    - The unfired blank is a semi-porous solid that is insoluble in water.
    - 'Treat the ground filler grade as a cancer hazard: it contains quartz.'
  good_for:
    - Blanks that are cut to shape before firing, as the supplier sells it.
    - Gaskets in high-pressure presses, where the mineral deforms to seal the cell and insulates heat (patent).
  watch_out:
    - Dust from machining can contain respirable crystalline silica, which can cause silicosis and cancer.
    - Natural composition and moisture vary between batches, which makes gasket behaviour less consistent.
    - Heating above about 500 °C drives off structural water, so the material changes in firing.
    - No space, vacuum or flight data were found.
  verdict: Pyrophyllite is a practical machinable blank and gasket mineral. Its dust is the main hazard, and no space result is on record.