Larnite - Occurrence, Properties, and Distribution

Scawt mountain Hill in Co. Antrim Northern Ireland.

Scawt mountain Hill in Co. Antrim Northern Ireland. Image Credit: Ballygally View Images/Shutterstock.com

Larnite is a member of the olivine group of minerals. It is a high temperature, monoclinic polymorph of calico-olivine. The mineral was first discovered from Scawt Hill, near Larne, Co., Antrim, Ireland in 1929. It was named after its locality.

Chemical Formula – Ca2SiO4

Larnite crystallizes at high temperatures and occurs in limestones or chalk zones in contact with molten basaltic rocks. Larnite is stable in the range of 520° to 670° C.

Properties of Larnite

The following are the key properties of larnite:

  • Cell Data
    • Space Group: P21/n
    • a = 5.502(1)
    • b = 6.745(1)
    • c = 9.297(1)
    • β = 94:59(2)°
    • Z = 4
  • Crystal Data
    • Monoclinic - prismatic
    • Point Group: 2/m
    • As anhedral grains flattened on {010}; subhedral interlocking grains; granular, massive. Twinning: Common, polysynthetic || {100}; a second set may be ⊥ the first
    • X-ray Powder Pattern: 2.795 (100), 2.744 (95), 2.780 (90), 2.608 (65), 2.188 (65), 2.731 (40), 2.716 (40)
    • Chemical Composition
      Elements Content 1 Content 2 Content 3
      CaO 64.98 65.56 65.12
      SiO2 31.00 33.89 34.88
      Al2O3 1.12 - -
      CO2 0.82 - -
      MgO 0.69 - -
      FeO 0.64 - -
      H2O+ 0.61 - -
      Na2O - 0.36 -
      P2O - 0.32 -
      Fe2O3 trace - -
      Total 99.86 100.13 100.00

Content 1: Scawt Hill, Ireland; minor spurrite impurity.
Content 2: Do.; by electron microprobe, an average of three analyses.
Content 3: Ca2SiO4

  • Optical Properties
    • Optical Class: Biaxial (+)
    • Orientation: Z = b
    • X ^ c = 13°-14°
    • Dispersion: r < v; weak.
    • α = 1.700-1.715
    • β = 1.715-1.723
    • γ = 1.725-1.740
    • 2V(meas.) = 60°-63°
  • Estimated Properties
    . .
    Electron density Bulk density (electron density)=3.34 g/cm3
    note: Specific gravity of larnite =3.34 g/cm3
    Photoelectric PELarnite = 6.32 barns/electron
    U= PELarnite x ρElectron density= 21.09 barns/cm3
    Fermion index Fermion index = 0.008
    Boson index = 0.99
    Radioactivity Larnite is not radioactive

How to Identify Larnite

Larnite is either white or gray in color, colorless in thin sections. It has a transparent to translucent appearance, white streak, good cleavage, and a vitreous luster. It generally forms anhedral to subhedral crystals in a matrix. The relative hardness of larnite is 6 on the Mohs scale, and its density is 3.28 g/cm3.

Global Distribution

Larnite is distributed in the following places:

  • Scawt Hill and Ballycraigy, near Larne, Co. Antrim, Ireland
  • Camas Mòr, Isle of Muck, Scotland
  • Camphouse, Ardnamurchan, Argyllshire, Scotland
  • Bellerberg volcano, 2 km north of Mayen, Eifel district, Germany
  • Hatrurim Formation, Negev, Israel
  • Rehia, and in Golden Gully, Tokatoka district, about 150 km north of Auckland, New Zealand
  • Gosse Pile, Tomkinson Ranges, and in the Deering Hills, South Australia
  • Marble Canyon, Culberson Co., Texas, USA.

The Occurrence of Larnite and Useful Mineral Association

Larnite occurs in contact metamorphic terranes involving sedimentary carbonate rocks. It is closely associated with spurrite, melilite, mayenite, brownmillerite, hydrogrossular, scawtite, kilchoanite, rankinite, wollastonite, perovskite, merwinite, and gehlenite.

This article was updated on 27th February, 2020.

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