Lawsonite

Lawsonite
Sample from the type locality in California with two elongated, lustrous and translucent crystals of pastel pink, lawsonite in mica schist. Size: 6.1 cm × 3.2 cm × 2.5 cm (2.4 in × 1.3 in × 1.0 in)
General
CategorySorosilicate
Formula
(repeating unit)
CaAl2Si2O7(OH)2·H2O
IMA symbolLws[1]
Strunz classification9.BE.05
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupCmcm
Unit cella = 5.847, b = 8.79,
c = 13.128 [Å]; Z = 4
Identification
ColorColorless, white, pale blue to grayish blue
Crystal habitCommonly prismatic, tabular; also granular, massive
TwinningCommon on {101} lamellar
CleavagePerfect on {100} and {010}, imperfect on {101}
TenacityBrittle
Mohs scale hardness7.5
LusterVitreous, greasy
StreakWhite
DiaphaneityTranslucent
Specific gravity3.05–3.12
Optical propertiesBiaxial (+)
Refractive indexnα = 1.665
nβ = 1.672–1.676
nγ = 1.684–1.686
Birefringenceδ = 0.019–0.021
PleochroismWeak; X = blue, pale brownish yellow;
Y = deep bluish green, yellowish green;
Z = colorless, yellowish
2V angleMeasured: 84°–85°
DispersionStrong, r > v
References[2][3][4]

Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl2Si2O7(OH)2·H2O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, pink, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα = 1.665, nβ = 1.672 – 1.676, and nγ = 1.684 – 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 7.5 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture. Not to be confused with Larsonite, a fossiliferous jasper mined in Nevada.

Lawsonite is a metamorphic mineral typical of the blueschist facies. It also occurs as a secondary mineral in altered gabbro and diorite. Associate minerals include epidote, titanite, glaucophane, garnet and quartz. It is an uncommon constituent of eclogite. Its scarcity in eclogite that has been exhumed to the Earth's surface does not reflect its abundance at depth in subduction zones but rather the fact that lawsonite is easily replaced by other minerals.

Lawsonite was first described in 1895[5] for occurrences on Ring Mountain of the Tiburon peninsula, Marin County, California and was named after geologist Andrew Lawson (1861–1952) of the University of California by two of Lawson's graduate students, Charles Palache and Frederick Leslie Ransome.[6]

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ Handbook of Mineralogy
  3. ^ Mindat.org
  4. ^ Webmineral data
  5. ^ Ransome, Frederick L. (1895). "On lawsonite, a new rock forming mineral from the Tiburon Peninsula, Marin County, California". University of California. Bulletin of the Department of Geology. 1 (10): 301–312.
  6. ^ Edson S. Bastin, "Biographical Memoir of Frederick Leslie Ransome, 1868-1935", National Academy of Sciences Biographic Memoirs XXII: 156 and Charles Palache with Frederick Leslie Ransome, "Uber Lawsonit, ein neues Gesteins-bildendes Mineral aus Californien" Zeits. Krist. 24 (1896): 588–592.