Searlesite is a sodium borosilicate mineral, with the chemical formula NaBSi₂O₅(OH)₂. It was discovered in 1914 at Searles Lake, California, and was named to honor John W. Searles (16 November 1828 – 7 October 1897), California pioneer, who drilled the well that yielded the first known Searlesite.

Searlesite
Searlesite, near Green River, Wyoming
General
Category	Phyllosilicate
Formula (repeating unit)	NaBSi2O5(OH)2
IMA symbol	Sle[1]
Strunz classification	9.EF.15
Crystal system	Monoclinic
Crystal class	Sphenoidal (2) (same H-M symbol)
Space group	P21
Unit cell	a = 7.98 Å, b = 7.06 Å c = 4.9 Å; β = 93.95°
Identification
Color	White, light brown
Mohs scale hardness	1–2
Luster	Vitreous
Optical properties	Biaxial (−)
Refractive index	nα = 1.516 nβ = 1.531 nγ = 1.535
Birefringence	δ = 0.019

Searlesite is usually found disseminated in fine-grained lacustrine strata and often associated with altering volcanic ash. It may be a minor component of borate deposits, but it is rarely found concentrated or in megascopic crystals and so has not been developed as an ore mineral of boron. It occurs interbedded with oil shales or marls (Green River Formation, US) and in boron-bearing evaporite deposits (California, US); rarely in vugs in phonolite (Point of Rocks, New Mexico).