The lunar samples consist largely of augite, calcic plagioclase, and ilmenite. Olivine is a minor constituent of some rocks, as is cristobalite. Other minerals present in small amounts include tridymite, chromite, kamacite, taenite, and troilite. The principal rock types can be broadly grouped into ilmenite basalts and breccias. Except for their high ilmenite content, the lunar rocks resemble the calcium-rich achondritic meteorites (eucrites and howardites) in composition and structure. Evidence of a meteoritic increment in the lunar soil is provided by the presence of nickel-iron particles in glass and breccia, and the occurrence of metal-troilite spheroids; the breccias contain occasional silicate aggregates that resemble meteoritic chondrules. The lunar fines contain 325 parts of water-soluble calcium per million.