Editing Strontianite (section)

== Physical properties ==
[[Cleavage (crystal)|Cleavage]] is nearly perfect parallel to one set of prism faces, [[Miller index|{110}]], and poor on {021}.  Traces of cleavage have been observed on {010}.<ref name=Mindat/><ref name=HOM/>
 
[[Crystal twinning|Twinning]] is very common, with twin plane {110}.  The twins are usually contact twins; in a contact twin the two individuals appear to be reflections of each other in the twin plane.  Penetration twins of strontainite are rarer; penetration twins are made up of interpenetrating individuals that are related to each other by rotation about a twin axis.  Repeated twins are made up of three or more individuals twinned according to the same law.  If all the twin planes are parallel then the twin is polysynthetic, otherwise it is cyclic.<ref name=MOM/>  In strontianite repeated twinning forms cyclic twins with three or four individuals, or polysynthetic twins.<ref name=Dana/><ref name=Mindat/><ref name=HOM/>

The mineral is brittle, and breaks with a subconchoidal to uneven [[Fracture (geology)|fracture]].  It is quite soft, with a [[Mohs hardness]] of {{frac|3|1|2}},<ref name=Mindat/> between [[calcite]] and [[fluorite]].  The [[specific gravity]] of the pure [[endmember]] with no calcium substituting for strontium is 3.78,<ref name=AM61/> but most samples contain some calcium, which is lighter than strontium, giving a lower specific gravity, in the range 3.74 to 3.78.<ref name=Mindat/>  Substitutions of the heavier ions [[barium]] and/or [[lead]] increase the specific gravity, although such substitutions are never very abundant.<ref name=AM61/>  Strontianite is soluble in dilute [[hydrochloric acid]] HCl<ref name=Dana/><ref name=Mindat/> and it is not [[Radioactive decay|radioactive]].<ref name=Webmin/>