▁m ica ▁family ▁of ▁ph yll os il ic ates . ▁It ▁is ▁also ▁known ▁as ▁mag nes ium ▁m ica . ▁ ▁Ph log op ite ▁is ▁the ▁mag nes ium ▁end member ▁of ▁the ▁bi ot ite ▁solid ▁solution ▁series , ▁with ▁the ▁chemical ▁formula ▁K M g 3 Al Si 3 O 1 0 ( F , OH ) 2 . ▁Iron ▁subst it utes ▁for ▁mag nes ium ▁in ▁variable ▁amounts ▁leading ▁to ▁the ▁more ▁common ▁bi ot ite ▁with ▁higher ▁iron ▁content . ▁For ▁physical ▁and ▁optical ▁identification , ▁it ▁shares ▁most ▁of ▁the ▁characteristic ▁properties ▁of ▁bi ot ite . ▁ ▁Par agen esis ▁Ph log op ite ▁is ▁an ▁important ▁and ▁relatively ▁common ▁end - member ▁composition ▁of ▁bi ot ite . ▁Ph log op ite ▁m icas ▁are ▁found ▁primarily ▁in ▁ igne ous ▁rocks , ▁although ▁it ▁is ▁also ▁common ▁in ▁contact ▁met am orph ic ▁a ure oles ▁of ▁intr us ive ▁ igne ous ▁rocks ▁with ▁mag nes ian ▁country ▁rocks ▁and ▁in ▁mar ble ▁formed ▁from ▁imp ure ▁dol om ite ▁( d ol om ite ▁with ▁some ▁sil ic last ic ▁sed iment ). ▁ ▁The ▁occurrence ▁of ▁ph log op ite ▁m ica ▁within ▁ igne ous ▁rocks ▁is ▁difficult ▁to ▁const rain ▁precisely ▁because ▁the ▁primary ▁control ▁is ▁rock ▁composition ▁as ▁expected , ▁but ▁ph log op ite ▁is ▁also ▁controlled ▁by ▁conditions ▁of ▁cry st all isation ▁such ▁as ▁temperature , ▁pressure , ▁and ▁v apor ▁content