Single　magnetite　crystals　from　the　Cihai　Fe　skarn　deposit　in　Northwestern　China　are　strongly　growth　zoned.　Magnetite　cores　are　in　equilibrium　with　garnet　whereas　magnetite　rims　are　enclosed　by　calcite.　The　chemical　zoning　in　magnetite　is　well　defined　by　an　abrupt　core-to-rim　Si　and　Ca　increase　as　well　as　Ti　decrease.　Electron　microprobe　analysis　(EMPA)　results　show　that　magnetites　rims　contain　from　2.24　to　5.70　wt%　SiO2　(averaging　4.56　wt%),　which　are　silician　magnetite.　Plots　of　EMPA　data　suggest　that　silician　magnetite　in　the　Cihai　skarn　deposit　presents　in　the　form　of　[Si4+]IV　+　[Fe2+](VI)　＜-＞　[Fe3+](IV)　+　[Fe3+](VI).　The　LA-ICP-MS　dating　results　show　that　the　garnet　coexisting　with　the　magnetite　core　has　a　U-Pb　age　of　282.5　+/-　2.6　Ma,　which　is　consistent　with　the　timing　of　Fe　mineralization　in　the　retrograde　skarn　stage,　indicating　that　silician　magnetite　was　formed　within　a　relatively　short　time　interval　after　magnetite　precipitation.　Based　on　coexisting　minerals,　chemical　compositions,　and　our　previous　fluid　inclusion　analyses,　we　propose　that　the　Si　zoning　of　magnetite　is　largely　temperature　dependent　and,　thus,　is　interpreted　as　a　retrograde　growth　zoning.　It　is　suggested　that　silician　magnetite　formed　as　a　result　of　changing　compatibility　due　to　decreasing　temperatures　and　can　potentially　be　used　to　trace　oreforming　temperatures　in　hydrothermal　deposits.　Our　study　provides　independent　constraints　on　the　conditions　of　hydrothermal　magnetite　formation　in　the　Cihai　Fe　skarn　deposit,　and　suggests　that　silician　magnetite　may　serve　as　a　potential　relatively　low-temperature　(＜300(　degrees)C)　indicator　in　other　less-well-constrained　magmatichydrothermal　systems.