Growth Mechanism And Polytype Genesis Of Synthetic Fluorophlogopite From Melt

Mica is one of major rock-forming minerals,and widely occurs in igneous,sedimentary,and metamorphic rocks.Mica can crystallize from the vapor,melt,hydrothermal solution,and solid state in variable of geological environments.During the process of crystal growth,micas can record important information such as geological history changes and environmental variation,and thus has attracted the interests of mineralogists and petrologists.As an important indicator,micas'structure?e.g.,polytypism?and growth micromorphology can potentially yield information about supersaturation,temperature,and pressure of environment during crystallization.However,at present,only growth processes of hydrothermal micas have been extensively investigated.The growth processes under other conditions have rarely been reported.It is well known that,mica formed by crystallization during magma cooling,an important part of the mica family,can form valuable mineral deposit.However,growth process of mica in the melt and related growth kinetics are poorly understood.In addition,mica often forms various polytypes during crystal growth due to some kinetics and/or thermodynamic factors.However,polytype formation remains controversial,especially for the trioctahedral micas?e.g.,biotite and phlogopite?.Such as:?1?whether structural control affects polymorphism and to what extent;?2?the relationship between growth mechanism?layer and spiral growth?and polytype formation;?3?does temperature rate change affect the forming of different polytypes;?4?whether the chemical composition controls the formation of different mica polytypes.Fluorophlogopite is one of trioctahedral micas,which is usually found in volcanic rocks,metamorphic rocks,and hydrothermal veins.Compared with natural samples,synthetic fluorophlogopite has the advantages of simple composition,large crystal size,and controllable reaction conditions.Previous studies showed that many polytypes?such as 1M,2M₁,and 3T?could coexist in fluorophlogopite.Therefore,fluorophlogopite is an ideal research object for the scientific issues studying including the growth process of mica in the melt and the formation mechanism of mica polytypes.The purpose of the present study is to reveal the relationship between the growth mechanism of fluorophlogopite in the melt and its driving force.The effects of structural control,growth mechanism,cooling rate,and Al concentration on the polytype formation of fluorophlogopite and the related mechanism are also discussed.The main conclusions are listed as below:1.Refined crystal structures of the 1M-and 2M₁-fluorophlogopite,and clarified the effect of structural control on the polytype formation.The space group of 1M-fluorophlogopite is C2/m,and the refined lattice parameters are a=5.2941?4??,b=9.1773?6??,c=10.1061?7??,?=100.141?7?°.The space group of the 2M₁-fluorophlogopite is C2/c,and the cell dimensions are a=5.3094?16??,b=9.1973?28??,c=20.0442?60??,and?=95.141?7?°.The 1M-and2M₁-fluorophlogopite have similar chemical compositions and were characterized by high Al₂O₃ content?15.60 wt%for 1M and 15.65 wt%for 2M₁?.The calculated chemicalformulasareK_1.00Na_0.01Mg_2.82Al_0.16(Si_2.88Al_1.12)O₁₀F₂and K_0.99Na_0.01Mg_2.84Al_0.15(Si_2.87Al_1.13)O₁₀F₂ for1M-and2M₁-fluorophlogopite,respectively.The isomorphous substitution of Al³⁺for Si⁴⁺in tetrahedral sites of fluorophlogopite causes distortion in the layer structure,and results in a larger tetrahedral rotation angle?,longer tetrahedral bond distance,larger bond length difference between inner and outer potassium-oxygen??K-O?,relatively higher value of octahedral flattening angle?,and smaller tetrahedral flattening angle?.The difference of structural control between 1M-and 2M₁-fluorophlogopite is very small,and the stacking sequences are easily affected by other environmental factors.Structural control shows positive influence on the formation of 2M₁ polytype,but probablely is not the main controlling factor.2.The realtions among surface micromorphology of fluorophlogopite growing in the melt,its growth mechanism and driving force are elucidated.The growth mechanism of fluorophlogopite in the melt is controlled by the driving force.The cooling rate between 3⁵?C/h is the transition regions of the layer and spiral growth.The spiral growth dominates for small driving force with the cooling rate of 3?C/h,whereas the layer growth prevails in the crystal growth when the cooling rate enlarges to 5?C/h.In addition,platelet deposition mechanism and negative spiral growth are also observed as well.The fluorophlogopite exhibits anisotropic growth,and growth speed along a-axis is apparently faster than that along b-axis direction.Pentagon morphology and pits can often form due to the concentration and anisotropic transportation.3.The influence of cooling rate on the polytype formation and its mechanism is explored.Cooling rate has an obvious influence on the polytype formation.Neither the slow cooling?3?C/h?nor the fast cooling favors the formation of2M₁-fluorophlogopite when the other conditions are kept constant.Between the cooling rate 5?C/h and 30?C/h,the content of 2M₁-fluorophlogopite increases from 9wt%to 14 wt%,and then decreases to 8 wt%.During the slow cooling process,only the 1M polytpe crystallizes because of its low energy minimum at equilibrium.The2M₁ polytype,with higher potential minima,can also crystallize metastably in the melt deviating from the equilibrium state when the cooling rate increases to 5³0?C/h.As cooling rate is above 50?C/h,no 2M₁ polytype crystallizes resulting from two reasons:1)the growth rate increases along c-axis direction;2)weak structural control cannot form ordered stacking of alternant?120?rotation at the rapid cooling conditions.In addition,spiral growth shows no relationship with polytype formation of fluorophlogopite.The 2M₁-fluorophlogopite crystallizes by the layer growth mechanism.4.The effect of Al on the polytype formation is investigated.There is no obvious correlation between the content of structural Al and the formation of 2M₁-fluorophlogopite.The content of 2M₁-fulorophlogopite has positive relation with the Al concentration in the initial reactants.This suggests that the growth condition with high Al concentration in the reactants favors the formation of2M₁-fluorophlogopite over 1M-fluorophlogopite.The reason lies on that Al ordering in the octahedral sheets probably affects the stacking-layer structure,and thus favours the formation of 2M₁-fluorophlogopite over 1M-fluorophlogopite.