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  <xs:element id="mg_OtherAmericasMember" abstract="true" name="OtherAmericasMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantFixedChargeCoverageRatio" abstract="false" name="DebtInstrumentCovenantFixedChargeCoverageRatio" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="mg_DebtCovenantPeriodSixMember" abstract="true" name="DebtCovenantPeriodSixMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantThresholdBusinessAcquisitionMinimumAmount" abstract="false" name="DebtInstrumentCovenantThresholdBusinessAcquisitionMinimumAmount" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_EffectiveIncomeTaxRateReconciliationDomesticNetOperatingLossCarrybackCARESActAmount" abstract="false" name="EffectiveIncomeTaxRateReconciliationDomesticNetOperatingLossCarrybackCARESActAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIncreaseDecreaseDueToChangesInAmount" abstract="false" name="FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityIncreaseDecreaseDueToChangesInAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_AccruedWorkersCompensationAndHealthBenefitsCurrent" abstract="false" name="AccruedWorkersCompensationAndHealthBenefitsCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantNonRecurringCashChargesAddedBackForPurposesOfDefiningEBITDA" abstract="false" name="DebtInstrumentCovenantNonRecurringCashChargesAddedBackForPurposesOfDefiningEBITDA" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_IndustrialsMember" abstract="true" name="IndustrialsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_CreditAgreementMember" abstract="true" name="CreditAgreementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_BoilermakersandPipefittersPlansMember" abstract="true" name="BoilermakersandPipefittersPlansMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_DefinedContributionPlanContinuousServicePeriodAfterWhichEmployeesBecomeEligibleToParticipateUnderPlan" abstract="false" name="DefinedContributionPlanContinuousServicePeriodAfterWhichEmployeesBecomeEligibleToParticipateUnderPlan" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_DefinedContributionPlanMinimumAgeForEmployeesToContributeCatchUpContributionsUnderInternalRevenueServicesLimits" abstract="false" name="DefinedContributionPlanMinimumAgeForEmployeesToContributeCatchUpContributionsUnderInternalRevenueServicesLimits" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_DeferredTaxAssetsDeferredTaxExpenseStockOptions" abstract="false" name="DeferredTaxAssetsDeferredTaxExpenseStockOptions" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DefinedContributionPlanEmployersMatchingContributionAnnualVestingPeriod" abstract="false" name="DefinedContributionPlanEmployersMatchingContributionAnnualVestingPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_PetrochemicalMember" abstract="true" name="PetrochemicalMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAwardMetrics" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfAwardMetrics" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="mg_ClassActionOnBehalfOfCurrentAndFormerEmployeesMember" abstract="true" name="ClassActionOnBehalfOfCurrentAndFormerEmployeesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_IncrementalCommonSharesAttributableToRestrictedStockUnitsOutstanding" abstract="false" name="IncrementalCommonSharesAttributableToRestrictedStockUnitsOutstanding" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="mg_DisposalOfForeignSubsidiariesMember" abstract="true" name="DisposalOfForeignSubsidiariesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ScheduleOfBusinessCombinationAcquisitionRelatedCostsTableTextBlock" abstract="false" name="ScheduleOfBusinessCombinationAcquisitionRelatedCostsTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="mg_VariableRateComponentDomain" abstract="true" name="VariableRateComponentDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_VariousPipelineProjectsForTexasCustomerMember" abstract="true" name="VariousPipelineProjectsForTexasCustomerMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ProceedsFromIncomeTaxRefundsCARESAct" abstract="false" name="ProceedsFromIncomeTaxRefundsCARESAct" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_OperatingLeasePercentageOfReductionOfRentalPayments" abstract="false" name="OperatingLeasePercentageOfReductionOfRentalPayments" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="mg_NetIncomeLossAndForeignCurrencyTranslationAdjustmentsAttributableToNoncontrollingInterest" abstract="false" name="NetIncomeLossAndForeignCurrencyTranslationAdjustmentsAttributableToNoncontrollingInterest" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrent" abstract="false" name="AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarningsChangeDueToChangeInTaxRateAmount" abstract="false" name="EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarningsChangeDueToChangeInTaxRateAmount" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DeferredTaxAssetsOperatingLeaseLiabilities" abstract="false" name="DeferredTaxAssetsOperatingLeaseLiabilities" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_OtherProcessIndustriesMember" abstract="true" name="OtherProcessIndustriesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_RestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsMember" abstract="true" name="RestrictedStockUnitsRSUsandPerformanceStockUnitsPSUsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarningsChangeDueToChangeInTaxRatePercent" abstract="false" name="EffectiveIncomeTaxRateReconciliationRepatriationOfForeignEarningsChangeDueToChangeInTaxRatePercent" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:percentItemType"/>
  <xs:element id="mg_CapitalManagementEnterpriseCMEMember" abstract="true" name="CapitalManagementEnterpriseCMEMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ProductsandSystemsMember" abstract="true" name="ProductsandSystemsMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ClassActionOnBehalfOfStateOfCaliforniaMember" abstract="true" name="ClassActionOnBehalfOfStateOfCaliforniaMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_AerospaceandDefenseMember" abstract="true" name="AerospaceandDefenseMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_LongTermPurchaseCommitmentExtensionPeriod" abstract="false" name="LongTermPurchaseCommitmentExtensionPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_IncreaseDecreaseInOtherAccruedLiabilitiesAndDeferredLiabilities" abstract="false" name="IncreaseDecreaseInOtherAccruedLiabilitiesAndDeferredLiabilities" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantConsolidatedDebtLeverageRatioLowestAvailableMarginApplicable" abstract="false" name="DebtInstrumentCovenantConsolidatedDebtLeverageRatioLowestAvailableMarginApplicable" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="mg_TermALoanFacilityMember" abstract="true" name="TermALoanFacilityMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_LongTermIncentivePlan2009Member" abstract="true" name="LongTermIncentivePlan2009Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_WeightedAverageDiscountRatePercentAbstract" abstract="true" name="WeightedAverageDiscountRatePercentAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="mg_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceConditionAdjustmentsNet" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsPerformanceConditionAdjustmentsNet" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="mg_LineOfCreditFacilityMaximumBorrowingCapacityInNonUSCurrencies" abstract="false" name="LineOfCreditFacilityMaximumBorrowingCapacityInNonUSCurrencies" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_RevenuePracticalExpedientIncrementalCostOfObtainingContractMaximumPeriod" abstract="false" name="RevenuePracticalExpedientIncrementalCostOfObtainingContractMaximumPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_DeferredIncomeTaxExpenseBenefitNet" abstract="false" name="DeferredIncomeTaxExpenseBenefitNet" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_ShareBasedCompensationArrangementByShareBasedPaymentAwardAverageSharePriceTradingPeriod" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardAverageSharePriceTradingPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_CashFlowLesseeAbstract" abstract="true" name="CashFlowLesseeAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="mg_OtherLongTermDebtMember" abstract="true" name="OtherLongTermDebtMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_LongTermIncentivePlan2016Member" abstract="true" name="LongTermIncentivePlan2016Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_LossContingencyNumberOfWeldsInspected" abstract="false" name="LossContingencyNumberOfWeldsInspected" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="mg_DeferredTaxAssetsInterestCarryforwards" abstract="false" name="DeferredTaxAssetsInterestCarryforwards" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantNumberOfDaysBeforeBusinessAcquisitionWrittenNoticeRequired" abstract="false" name="DebtInstrumentCovenantNumberOfDaysBeforeBusinessAcquisitionWrittenNoticeRequired" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_DebtCovenantPeriodFourMember" abstract="true" name="DebtCovenantPeriodFourMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_OperatingLeaseArrangementMember" abstract="true" name="OperatingLeaseArrangementMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_VariableRateComponentMember" abstract="true" name="VariableRateComponentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_AccruedSalariesWagesAndEmployeeRelatedBenefitsCurrent" abstract="false" name="AccruedSalariesWagesAndEmployeeRelatedBenefitsCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationPeriodFollowingDateNoLongerServingAsOfficer" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardExpirationPeriodFollowingDateNoLongerServingAsOfficer" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_PaymentsOfContingentConsiderationForBusinessAcquisitions" abstract="false" name="PaymentsOfContingentConsiderationForBusinessAcquisitions" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_ShareBasedCompensationArrangementByShareBasedPaymentAwardLastDayTradingPeriod" abstract="false" name="ShareBasedCompensationArrangementByShareBasedPaymentAwardLastDayTradingPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_DeferredIncomeTaxExpenseBenefitReserveForUncertainTaxPositions" abstract="false" name="DeferredIncomeTaxExpenseBenefitReserveForUncertainTaxPositions" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_ExecutiveAndSeniorOfficersMember" abstract="true" name="ExecutiveAndSeniorOfficersMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_LitigationSettlementReleaseOfClaim" abstract="false" name="LitigationSettlementReleaseOfClaim" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_AuditInformationAbstract" abstract="true" name="AuditInformationAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="mg_PurchaseCommitmentCumulativeAmountCommitted" abstract="false" name="PurchaseCommitmentCumulativeAmountCommitted" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtCovenantPeriodThreeMember" abstract="true" name="DebtCovenantPeriodThreeMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_PaymentsToAcquireIntangibleAssetsAndSoftware" abstract="false" name="PaymentsToAcquireIntangibleAssetsAndSoftware" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_AcquisitionRelatedContingenciesMember" abstract="true" name="AcquisitionRelatedContingenciesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_PaymentOfFinancingCostsOperatingActivities" abstract="false" name="PaymentOfFinancingCostsOperatingActivities" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtCovenantPeriodDomain" abstract="true" name="DebtCovenantPeriodDomain" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrentDisclosureTextBlock" abstract="false" name="AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrentDisclosureTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="mg_AcquisitionRelatedCostsAndFairValueAdjustmentsToInitialPurchasePriceAllocation" abstract="false" name="AcquisitionRelatedCostsAndFairValueAdjustmentsToInitialPurchasePriceAllocation" nillable="true" xbrli:periodType="duration" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtCovenantPeriodFiveMember" abstract="true" name="DebtCovenantPeriodFiveMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_OfficeFurnitureAndEquipmentMember" abstract="true" name="OfficeFurnitureAndEquipmentMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_ScheduleOfAccruedLiabilitiesDeferredRevenueAndContingentConsiderationLiabilitiesCurrentTableTextBlock" abstract="false" name="ScheduleOfAccruedLiabilitiesDeferredRevenueAndContingentConsiderationLiabilitiesCurrentTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="mg_WeightedAverageRemainingLeaseTermAbstract" abstract="true" name="WeightedAverageRemainingLeaseTermAbstract" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:stringItemType"/>
  <xs:element id="mg_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareDueToNetLossDuringPeriodAmount" abstract="false" name="AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareDueToNetLossDuringPeriodAmount" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:sharesItemType"/>
  <xs:element id="mg_DeferredTaxAssetsFinanceLeaseObligation" abstract="false" name="DeferredTaxAssetsFinanceLeaseObligation" nillable="true" xbrli:periodType="instant" xbrli:balance="debit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityAccretion" abstract="false" name="FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityAccretion" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityForeignCurrencyTranslationAdjustments" abstract="false" name="FairValueMeasurementWithUnobservableInputsReconciliationRecurringBasisLiabilityForeignCurrencyTranslationAdjustments" nillable="true" xbrli:periodType="duration" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_TheCreditAgreementJPMorganChaseBankMember" abstract="true" name="TheCreditAgreementJPMorganChaseBankMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_AccruedExpensesAndOtherLiabilitiesCurrent" abstract="false" name="AccruedExpensesAndOtherLiabilitiesCurrent" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
  <xs:element id="mg_DebtCovenantPeriodTwoMember" abstract="true" name="DebtCovenantPeriodTwoMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:domainItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantComplianceTotalConsolidatedDebtLeverageRatio" abstract="false" name="DebtInstrumentCovenantComplianceTotalConsolidatedDebtLeverageRatio" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="mg_SelfInsurancePolicyTextBlock" abstract="false" name="SelfInsurancePolicyTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="mg_EmployeeStockOwnershipPlanNumberOfPlans" abstract="false" name="EmployeeStockOwnershipPlanNumberOfPlans" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:integerItemType"/>
  <xs:element id="mg_AssetsAndLiabilitiesLesseeTableTextBlock" abstract="false" name="AssetsAndLiabilitiesLesseeTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types1:textBlockItemType"/>
  <xs:element id="mg_MultiemployerPlanPensionSignificantFundedStatusPercentage" abstract="false" name="MultiemployerPlanPensionSignificantFundedStatusPercentage" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
</xs:schema>
